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Monday, April 14, 2008

.NET 3.0

Microsoft’s .NET Framework 3.0 (formerly codenamed “WinFX”), is the new managed code programming model for Windows. It combines the power of the .NET Framework 2.0 with four new technologies:
  1. Windows Presentation Foundation (WPF)
  2. Windows Communication Foundation (WCF)
  3. Windows Workflow Foundation (WF)
  4. Windows CardSpace
Use the .NET Framework 3.0 to build applications that have visually compelling user experiences, seamless communication across technology boundaries, the ability to support a wide range of business processes, and an easier way to manage your personal information online.
Figure - The .NET Framework 3.0.

Windows Communication Foundation

Discover the new breed of communications infrastructure built around the Web services. This suite of .NET technologies for building and running connected systems unifies a broad array of distributed systems capabilities in a composable and extensible architecture to provide secure, reliable, and transacted messaging along with interoperability.

Windows Presentation Foundation

Find information to help you learn about the Windows Presentation Foundation, Microsoft's technology for building applications and high fidelity experiences in Windows Vista, blending together application UI, documents, and media content, while exploiting the full power of your computer. Get hands-on labs, samples, and more.

Windows Workflow Foundation

Learn all about Microsoft's new programming model, engine and tools for quickly building workflow-enabled applications on Windows. Windows Workflow Foundation includes support for both system workflow and human workflow across a wide range of scenarios.

Windows CardSpace

Find out how to build applications with Windows Cardspace (formerly "InfoCard"), Microsoft's technology for managing digital identities.

Note: For More Details about Framework 3.0, Please Refer the following Link

Sunday, April 13, 2008

Resources

Recommended books

I recommend the following books, either because I personally like them, or because I think they are well regarded by other .NET developers. (Note that I get a commission from Amazon if you buy a book after following one of these links.)


Web sites

Blogs

The following Weblogs ('blogs') have regular .NET content:

Free tools

.NET 2.0

What are the new features of .NET 2.0?

Generics, anonymous methods, partial classes, iterators, property visibility (separate visibility for get and set) and static classes. See http://msdn.microsoft.com/msdnmag/issues/04/05/C20/default.aspx for more information about these features.

What are the new 2.0 features useful for?

Generics are useful for writing efficient type-independent code, particularly where the types might include value types. The obvious application is container classes, and the .NET 2.0 class library includes a suite of generic container classes in the System.Collections.Generic namespace. Here's a simple example of a generic container class being used:

    List myList = new List();
myList.Add( 10 );

Anonymous methods reduce the amount of code you have to write when using delegates, and are therefore especially useful for GUI programming. Here's an example

    AppDomain.CurrentDomain.ProcessExit += delegate { Console.WriteLine("Process ending ..."); };

Partial classes is a useful feature for separating machine-generated code from hand-written code in the same class, and will therefore be heavily used by development tools such as Visual Studio.

Iterators reduce the amount of code you need to write to implement IEnumerable/IEnumerator. Here's some sample code:

    static void Main()
{
RandomEnumerator re = new RandomEnumerator( 5 );
foreach( double r in re )
Console.WriteLine( r );
Console.Read();
}

class RandomEnumerator : IEnumerable
{
public RandomEnumerator(int size) { m_size = size; }

public IEnumerator GetEnumerator()
{
Random rand = new Random();
for( int i=0; i <>
yield return rand.NextDouble();
}

int m_size = 0;
}

The use of 'yield return' is rather strange at first sight. It effectively synthethises an implementation of IEnumerator, something we had to do manually in .NET 1.x.

What's the problem with .NET generics?

.NET generics work great for container classes. But what about other uses? Well, it turns out that .NET generics have a major limitation - they require the type parameter to be constrained. For example, you cannot do this:

    static class Disposer
{
public static void Dispose(T obj) { obj.Dispose(); }
}

The C# compiler will refuse to compile this code, as the type T has not been constrained, and therefore only supports the methods of System.Object. Dispose is not a method on System.Object, so the compilation fails. To fix this code, we need to add a where clause, to reassure the compiler that our type T does indeed have a Dispose method

    static class Disposer where T : IDisposable
{
public static void Dispose(T obj) { obj.Dispose(); }
}

The problem is that the requirement for explicit contraints is very limiting. We can use constraints to say that T implements a particular interface, but we can't dilute that to simply say that T implements a particular method. Contrast this with C++ templates (for example), where no constraint at all is required - it is assumed (and verified at compile time) that if the code invokes the Dispose() method on a type, then the type will support the method.

In fact, after writing generic code with interface constraints, we quickly see that we haven't gained much over non-generic interface-based programming. For example, we can easily rewrite the Disposer class without generics:

    static class Disposer
{
public static void Dispose( IDisposable obj ) { obj.Dispose(); }
}

For more on this topic, start by reading the following articles:

Bruce Eckel: http://www.mindview.net/WebLog/log-0050
Ian Griffiths: http://www.interact-sw.co.uk/iangblog/2004/03/14/generics
Charles Cook: http://www.cookcomputing.com/blog/archives/000425.html

What's new in the .NET 2.0 class library?

Here is a selection of new features in the .NET 2.0 class library:

  • Generic collections in the System.Collections.Generic namespace.
  • The System.Nullable type. (Note that C# has special syntax for this type, e.g. int? is equivalent to Nullable)
  • The GZipStream and DeflateStream classes in the System.IO.Compression namespace.
  • The Semaphore class in the System.Threading namespace.
  • Wrappers for DPAPI in the form of the ProtectedData and ProtectedMemory classes in the System.Security.Cryptography namespace.
  • The IPC remoting channel in the System.Runtime.Remoting.Channels.Ipc namespace, for optimised intra-machine communication.

and many, many more. See http://msdn2.microsoft.com/en-us/library/t357fb32(en-US,VS.80).aspx for a comprehensive list of changes.

Miscellaneous

How does .NET remoting work?

.NET remoting involves sending messages along channels. Two of the standard channels are HTTP and TCP. TCP is intended for LANs only - HTTP can be used for LANs or WANs (internet).

Support is provided for multiple message serializarion formats. Examples are SOAP (XML-based) and binary. By default, the HTTP channel uses SOAP (via the .NET runtime Serialization SOAP Formatter), and the TCP channel uses binary (via the .NET runtime Serialization Binary Formatter). But either channel can use either serialization format.

There are a number of styles of remote access:

  • SingleCall. Each incoming request from a client is serviced by a new object. The object is thrown away when the request has finished.
  • Singleton. All incoming requests from clients are processed by a single server object.
  • Client-activated object. This is the old stateful (D)COM model whereby the client receives a reference to the remote object and holds that reference (thus keeping the remote object alive) until it is finished with it.

Distributed garbage collection of objects is managed by a system called 'leased based lifetime'. Each object has a lease time, and when that time expires the object is disconnected from the .NET runtime remoting infrastructure. Objects have a default renew time - the lease is renewed when a successful call is made from the client to the object. The client can also explicitly renew the lease.

If you're interested in using XML-RPC as an alternative to SOAP, take a look at Charles Cook's XML-RPC.Net.

How can I get at the Win32 API from a .NET program?

Use P/Invoke. This uses similar technology to COM Interop, but is used to access static DLL entry points instead of COM objects. Here is an example of C# calling the Win32 MessageBox function:

    using System; 
using System.Runtime.InteropServices;

class MainApp
{
[DllImport("user32.dll", EntryPoint="MessageBox", SetLastError=true, CharSet=CharSet.Auto)]
public static extern int MessageBox(int hWnd, String strMessage, String strCaption, uint uiType);

public static void Main()
{
MessageBox( 0, "Hello, this is PInvoke in operation!", ".NET", 0 );
}
}

Pinvoke.net is a great resource for off-the-shelf P/Invoke signatures.

How do I write to the application configuration file at runtime?

You don't. See http://www.interact-sw.co.uk/iangblog/2004/11/25/savingconfig.

What is the difference between an event and a delegate?

An event is just a wrapper for a multicast delegate. Adding a public event to a class is almost the same as adding a public multicast delegate field. In both cases, subscriber objects can register for notifications, and in both cases the publisher object can send notifications to the subscribers. However, a public multicast delegate has the undesirable property that external objects can invoke the delegate, something we'd normally want to restrict to the publisher. Hence events - an event adds public methods to the containing class to add and remove receivers, but does not make the invocation mechanism public.

See this post by Julien Couvreur for more discussion.

What size is a .NET object?

Each instance of a reference type has two fields maintained by the runtime - a method table pointer and a sync block. These are 4 bytes each on a 32-bit system, making a total of 8 bytes per object overhead. Obviously the instance data for the type must be added to this to get the overall size of the object. So, for example, instances of the following class are 12 bytes each:

    class MyInt
{
...
private int x;
}

However, note that with the current implementation of the CLR there seems to be a minimum object size of 12 bytes, even for classes with no data (e.g. System.Object).

Values types have no equivalent overhead.

Will my .NET app run on 64-bit Windows?

64-bit (x64) versions of Windows support both 32-bit and 64-bit processes, and corresponding 32-bit and 64-bit versions of .NET 2.0. (.NET 1.1 is 32-bit only).

.NET 1.x apps automatically run as 32-bit processes on 64-bit Windows.

.NET 2.0 apps can either run as 32-bit processes or as 64-bit processes. The OS decides which to use based on the PE header of the executable. The flags in the PE header are controlled via the compiler /platform switch, which allows the target of the app to be specified as 'x86', 'x64' or 'any cpu'. Normally you specify 'any cpu', and your app will run as 32-bit on 32-bit Windows and 64-bit on 64-bit Windows. However if you have some 32-bit native code in your app (loaded via COM interop, for example), you will need to specify 'x86', which will force 64-bit Windows to load your app in a 32-bit process. You can also tweak the 32-bit flag in the PE header using the SDK corflags utility.

Some more explanation here:

http://blogs.msdn.com/gauravseth/archive/2006/03/07/545104.aspx
http://blogs.msdn.com/joshwil/archive/2005/04/08/406567.aspx
http://msdn.microsoft.com/netframework/programming/64bit/gettingstarted/

What is reflection?

All .NET compilers produce metadata about the types defined in the modules they produce. This metadata is packaged along with the module (modules in turn are packaged together in assemblies), and can be accessed by a mechanism called reflection. The System.Reflection namespace contains classes that can be used to interrogate the types for a module/assembly.

Using reflection to access .NET metadata is very similar to using ITypeLib/ITypeInfo to access type library data in COM, and it is used for similar purposes - e.g. determining data type sizes for marshaling data across context/process/machine boundaries.

Reflection can also be used to dynamically invoke methods (see System.Type.InvokeMember), or even create types dynamically at run-time (see System.Reflection.Emit.TypeBuilder).

Tracing

Is there built-in support for tracing/logging?

Yes, in the System.Diagnostics namespace. There are two main classes that deal with tracing - Debug and Trace. They both work in a similar way - the difference is that tracing from the Debug class only works in builds that have the DEBUG symbol defined, whereas tracing from the Trace class only works in builds that have the TRACE symbol defined. Typically this means that you should use System.Diagnostics.Trace.WriteLine for tracing that you want to work in debug and release builds, and System.Diagnostics.Debug.WriteLine for tracing that you want to work only in debug builds.

Can I redirect tracing to a file?

Yes. The Debug and Trace classes both have a Listeners property, which is a collection of sinks that receive the tracing that you send via Debug.WriteLine and Trace.WriteLine respectively. By default the Listeners collection contains a single sink, which is an instance of the DefaultTraceListener class. This sends output to the Win32 OutputDebugString() function and also the System.Diagnostics.Debugger.Log() method. This is useful when debugging, but if you're trying to trace a problem at a customer site, redirecting the output to a file is more appropriate. Fortunately, the TextWriterTraceListener class is provided for this purpose.

Here's how to use the TextWriterTraceListener class to redirect Trace output to a file:

    Trace.Listeners.Clear();
FileStream fs = new FileStream( @"c:\log.txt", FileMode.Create, FileAccess.Write );
Trace.Listeners.Add( new TextWriterTraceListener( fs ) );

Trace.WriteLine( @"This will be writen to c:\log.txt!" );
Trace.Flush();

Note the use of Trace.Listeners.Clear() to remove the default listener. If you don't do this, the output will go to the file and OutputDebugString(). Typically this is not what you want, because OutputDebugString() imposes a big performance hit.

Can I customise the trace output?

Yes. You can write your own TraceListener-derived class, and direct all output through it. Here's a simple example, which derives from TextWriterTraceListener (and therefore has in-built support for writing to files, as shown above) and adds timing information and the thread ID for each trace line:

    class MyListener : TextWriterTraceListener
{
public MyListener( Stream s ) : base(s)
{
}

public override void WriteLine( string s )
{
Writer.WriteLine( "{0:D8} [{1:D4}] {2}",
Environment.TickCount - m_startTickCount,
AppDomain.GetCurrentThreadId(),
s );
}

protected int m_startTickCount = Environment.TickCount;
}

(Note that this implementation is not complete - the TraceListener.Write method is not overridden for example.)

The beauty of this approach is that when an instance of MyListener is added to the Trace.Listeners collection, all calls to Trace.WriteLine() go through MyListener, including calls made by referenced assemblies that know nothing about the MyListener class.

Are there any third party logging components available?

Log4net is a port of the established log4j Java logging component.

Threads

How do I spawn a thread?

Create an instance of a System.Threading.Thread object, passing it an instance of a ThreadStart delegate that will be executed on the new thread. For example:

    class MyThread
{
public MyThread( string initData )
{
m_data = initData;
m_thread = new Thread( new ThreadStart(ThreadMain) );
m_thread.Start();
}

// ThreadMain() is executed on the new thread.
private void ThreadMain()
{
Console.WriteLine( m_data );
}

public void WaitUntilFinished()
{
m_thread.Join();
}

private Thread m_thread;
private string m_data;
}

In this case creating an instance of the MyThread class is sufficient to spawn the thread and execute the MyThread.ThreadMain() method:

    MyThread t = new MyThread( "Hello, world." );
t.WaitUntilFinished();

How do I stop a thread?

There are several options. First, you can use your own communication mechanism to tell the ThreadStart method to finish. Alternatively the Thread class has in-built support for instructing the thread to stop. The two principle methods are Thread.Interrupt() and Thread.Abort(). The former will cause a ThreadInterruptedException to be thrown on the thread when it next goes into a WaitJoinSleep state. In other words, Thread.Interrupt is a polite way of asking the thread to stop when it is no longer doing any useful work. In contrast, Thread.Abort() throws a ThreadAbortException regardless of what the thread is doing. Furthermore, the ThreadAbortException cannot normally be caught (though the ThreadStart's finally method will be executed). Thread.Abort() is a heavy-handed mechanism which should not normally be required.

How do I use the thread pool?

By passing an instance of a WaitCallback delegate to the ThreadPool.QueueUserWorkItem() method

    class CApp
{
static void Main()
{
string s = "Hello, World";
ThreadPool.QueueUserWorkItem( new WaitCallback( DoWork ), s );

Thread.Sleep( 1000 ); // Give time for work item to be executed
}

// DoWork is executed on a thread from the thread pool.
static void DoWork( object state )
{
Console.WriteLine( state );
}
}

How do I know when my thread pool work item has completed?

There is no way to query the thread pool for this information. You must put code into the WaitCallback method to signal that it has completed. Events are useful for this.

How do I prevent concurrent access to my data?

Each object has a concurrency lock (critical section) associated with it. The System.Threading.Monitor.Enter/Exit methods are used to acquire and release this lock. For example, instances of the following class only allow one thread at a time to enter method f():

    class C
{
public void f()
{
try
{
Monitor.Enter(this);
...
}
finally
{
Monitor.Exit(this);
}
}
}

C# has a 'lock' keyword which provides a convenient shorthand for the code above:

    class C
{
public void f()
{
lock(this)
{
...
}
}
}

Note that calling Monitor.Enter(myObject) does NOT mean that all access to myObject is serialized. It means that the synchronisation lock associated with myObject has been acquired, and no other thread can acquire that lock until Monitor.Exit(o) is called. In other words, this class is functionally equivalent to the classes above:

    class C
{
public void f()
{
lock( m_object )
{
...
}
}

private m_object = new object();
}

Actually, it could be argued that this version of the code is superior, as the lock is totally encapsulated within the class, and not accessible to the user of the object.

Should I use ReaderWriterLock instead of Monitor.Enter/Exit?

Maybe, but be careful. ReaderWriterLock is used to allow multiple threads to read from a data source, while still granting exclusive access to a single writer thread. This makes sense for data access that is mostly read-only, but there are some caveats. First, ReaderWriterLock is relatively poor performing compared to Monitor.Enter/Exit, which offsets some of the benefits. Second, you need to be very sure that the data structures you are accessing fully support multithreaded read access. Finally, there is apparently a bug in the v1.1 ReaderWriterLock that can cause starvation for writers when there are a large number of readers.

Ian Griffiths has some interesting discussion on ReaderWriterLock here and here.

Implications for COM

Does .NET replace COM?

This subject causes a lot of controversy, as you'll see if you read the mailing list archives. Take a look at the following two threads:

http://discuss.develop.com/archives/wa.exe?A2=ind0007&L=DOTNET&D=0&P=68241
http://discuss.develop.com/archives/wa.exe?A2=ind0007&L=DOTNET&P=R60761

The bottom line is that .NET has its own mechanisms for type interaction, and they don't use COM. No IUnknown, no IDL, no typelibs, no registry-based activation. This is mostly good, as a lot of COM was ugly. Generally speaking, .NET allows you to package and use components in a similar way to COM, but makes the whole thing a bit easier.

DCOM dead?

Pretty much, for .NET developers. The .NET Framework has a new remoting model which is not based on DCOM. DCOM was pretty much dead anyway, once firewalls became widespread and Microsoft got SOAP fever. Of course DCOM will still be used in interop scenarios.

Is COM+ dead?

Not immediately. The approach for .NET 1.0 was to provide access to the existing COM+ services (through an interop layer) rather than replace the services with native .NET ones. Various tools and attributes were provided to make this as painless as possible. Over time it is expected that interop will become more seamless - this may mean that some services become a core part of the CLR, and/or it may mean that some services will be rewritten as managed code which runs on top of the CLR.

For more on this topic, search for postings by Joe Long in the archives - Joe is the MS group manager for COM+. Start with this message:

http://discuss.develop.com/archives/wa.exe?A2=ind0007&L=DOTNET&P=R68370

Can I use COM components from .NET programs?

Yes. COM components are accessed from the .NET runtime via a Runtime Callable Wrapper (RCW). This wrapper turns the COM interfaces exposed by the COM component into .NET-compatible interfaces. For oleautomation interfaces, the RCW can be generated automatically from a type library. For non-oleautomation interfaces, it may be necessary to develop a custom RCW which manually maps the types exposed by the COM interface to .NET-compatible types.

Here's a simple example for those familiar with ATL. First, create an ATL component which implements the following IDL:

    import "oaidl.idl";
import "ocidl.idl";

[
object,
uuid(EA013F93-487A-4403-86EC-FD9FEE5E6206),
helpstring("ICppName Interface"),
pointer_default(unique),
oleautomation
]

interface ICppName : IUnknown
{
[helpstring("method SetName")] HRESULT SetName([in] BSTR name);
[helpstring("method GetName")] HRESULT GetName([out,retval] BSTR *pName );
};

[
uuid(F5E4C61D-D93A-4295-A4B4-2453D4A4484D),
version(1.0),
helpstring("cppcomserver 1.0 Type Library")
]
library CPPCOMSERVERLib
{
importlib("stdole32.tlb");
importlib("stdole2.tlb");
[
uuid(600CE6D9-5ED7-4B4D-BB49-E8D5D5096F70),
helpstring("CppName Class")
]
coclass CppName
{
[default] interface ICppName;
};
};

When you've built the component, you should get a typelibrary. Run the TLBIMP utility on the typelibary, like this:

    tlbimp cppcomserver.tlb

If successful, you will get a message like this:

    Typelib imported successfully to CPPCOMSERVERLib.dll

You now need a .NET client - let's use C#. Create a .cs file containing the following code:

    using System;
using CPPCOMSERVERLib;

public class MainApp
{
static public void Main()
{
CppName cppname = new CppName();
cppname.SetName( "bob" );
Console.WriteLine( "Name is " + cppname.GetName() );
}
}

Compile the C# code like this:

    csc /r:cppcomserverlib.dll csharpcomclient.cs

Note that the compiler is being told to reference the DLL we previously generated from the typelibrary using TLBIMP. You should now be able to run csharpcomclient.exe, and get the following output on the console:

    Name is bob

Can I use .NET components from COM programs?

Yes. .NET components are accessed from COM via a COM Callable Wrapper (CCW). This is similar to a RCW (see previous question), but works in the opposite direction. Again, if the wrapper cannot be automatically generated by the .NET development tools, or if the automatic behaviour is not desirable, a custom CCW can be developed. Also, for COM to 'see' the .NET component, the .NET component must be registered in the registry.

Here's a simple example. Create a C# file called testcomserver.cs and put the following in it:

    using System;
using System.Runtime.InteropServices;

namespace AndyMc
{
[ClassInterface(ClassInterfaceType.AutoDual)]
public class CSharpCOMServer
{
public CSharpCOMServer() {}
public void SetName( string name ) { m_name = name; }
public string GetName() { return m_name; }
private string m_name;
}
}

Then compile the .cs file as follows:

    csc /target:library testcomserver.cs

You should get a dll, which you register like this:

    regasm testcomserver.dll /tlb:testcomserver.tlb /codebase

Now you need to create a client to test your .NET COM component. VBScript will do - put the following in a file called comclient.vbs:

    Dim dotNetObj
Set dotNetObj = CreateObject("AndyMc.CSharpCOMServer")
dotNetObj.SetName ("bob")
MsgBox "Name is " & dotNetObj.GetName()

and run the script like this:

    wscript comclient.vbs

And hey presto you should get a message box displayed with the text "Name is bob".

An alternative to the approach above it to use the dm.net moniker developed by Jason Whittington and Don Box.

Is ATL redundant in the .NET world?

Yes. ATL will continue to be valuable for writing COM components for some time, but it has no place in the .NET world.

Intermediate Language (IL)

Can I look at the IL for an assembly?

Yes. MS supply a tool called ILDASM that can be used to view the metadata and IL for an assembly.

Source code be reverse-engineered from IL?

Yes, it is often relatively straightforward to regenerate high-level source from IL. Lutz Roeder's Reflector does a very good job of turning IL into C# or VB.NET.

How can I stop my code being reverse-engineered from IL?

You can buy an IL obfuscation tool. These tools work by 'optimising' the IL in such a way that reverse-engineering becomes much more difficult.

Of course if you are writing web services then reverse-engineering is not a problem as clients do not have access to your IL.

Can I write IL programs directly?

Yes. Peter Drayton posted this simple example to the DOTNET mailing list:

    .assembly MyAssembly {}

.class MyApp {

.method static void Main() {
.entrypoint
ldstr "Hello, IL!"
call void System.Console::WriteLine(class System.Object)
ret
}
}

Just put this into a file called hello.il, and then run ilasm hello.il. An exe assembly will be generated.

Can I do things in IL that I can't do in C#?

Yes. A couple of simple examples are that you can throw exceptions that are not derived from System.Exception, and you can have non-zero-based arrays.


Code Access Security

What is Code Access Security (CAS)?

CAS is the part of the .NET security model that determines whether or not code is allowed to run, and what resources it can use when it is running. For example, it is CAS that will prevent a .NET web applet from formatting your hard disk.

How does CAS work?

The CAS security policy revolves around two key concepts - code groups and permissions. Each .NET assembly is a member of a particular code group, and each code group is granted the permissions specified in a named permission set.
For example, using the default security policy, a control downloaded from a web site belongs to the 'Zone - Internet' code group, which adheres to the permissions defined by the 'Internet' named permission set. (Naturally the 'Internet' named permission set represents a very restrictive range of permissions.)

Who defines the CAS code groups?

Microsoft defines some default ones, but you can modify these and even create your own. To see the code groups defined on your system, run 'caspol -lg' from the command-line. On my system it looks like this:


Level = Machine
Code Groups:
1. All code: Nothing
1.1. Zone - MyComputer: FullTrust
1.1.1. Honor SkipVerification requests: SkipVerification
1.2. Zone - Intranet: LocalIntranet
1.3. Zone - Internet: Internet
1.4. Zone - Untrusted: Nothing
1.5. Zone - Trusted: Internet
1.6. StrongName -

0024000004800000940000000602000000240000525341

310004000003


000000CFCB3291AA715FE99D40D49040336F9056D7886

FED46775BC7BB5430BA4444FEF8348EBD06


F962F39776AE4DC3B7B04A7FE6F49F25F740423EBF2C0B

89698D8D08AC48D69CED0FC8F83B465E08


07AC11EC1DCC7D054E807A43336DDE408A5393A485561

23272CEEEE72F1660B71927D38561AABF5C


AC1DF1734633C602F8F2D5: Everything

Note the hierarchy of code groups - the top of the hierarchy is the most general ('All code'), which is then sub-divided into several groups, each of which in turn can be sub-divided. Also note that (somewhat counter-intuitively) a sub-group can be associated with a more permissive permission set than its parent.

How do I define my own code group?

Use caspol. For example, suppose you trust code from www.mydomain.com and you want it have full access to your system, but you want to keep the default restrictions for all other internet sites. To achieve this, you would add a new code group as a sub-group of the 'Zone - Internet' group, like this:

caspol -ag 1.3 -site www.mydomain.com FullTrust

Now if you run caspol -lg you will see that the new group has been added as group 1.3.1:

... 1.3. Zone - Internet: Internet

1.3.1. Site - www.mydomain.com: FullTrust

...

Note that the numeric label (1.3.1) is just a caspol invention to make the code groups easy to manipulate from the command-line. The underlying runtime never sees it.

How do I change the permission set for a code group?

Use caspol. If you are the machine administrator, you can operate at the 'machine' level - which means not only that the changes you make become the default for the machine, but also that users cannot change the permissions to be more permissive. If you are a normal (non-admin) user you can still modify the permissions, but only to make them more restrictive. For example, to allow intranet code to do what it likes you might do this:

caspol -cg 1.2 FullTrust

Note that because this is more permissive than the default policy (on a

standard system), you should only do this at the machine level - doing it at the user level will have no effect.

Can I create my own permission set?

Yes. Use caspol -ap, specifying an XML file containing the permissions in the permission set. To save you some time, here is a sample file corresponding to the 'Everything' permission set - just edit to suit your needs. When you have edited the sample, add it to the range of available permission sets like this:
caspol -ap samplepermset.xmlThen, to apply the permission set to a code group, do something like this:

caspol -cg 1.3 SamplePermSet

(By default, 1.3 is the 'Internet' code group)

I'm having some trouble with CAS. How can I troubleshoot the problem?

Caspol has a couple of options that might help. First, you can ask caspol to tell you what code group an assembly belongs to, using caspol -rsg. Similarly, you can ask what permissions are being applied to a particular assembly using caspol -rsp.

I can't be bothered with CAS. Can I turn it off?
Yes, as long as you are an administrator. Just run:
caspol -s off

Attributes

What are attributes?

There are at least two types of .NET attribute. The first type I will refer to as a metadata attribute - it allows some data to be attached to a class or method. This data becomes part of the metadata for the class, and (like other class metadata) can be accessed via reflection. An example of a metadata attribute is [serializable], which can be attached to a class and means that instances of the class can be serialized.

[serializable] public class CTest {}

The other type of attribute is a context attribute. Context attributes use a similar syntax to metadata attributes but they are fundamentally different. Context attributes provide an interception mechanism whereby instance activation and method calls can be pre- and/or post-processed. If you have encountered Keith Brown's universal delegator you'll be familiar with this idea.

7.2 Can I create my own metadata attributes?

Yes. Simply derive a class from System.Attribute and mark it with the AttributeUsage attribute. For example:

[AttributeUsage(AttributeTargets.Class)]

public class InspiredByAttribute : System.Attribute
{
public string InspiredBy;

public InspiredByAttribute( string inspiredBy )
{
InspiredBy = inspiredBy;
}
}

[InspiredBy("Andy Mc's brilliant .NET FAQ")]

class CTest
{
}

class CApp
{

public static void Main()
{
object[] atts = typeof(CTest).GetCustomAttributes(true);
foreach( object att in atts )

if( att is InspiredByAttribute )
Console.WriteLine( "Class CTest was inspired by{0}", ((InspiredByAttribute) att).InspiredBy );
}
}

Serialization

What is serialization?
Serialization is the process of converting an object into a stream of bytes. Deserialization is the opposite process of creating an object from a stream of bytes. Serialization/Deserialization is mostly used to transport objects (e.g. during remoting), or to persist objects (e.g. to a file or database).

Does the .NET Framework have in-built support for serialization?
There are two separate mechanisms provided by the .NET class library - XmlSerializer and SoapFormatter/BinaryFormatter. Microsoft uses XmlSerializer for Web Services, and uses SoapFormatter/BinaryFormatter for remoting. Both are available for use in your own code.

I want to serialize instances of my class. Should I use XmlSerializer, SoapFormatter or BinaryFormatter?
It depends. XmlSerializer has severe limitations such as the requirement that the target class has a parameterless constructor, and only public read/write properties and fields can be serialized. However, on the plus side, XmlSerializer has good support for customising the XML document that is produced or consumed. XmlSerializer's features mean that it is most suitable for cross-platform work, or for constructing objects from existing XML documents.SoapFormatter and BinaryFormatter have fewer limitations than XmlSerializer. They can serialize private fields, for example. However they both require that the target class be marked with the [Serializable] attribute, so like XmlSerializer the class needs to be written with serialization in mind. Also there are some quirks to watch out for - for example on deserialization the constructor of the new object is not invoked.The choice between SoapFormatter and BinaryFormatter depends on the application. BinaryFormatter makes sense where both serialization and deserialization will be performed on the .NET platform and where performance is important. SoapFormatter generally makes more sense in all other cases, for ease of debugging if nothing else.

Can I customise the serialization process?
Yes. XmlSerializer supports a range of attributes that can be used to configure serialization for a particular class. For example, a field or property can be marked with the [XmlIgnore] attribute to exclude it from serialization. Another example is the [XmlElement] attribute, which can be used to specify the XML element name to be used for a particular property or field.Serialization via SoapFormatter/BinaryFormatter can also be controlled to some extent by attributes. For example, the [NonSerialized] attribute is the equivalent of XmlSerializer's [XmlIgnore] attribute. Ultimate control of the serialization process can be acheived by implementing the the ISerializable interface on the class whose instances are to be serialized.

Why is XmlSerializer so slow?
There is a once-per-process-per-type overhead with XmlSerializer. So the first time you serialize or deserialize an object of a given type in an application, there is a significant delay. This normally doesn't matter, but it may mean, for example, that XmlSerializer is a poor choice for loading configuration settings during startup of a GUI application.

Why do I get errors when I try to serialize a Hashtable?
XmlSerializer will refuse to serialize instances of any class that implements IDictionary, e.g. Hashtable. SoapFormatter and BinaryFormatter do not have this restriction.

XmlSerializer is throwing a generic "There was an error reflecting MyClass" error. How do I find out what the problem is?
Look at the InnerException property of the exception that is thrown to get a more specific error message.

Why am I getting an InvalidOperationException when I serialize an ArrayList?
XmlSerializer needs to know in advance what type of objects it will find in an ArrayList. To specify the type, use the XmlArrayItem attibute like this:

public class Person
{
public string Name;
public int Age;
}

public class Population
{
[XmlArrayItem(typeof(Person))] public ArrayList People;
}

Garbage Collection

What is garbage collection?

Garbage collection is a heap-management strategy where a run-time component takes responsibility for managing the lifetime of the memory used by objects. This concept is not new to .NET - Java and many other languages/runtimes have used garbage collection for some time.

Is it true that objects don't always get destroyed immediately when the last reference goes away?

Yes. The garbage collector offers no guarantees about the time when an object will be destroyed and its memory reclaimed.

Why doesn't the .NET runtime offer deterministic destruction?

Because of the garbage collection algorithm. The .NET garbage collector works by periodically running through a list of all the objects that are currently being referenced by an application. All the objects that it doesn't find during this search are ready to be destroyed and the memory reclaimed. The implication of this algorithm is that the runtime doesn't get notified immediately when the final reference on an object goes away - it only finds out during the next sweep of the heap.Futhermore, this type of algorithm works best by performing the garbage collection sweep as rarely as possible. Normally heap exhaustion is the trigger for a collection sweep.

Is the lack of deterministic destruction in .NET a problem?

It's certainly an issue that affects component design. If you have objects that maintain expensive or scarce resources (e.g. database locks), you need to provide some way for the client to tell the object to release the resource when it is done. Microsoft recommend that you provide a method called Dispose() for this purpose. However, this causes problems for distributed objects - in a distributed system who calls the Dispose() method? Some form of reference-counting or ownership-management mechanism is needed to handle distributed objects - unfortunately the runtime offers no help with this.

Does non-deterministic destruction affect the usage of COM objects from managed code?

Yes. When using a COM object from managed code, you are effectively relying on the garbage collector to call the final release on your object. If your COM object holds onto an expensive resource which is only cleaned-up after the final release, you may need to provide a new interface on your object which supports an explicit Dispose() method.

Do I have any control over the garbage collection algorithm?

A little. For example, the System.GC class exposes a Collect method - this forces the garbage collector to collect all unreferenced objects immediately.

How can I find out what the garbage collector is doing?

Lots of interesting statistics are exported from the .NET runtime via the '.NET CLR xxx' performance counters. Use Performance Monitor to view them.

I've heard that Finalize methods should be avoided. Should I implement Finalize on my class?

An object with a Finalize method is more work for the garbage collector than an object without one. Also there are no guarantees about the order in which objects are Finalized, so there are issues surrounding access to other objects from the Finalize method. Finally, there is no guarantee that a Finalize method will get called on an object, so it should never be relied upon to do clean-up of an object's resources.Microsoft recommend the following pattern:

public class CTest : IDisposable

{

public void Dispose()

{... // Cleanup activities

GC.SuppressFinalize(this);

}

~CTest() // C# syntax hiding the Finalize() method

{

Dispose();

}

}

In the normal case the client calls Dispose(), the object's resources are freed, and the garbage collector is relieved of its Finalizing duties by the call to SuppressFinalize() In the worst case, ie, the client forgets to call Dispose(), there is a reasonable chance that the object's resources will eventually get freed by the garbage collector calling Finalize() Given the limitations of the garbage collection algorithm this seems like a pretty reasonable approach

What is the lapsed listener problem?

The lapsed listener problem is one of the primary causes of leaks in .NET applications. It occurs when a subscriber (or ‘listener’) signs up for a publisher’s event, but fails to unsubscribe. The failure to unsubscribe means that the publisher maintains a reference to the subscriber as long as the publisher is alive. For some publishers, this may be the duration of the application.
This situation causes two problems. The obvious problem is the leakage of the subscriber object. The other problem is the performance degredation due to the publisher sending redundant notifications to ‘zombie’ subscribers.
There are at least a couple of solutions to the problem. The simplest is to make sure the subscriber is unsubscribed from the publisher, typically by adding an Unsubscribe() method to the subscriber.

When do I need to use GC.KeepAlive?

It's very unintuitive, but the runtime can decide that an object is garbage much sooner than you expect. More specifically, an object can become garbage while a method is executing on the object, which is contrary to most developers' expectations.

Example:

using System;

using System.Runtime.InteropServices;

class Win32

{

[DllImport("kernel32.dll")]

public static extern IntPtr CreateEvent( IntPtr lpEventAttributes,bool bManualReset,bool bInitialState, string lpName);

[DllImport("kernel32.dll", SetLastError=true)]

public static extern bool CloseHandle(IntPtr hObject);

[DllImport("kernel32.dll")]

public static extern bool SetEvent(IntPtr hEvent);

}

class EventUser

{

public EventUser()

{

hEvent = Win32.CreateEvent( IntPtr.Zero, false, false, null );

}

~EventUser()

{

Win32.CloseHandle( hEvent );

Console.WriteLine("EventUser finalized");

}

public void UseEvent()

{

UseEventInStatic( this.hEvent );

}

static void UseEventInStatic( IntPtr hEvent )

{ //GC.Collect();

bool bSuccess = Win32.SetEvent( hEvent );

Console.WriteLine( "SetEvent " + (bSuccess ? "succeeded" : "FAILED!") );

}

IntPtr hEvent;

}

class App

{

static void Main(string[] args)

{

EventUser eventUser = new EventUser();

eventUser.UseEvent();

}

}
If you run this code, it'll probably work fine, and you'll get the following output:

SetEvent succeeded

EventDemo finalized
However, if you uncomment the GC.Collect() call in the UseEventInStatic() method, you'll get this output:

EventDemo finalized

SetEvent FAILED!
(Note that you need to use a release build to reproduce this problem.)
So what's happening here? Well, at the point where UseEvent() calls UseEventInStatic(), a copy is taken of the hEvent field, and there are no further references to the EventUser object anywhere in the code. So as far as the runtime is concerned, the EventUser object is garbage and can be collected. Normally of course the collection won't happen immediately, so you'll get away with it, but sooner or later a collection will occur at the wrong time, and your app will fail.

A solution to this problem is to add a call to GC.KeepAlive(this) to the end of the UseEvent method

Friday, April 11, 2008

Application Domains

What is an application domain?

An AppDomain can be thought of as a lightweight process. Multiple AppDomains can exist inside a Win32 process. The primary purpose of the AppDomain is to isolate applications from each other, and so it is particularly useful in hosting scenarios such as ASP.NET. An AppDomain can be destroyed by the host without affecting other AppDomains in the process.Win32 processes provide isolation by having distinct memory address spaces. This is effective, but expensive. The .NET runtime enforces AppDomain isolation by keeping control over the use of memory - all memory in the AppDomain is managed by the .NET runtime, so the runtime can ensure that AppDomains do not access each other's memory.One non-obvious use of AppDomains is for unloading types. Currently the only way to unload a .NET type is to destroy the AppDomain it is loaded into. This is particularly useful if you create and destroy types on-the-fly via reflection.

How does an AppDomain get created?

AppDomains are usually created by hosts. Examples of hosts are the Windows Shell, ASP.NET and IE. When you run a .NET application from the command-line, the host is the Shell. The Shell creates a new AppDomain for every application.AppDomains can also be explicitly created by .NET applications. Here is a C# sample which creates an AppDomain, creates an instance of an object inside it, and then executes one of the object's methods:

using System;
using System.Runtime.Remoting;
using System.Reflection;

public class CAppDomainInfo : MarshalByRefObject
{

public string GetName()
{
return AppDomain.CurrentDomain.FriendlyName;
}
}
public class App
{
public static int Main()
{
AppDomain ad = AppDomain.CreateDomain( "Andy's new domain" ); CAppDomainInfo adInfo = (CAppDomainInfo)ad.CreateInstanceAndUnwrap(Assembly.GetCallingAssembly().GetName().Name, "CAppDomainInfo" );
Console.WriteLine( "Created AppDomain name = " + adInfo.GetName()
return 0;
}
}

Assemblies

What is an assembly?

An assembly is sometimes described as a logical .EXE or .DLL, and can be an application (with a main entry point) or a library. An assembly consists of one or more files (dlls, exes, html files etc), and represents a group of resources, type definitions, and implementations of those types. An assembly may also contain references to other assemblies. These resources, types and references are described in a block of data called a manifest. The manifest is part of the assembly, thus making the assembly self-describing.

An important aspect of assemblies is that they are part of the identity of a type. The identity of a type is the assembly that houses it combined with the type name. This means, for example, that if assembly A exports a type called T, and assembly B exports a type called T, the .NET runtime sees these as two completely different types. Furthermore, don't get confused between assemblies and namespaces - namespaces are merely a hierarchical way of organising type names. To the runtime, type names are type names, regardless of whether namespaces are used to organise the names. It's the assembly plus the typename (regardless of whether the type name belongs to a namespace) that uniquely indentifies a type to the runtime.

Assemblies are also important in .NET with respect to security - many of the security restrictions are enforced at the assembly boundary.

How can I produce an assembly?

The simplest way to produce an assembly is directly from a .NET compiler.

For example, the following C# program:

public class CTest
{
public CTest()
{
System.Console.WriteLine( "Hello from CTest" );
}
}

can be compiled into a library assembly (dll) like this:

csc /t:library ctest.cs

You can then view the contents of the assembly by running the "IL Disassembler" tool that comes with the .NET SDK.

Alternatively you can compile your source into modules, and then combine the modules into an assembly using the assembly linker (al.exe). For the C# compiler, the /target:module switch is used to generate a module instead of an assembly.


What is the difference between a private assembly and a shared assembly?

The terms 'private' and 'shared' refer to how an assembly is deployed, not any intrinsic attributes of the assembly.

  • Location and Visiblity:
A private assembly is normally used by a single application, and is stored in the application's directory, or a sub-directory beneath. A shared assembly is intended to be used by multiple applications, and is normally stored in the global assembly cache (GAC), which is a central repository for assemblies maintained by the CLR. (A shared assembly can also be stored outside the GAC, in which case each application must be pointed to its location via a codebase entry in the application's configuration file.) shared assemblies are usually libraries of code which many applications will find useful. E.g. the .NET framework classes.

The main advantage of deploying assemblies to the GAC is that the GAC can support multiple versions of the same assembly side-by-side. Assemblies deployed to the GAC must be strong-named. Outside the GAC, strong-naming is optional.
  • Versioning:
The runtime enforces versioning constraints only on shared assemblies, not on private assemblies


How do assemblies find each other?

By searching directory paths. There are several factors that can affect the path (such as the AppDomain host, and application configuration files), but for weakly named assemblies (Private Assembly) the search path is normally the application's directory and its sub-directories. For strongly named assemblies(shared Assembly), the search path is the GAC followed by the private assembly path

How does assembly versioning work?

Each assembly has a version number called the compatiblity version. Also each reference to an assembly (from another assembly) includes both the name and versions of referenced assembly.

An assembly has a version number consisting of four parts, e.g. 2.2.150.2. These are typically interpreted as Major.Minor.Build.Revision, but this is just a convention.

The CLR applies no version constraints on weakly named assemblies, so the assembly version has no real significance.

For strongly named assemblies, the version of a referenced assembly is stored in the referring assembly, and by default only this exact version will be loaded at run-time. If the exact version is not available, the referring assembly will fail to load. It is possible to override this behaviour in the config file for the referring assembly - references to a single version or a range of versions of the referenced assembly can be redirected to a specific version. For example, versions 1.0.0.0 to 2.0.0.0 can be redirected to version 3.0.125.3. However note that there is no way to specify a range of versions to be redirected to. Publisher policy files offer an alternative mechanism for redirecting to a different version for assemblies deployed to the GAC - a publisher policy file allows the publisher of the assembly to redirect all applications to a new version of an assembly in one operation, rather than having to modify all of the application configuration files.

The restrictions on version policy for strongly named assemblies can cause problems when providing patches or 'hot fixes' for individual assemblies within an application. To avoid having to deploy config file changes or publisher policy files along with the hot fix, it makes sense to reuse the same assembly version for the hot fix. If desired, the assemblies can be distinguised by altering the assembly file version, which is not used at all by the CLR for applying version policy.

Note that the versioning of strongly named assemblies applies whether the assemblies are deployed privately or to the GAC.