MSIL allows for overloads which differ only in return types because of

call void [mscorlib]System.Console::Write(string)

or

callvirt int32 ...

Most .Net languages including C# and VB do not use the tail recursion feature of MSIL code.

Tail recursion is an optimization that is common in functional languages. It occurs when a method A ends by returning the value of method B such that method A's stack can be deallocated once the call to method B is made.

MSIL code supports tail recursion explicitly, and for some algorithms this could be a important optimization to make. But since C# and VB do not generate the instructions to do this, it must be done manually (or using F# or some other language).

Here is an example of how tail-recursion may be implemented manually in C#:

private static int RecursiveMethod(int myParameter)
{
    // Body of recursive method
    if (BaseCase(details))
        return result;
    // ...

    return RecursiveMethod(modifiedParameter);
}

// Is transformed into:

private static int RecursiveMethod(int myParameter)
{
    while (true)
    {
        // Body of recursive method
        if (BaseCase(details))
            return result;
        // ...

        myParameter = modifiedParameter;
    }
}

It is common practice to remove recursion by moving the local data from the hardware stack onto a heap-allocated stack data structure. In the tail-call recursion elimination as shown above, the stack is eliminated completely, which is a pretty good optimization. Also, the return value does not have to walk up a long call-chain, but it is returned directly.

But, anyway, the CIL provides this feature as part of the language, but with C# or VB it has to be implemented manually. (The jitter is also free to make this optimization on its own, but that is a whole other issue.)

In MSIL, you can have a class which cannot inherit from System.Object.

Sample code: compile it with ilasm.exe UPDATE: You must use "/NOAUTOINHERIT" to prevent assembler from auto inheriting.

// Metadata version: v2.0.50215
.assembly extern mscorlib
{
  .publickeytoken = (B7 7A 5C 56 19 34 E0 89 )                         // .z\V.4..
  .ver 2:0:0:0
}
.assembly sample
{
  .custom instance void [mscorlib]System.Runtime.CompilerServices.CompilationRelaxationsAttribute::.ctor(int32) = ( 01 00 08 00 00 00 00 00 ) 
  .hash algorithm 0x00008004
  .ver 0:0:0:0
}
.module sample.exe
// MVID: {A224F460-A049-4A03-9E71-80A36DBBBCD3}
.imagebase 0x00400000
.file alignment 0x00000200
.stackreserve 0x00100000
.subsystem 0x0003       // WINDOWS_CUI
.corflags 0x00000001    //  ILONLY
// Image base: 0x02F20000


// =============== CLASS MEMBERS DECLARATION ===================

.class public auto ansi beforefieldinit Hello
{
  .method public hidebysig static void  Main(string[] args) cil managed
  {
    .entrypoint
    // Code size       13 (0xd)
    .maxstack  8
    IL_0000:  nop
    IL_0001:  ldstr      "Hello World!"
    IL_0006:  call       void [mscorlib]System.Console::WriteLine(string)
    IL_000b:  nop
    IL_000c:  ret
  } // end of method Hello::Main
} // end of class Hello

It's possible to combine the protected and internal access modifiers. In C#, if you write protected internal a member is accessible from the assembly and from derived classes. Via MSIL you can get a member which is accessible from derived classes within the assembly only. (I think that could be pretty useful!)

Ooh, I didn't spot this at the time. (If you add the jon-skeet tag it's more likely, but I don't check it that often.)

It looks like you've got pretty good answers already. In addition:

• You can't get a handle on the boxed version of a value type in C#. You can in C++/CLI
• You can't do a try/fault in C# ("fault" is a like a "catch everything and rethrow at the end of the block" or "finally but only on failure")
• There are lots of names which are forbidden by C# but legal IL
• IL allows you to define your own parameterless constructors for value types ^[1].
• You can't define events with a "raise" element in C#. (In VB you have to for custom events, but "default" events don't include one.)
• Some conversions are allowed by the CLR but not by C#. If you go via object in C#, these will sometimes work. See a uint[]/int[] SO question ^[2] for an example.

I'll add to this if I think of anything else...

The CLR supports generic co/contravariance already, but C# is not getting this feature until 4.0

• C# 4.0 Features ^[1]
• Co/Contravariance ^[2]

In IL you can throw and catch any type at all, not just types derived from System.Exception.

IL has the distinction between call and callvirt for virtual method calls. By using the former you can force calling a virtual method of the current static class type instead of the virtual function in the dynamic class type.

C# has no way of doing this:

abstract class Foo {
    public void F() {
        Console.WriteLine(ToString()); // Always a virtual call!
    }

    public override string ToString() { System.Diagnostics.Debug.Assert(false); }
};

sealed class Bar : Foo {
    public override string ToString() { return "I'm called!"; }
}

VB, like IL, can issue nonvirtual calls by using the MyClass.Method() syntax. In the above, this would be MyClass.ToString().

With IL and VB.NET you can add filters when catching exceptions, but C# v3 does not support this feature.

This VB.NET example is taken from http://blogs.msdn.com/clrteam/archive/2009/02/05/catch-rethrow-and-filters-why-you-should-care.aspx (note the When ShouldCatch(ex) = True in the Catch clause):

Try
   Foo()
Catch ex As CustomBaseException When ShouldCatch(ex)
   Console.WriteLine("Caught exception!")
End Try

In a try/catch, you can re-enter the try block from its own catch block. So, you can do this:

.try {
    // ...

  MidTry:
    // ...

    leave.s RestOfMethod
}
catch [mscorlib]System.Exception {
    leave.s MidTry  // branching back into try block!
}

RestOfMethod:
    // ...

AFAIK you can't do this in C# or VB

As far as I know, there's no way to make module initializers (static constructors for an entire module) directly in C#:

http://blogs.msdn.com/junfeng/archive/2005/11/19/494914.aspx

Native types
You can work with the native int and native unsigned int types directly (in c# you can only work on an IntPtr which is not the same.

Transient Pointers
You can play with transient pointers, which are pointers to managed types but guaranteed not to move in memory since they are not in the managed heap. Not entirely sure how you could usefully use this without messing with unmanaged code but it's not exposed to the other languages directly only through things like stackalloc.

<Module>
you can mess about with the class if you so desire (you can do this by reflection without needing IL)

.emitbyte

15.4.1.1 The .emitbyte directive MethodBodyItem ::= … | .emitbyte Int32 This directive causes an unsigned 8-bit value to be emitted directly into the CIL stream of the method, at the point at which the directive appears. [Note: The .emitbyte directive is used for generating tests. It is not required in generating regular programs. end note]

.entrypoint
You have a bit more flexibility on this, you can apply it to methods not called Main for example.

have a read of the spec ^[1] I'm sure you'll find a few more.

You can hack method override co/contra-variance, which C# doesn't allow (this is NOT the same as generic variance!). I've got more information on implementing this here ^[1], and parts 1 ^[2] and 2 ^[3]

I think the one I kept wishing for (with entirely the wrong reasons) was inheritance in Enums. It doesn't seem like a hard thing to do in SMIL (since Enums are just classes) but it's not something the C# syntax wants you to do.

Here's some more:

1. You can have extra instance methods in delegates.
2. Delegates can implement interfaces.
3. You can have static members in delegates and interfaces.

20) You can treat an array of bytes as a (4x smaller) array of ints.

I used this recently to do a fast XOR implementation, since the CLR xor function operates on ints and I needed to do XOR on a byte stream.

The resulting code measured to be ~10x faster than the equivalent done in C# (doing XOR on each byte).

===

I don't have enough stackoverflow street credz to edit the question and add this to the list as #20, if someone else could that would be swell ;-)

Something obfuscators use - you can have a field/method/property/event all have the same name.

Enum inheritance is not really possible:

You can inherit from an Enum class. But the result doesn't behave like an Enum in particular. It behaves not even like a value type, but like an ordinary class. The srange thing is: IsEnum:True, IsValueType:True, IsClass:False

But thats not particulary useful (unless you want to confuse a person or the runtime itself.)

You can also derive a class from System.Multicast delegate in IL, but you can't do this in C#:

// The following class definition is illegal:

public class YourCustomDelegate : MulticastDelegate { }

You can also define module-level (aka global) methods in IL, and C#, in contrast, only allows you to define methods as long as they are attached to at least one type.