System.Reflection.Emit - If Statement Generation

Question

I'm trying to learn how If statements are generated with ILGenerator in C#.

Here's my code: (ilg is an ILGenerator)

ilg.Emit(OpCodes.Ldc_I4_1);
Label start = ilg.DefineLabel();
ilg.Emit(OpCodes.Brfalse, start);
ilg.Emit(OpCodes.Ldstr, "Hello");
ilg.Emit(OpCodes.Call, typeof(Console).GetMethod("WriteLine", new Type[] { typeof(string) }));
ilg.MarkLabel(start);

Some notes:

• Alone, calling Console.WriteLine works perfectly.
• This is the error that I get when I run the generated exe:

Unhandled Exception: System.InvalidProgramException: Common Language Runtime detected an invalid program. at Testing.Test.Main(String[] )

Answer 1

You code works almost fine, you just forgot to put OpCodes.Ret at end, see the code that works:

public static void Main(string[] args)
{
    var dynamicMethod = new DynamicMethod("PrintHello", typeof(void), null);
    var ilGenerator = dynamicMethod.GetILGenerator();
    ilGenerator.Emit(OpCodes.Ldc_I4_1);
    var toEnd = ilGenerator.DefineLabel();
    ilGenerator.Emit(OpCodes.Brfalse, toEnd);
    ilGenerator.Emit(OpCodes.Ldstr, "Hello");
    ilGenerator.Emit(OpCodes.Call, typeof(Console).GetMethod("WriteLine", new Type[] { typeof(string) }));
    ilGenerator.MarkLabel(toEnd);
    ilGenerator.Emit(OpCodes.Ret);

    var @delegate = (Action)dynamicMethod.CreateDelegate(typeof(Action));
    @delegate();
}

Then you'll have this output

Hello

If you comment this line ilGenerator.Emit(OpCodes.Ret); you will get this exception

System.InvalidProgramException: Common Language Runtime detected an invalid program.

Remember, every method in C# must have the OpCode.Ret

If your Main method is empty, like this

public static void Main(string[] args)
{
}

The IL generated (in debug mode, without optimization) will be this

.method public hidebysig static void Main (string[] args) cil managed 
{
    // Method begins at RVA 0x2050
    // Code size 2 (0x2)
    .maxstack 8
    .entrypoint

    IL_0000: nop
    IL_0001: ret
} // end of method Program::Main

The IL generated (in release mode, with optimization) will be this

.method public hidebysig static void Main (string[] args) cil managed 
{
    // Method begins at RVA 0x2050
    // Code size 1 (0x1)
    .maxstack 8
    .entrypoint

    IL_0000: ret
} // end of method Program::Main