Question

I’m currently facing new problem with operators. Using following code, I want to make output that would be same as when using if ... else pair in C#.

var method = new DynamicMethod("dummy", null, Type.EmptyTypes);
var g = method.GetILGenerator();

g.Emit(OpCodes.Ldstr, "string");
g.Emit(OpCodes.Ldstr, "string");
g.Emit(OpCodes.Call, typeof(String).GetMethod("op_Equality", new Type[]{typeof(string), typeof(string)}));
g.Emit(OpCodes.Ldc_I4, 0);
g.Emit(OpCodes.Ceq);
g.Emit(OpCodes.Brtrue_S, );

var action = (Action)method.CreateDelegate(typeof(Action));
action();

Console.Read();

My questions are:

1. How can I get the address of an instruction to pass it as a parameter for branch opcodes?
2. Is there any difference between BR and BR_S, Brtrue and Brtrue_S, Brfalse and Brfalse_S and similar instructions?

Thanks.

Answer 1

ILGenerator.ILOffset gives you the current offset in the IL stream, if that’s what you want. You can use DefineLabel and MarkLabel, too, as goric suggested.

The only difference between brtrue.s and brtrue is that brtrue.s is the short version of brtrue. brtrue uses a 4-byte offset and brtrue.s uses a 1-byte offset. The same applies for brfalse and brfalse.s (and br/br.s).

Those are not the only short versions of an IL instruction, there are also other short instructions, like ldc.i4.0 – ldc.i4.8 for loading integers. Those are mainly useful for generating smaller binaries, but i don’t think there is a big difference otherwise.