Emit local variable and assign a value to it

Question

I'm initializing an integer variable like this:

LocalBuilder a = ilGen.DeclareLocal(typeof(Int32));

How can I access it and assign a value to it? I want to do something like this:

int a, b;
a = 5;
b = 6;
return a + b;

Answer 1

Use the Ldloc and Stloc opcodes to read and write local variables:

LocalBuilder a = ilGen.DeclareLocal(typeof(Int32));
LocalBuilder b = ilGen.DeclareLocal(typeof(Int32));
ilGen.Emit(OpCodes.Ldc_I4, 5); // Store "5" ...
ilGen.Emit(OpCodes.Stloc, a);  // ... in "a".
ilGen.Emit(OpCodes.Ldc_I4, 6); // Store "6" ...
ilGen.Emit(OpCodes.Stloc, b);  // ... in "b".
ilGen.Emit(OpCodes.Ldloc, a);  // Load "a" ...
ilGen.Emit(OpCodes.Ldloc, b);  // ... and "b".
ilGen.Emit(OpCodes.Add);       // Sum them ...
ilGen.Emit(OpCodes.Ret);       // ... and return the result.

Note that the C# compiler uses the shorthand form of some of the opcodes (via .NET Reflector):

.locals init (
    [0] int32 a,
    [1] int32 b)

ldc.i4.5 
stloc.0 
ldc.i4.6 
stloc.1 
ldloc.0 
ldloc.1 
add 
ret