Use emit to return null

Question

I've been using the FastMember project. It contains this code:

il.Emit(OpCodes.Ldarg, 2); 
il.Emit(OpCodes.Newobj, typeof(ArgumentOutOfRangeException).GetConstructor(new[] { typeof(string) }));
il.Emit(OpCodes.Throw);

I would like to change that to just return null instead. I tried replacing it with a single line il.Emit(OpCodes.Ret);. However, I get invalid program errors using that. How do I set the return value to null using emitted code?

From a simple test program I notice you also have to use `ldnull` before returning it. Have you done so? (IL is stackbased, you have to add something to the stack so you can return it) — Jeroen Vannevel, May 05 '14 at 19:37
Almost any question in the form of "How do I do `x` in IL" can be answered with "write a simple C# program doing `x`, and use ildasm to look at the compiled IL". — Mitch, May 05 '14 at 19:53

score 4 · Accepted Answer · answered May 05 '14 at 19:48

4

If you just emit ret, that's like return; in C#. But you need return null;

You should use

il.Emit(OpCodes.Ldnull);
il.Emit(OpCodes.Ret);

answered May 05 '14 at 19:48

Kris Vandermotten

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Use emit to return null

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