Reflection.Emit - access topmost-but-one item from stack

问题

Is there a way in .NET, using Reflection.Emit, to access the topmost-but-one item from the stack? So if A is topmost, and B next - I want to process B then A. It would be fine to duplicate B above A (since I can simply "pop" the second B when I get to it).

Currently, I am declaring a local:

    LocalBuilder loc = il.DeclareLocal(typeof(Foo));
    il.Emit(OpCodes.Stloc, loc); // store and pop topmost stack item
    // work with (pop) previous stack item 
    il.Emit(OpCodes.Ldloc, loc); // push old topmost stack item

Is there a route that doesn't need the explicit local?

回答1:

I don't think so. In IL there aren't any instructions like swap which would allow you to do what you want. Why do you see using a local as objectionable? If the JIT compiler is good enough this won't result in any slower machine code than using a hypothetical swap operation in IL.

回答2:

+1 for kvbs answer, see: http://www.codeproject.com/KB/msil/msilenhancement.aspx

回答3:

Inline with what kvb said, you could try a small function to do some reordering. Not sure if it would be any faster.

回答4:

I encountered this same problem. I wanted to generate a rather large method and I often wanted to 'swap' in order to store a calculated value. I was unhappy with the large volume of locals showing up in ildasm and noticed that BeginScope/EndScope wasn't any help. I wound up creating a local 'swap' for the context of my method and reusing it for every swap operation. It makes the generated IL cleaner; not sure if it has any meaningful impact on performance.

来源：https://stackoverflow.com/questions/718003/reflection-emit-access-topmost-but-one-item-from-stack