How do you generate a "yield return null;" for an IEnumerator method?

[rickomax]

Hi!

I'm trying to use Mono.Cecil to duplicate some methods based on custom Attributes to make these copies use IEnumerator (so I can use them as Unity Coroutines).

I can load the original libraries with Cecil, find the methods, duplicate them, etc.

The problem is:

I have to replace some placeholder method calls with a yield return null;, which seems to require generating more IL code than I expected.

Does anyone know how that could be represented as IL code with Mono.Cecil?

I've been using https://sharplab.io/ to understand the pattern, but it seems like it could be more complex.

[rickomax]

So, something like:

[CoroutineMethod]
private int TestMethod(AssetLoaderContext assetLoaderContext, int a, int b) {
    if (assetLoaderContext.CheckMainThreadStall()) {

    }
    return a + b;
}

Could be duplicated to:

private IEnumerator TestMethod_Coroutine(Stack result, AssetLoaderContext assetLoaderContext, int a, int b) {
    if (assetLoaderContext.CheckMainThreadStall()) {
        yield return null;
    }
    result.Push(a + b);
}

[ltrzesniewski]

There is no direct IL equivalent to yield return. Methods that use yield are rewritten by the compiler as finite state machines, with state changes on yield statements.

I'm afraid this transformation is not exactly straightforward (especially starting from the low-level IL representation), but you can take a look at the Roslyn implementation if you'd like to replicate it. Comments there can be insightful, like this one:

//     yield return expression;
// is translated to
//     this.current = expression;
//     this.state = <next_state>;
//     return true;
//     <next_state_label>: ;
//     <hidden sequence point>
//     this.state = finalizeState;

[rickomax]

I appreciate it!
I ended up using Roslyn Code Generator for the task. It does not provide me strong type references (or at least, I still need help finding how to get them from parsed code), but it makes such compiler-specific conversion more trivial.