DynamicMethod Reflection Emit a call to a Func

Question

I am figuring out Reflection.Emit for some internal libraries and got stuck with calling a Func passed in as an argument. My scenario test uses the code transferred to IL with Linqpad in the picture My code to duplicate the IL in a DynamicMethod is the following

public class ScopeTest
{
    public delegate Task WrapScope(Func<Task> value);
    public (WrapScope scope,string id) WrapScopeInId()
    {
        var id = $"wrap~{Guid.NewGuid().ToString().Replace("-",string.Empty)}";

        var mi = typeof(Func<Task>).GetMethod("Invoke");
        var d = new DynamicMethod(id, typeof(Task), new[] { typeof(Func<Task>) });
        var gen = d.GetILGenerator();
        var lab = gen.DefineLabel();

        gen.Emit(OpCodes.Nop);
        gen.Emit(OpCodes.Ldarg_0);
        gen.Emit(OpCodes.Callvirt, mi);
        gen.Emit(OpCodes.Stloc_0);
        gen.Emit(OpCodes.Br_S, lab);
        gen.MarkLabel(lab);
        gen.Emit(OpCodes.Ldloc_0);
        gen.Emit(OpCodes.Ret);

        WrapScope del = (WrapScope)d.CreateDelegate(typeof(WrapScope));
        return (del,id);
    }
    
}

The code compiles and returns but when you call the WrapScope delegate del(Func<Task>) it throws a System.InvalidProgramException: Common Language Runtime detected an invalid program. What could be the problem running this DynamicMethod? Thanks

It won't solve your problem but [Nop](https://stackoverflow.com/questions/19206428/what-is-the-point-of-nop-in-cil) means no operation so you can remove the code emiting them — nalka, Mar 25 '21 at 17:16
1) [SharpLab is good](https://sharplab.io/#v2:C4LglgNgNAJiDUAfAAgJgIwFgBQyAMABMugKwDcOyAzEagQMIEDeOBbRNyAbAQCoCmAZ2AAKZCQA83AHwEATgEpmrdquQB2eSIUVsqgL459QA===), 2) You don't need the NOPs 3) You haven't defined slot 0? `ILGenerate.DeclareLocal` — canton7, Mar 25 '21 at 17:18
That `br.s` is utterly pointless as well, not sure why that's there — canton7, Mar 25 '21 at 17:26
You don't even need the stloc/ldloc in your example. Just ldarg.0, callvirt, ret — canton7, Mar 25 '21 at 17:28

canton7 · Accepted Answer · 2021-03-25T17:44:42.077

Your main problem is that you're storing a variable in slot 0, but you never declared slot 0. If we look at your code on SharpLab, we can see that the IL declares each of the slots that the method uses:

.locals init (
    [0] class [System.Private.CoreLib]System.Threading.Tasks.Task
)

(That says we've got 1 slot, index 0, of type Task).

You do this with ILGenerator using ILGenerator.DeclareLocal. We can use ldloc/stloc and pass in the LocalBuilder which is returned, rather than using the numbered ldloc.0/stloc.0.

var taskLocal = gen.DeclareLocal(typeof(Task));

gen.Emit(OpCodes.Nop);
gen.Emit(OpCodes.Ldarg_0);
gen.Emit(OpCodes.Callvirt, mi);
gen.Emit(OpCodes.Stloc, taskLocal);
gen.Emit(OpCodes.Br_S, lab);
gen.MarkLabel(lab);
gen.Emit(OpCodes.Ldloc, taskLocal);
gen.Emit(OpCodes.Ret);

That's all fine and works, but it contains a lot of unnecessary instructions. Linqpad is giving you the output of the compiler in Debug, which emits lots of unnecessary NOPs, etc. You'll see that SharpLab in Release mode doesn't show those, and we can simply remove them. That br.s is also irrelevant, as it's unconditionally jumping to the next instruction, so we can remove that too:

var taskLocal = gen.DeclareLocal(typeof(Task));

gen.Emit(OpCodes.Ldarg_0);
gen.Emit(OpCodes.Callvirt, mi);
gen.Emit(OpCodes.Stloc, taskLocal);
gen.Emit(OpCodes.Ldloc, taskLocal);
gen.Emit(OpCodes.Ret);

Now the stloc/ldloc looks pointless: we're taking a variable from the stack, moving it into a local, then immediately copying it back out of the local and onto the stack in order to return it. Just ditch the local altogether:

gen.Emit(OpCodes.Ldarg_0);
gen.Emit(OpCodes.Callvirt, mi);
gen.Emit(OpCodes.Ret);

Brilliant works great. Thanks for the help. The SharpLab IL has a lot of ceremony around it but now I know what to look at. Also good to know about the Debug vs Release switch. — MiddleTommy, Mar 25 '21 at 18:08

DynamicMethod Reflection Emit a call to a Func

1 Answers1