Resolving GenericParameter

[m-carrasco]

Hi everyone,

I just want to ask for some help with this problem. I've been reading old issues but I am still uncertain if my understanding is correct.

In my code, there is a MethodReference method to TResult System.Func2<Interop/ErrorInfo,Interop/ErrorInfo>::Invoke(T). This reference comes from the callvirt instruction at offset IL_0005 (see monodis output at the bottom).

My problem is that method.ReturnType.Resolve() is returning null, and in case it is relevant, ReturnType is pointing to a GenericParameter instance.

Ultimately, I'd like method.ReturnType.Resolve() to return a TypeDefinition reference to Interop/ErrorInfo.

However, I think I may be dealing with two different issues (on my side - not cecil) here.

1. The fact that method.ReturnType.Resolve() is returning null could be because my application is a net6 one while I'm telling Cecil (running in net6) to load a mono's DLL. Does this make any sense?
2. Even if I were correctly doing point 1, would Resolve() ever return a TypeDefinition reference to Interop/ErrorInfo?

Any suggestion/feedback/explanation regarding these points would be illuminating.

As a related issue, I found this old question https://groups.google.com/g/mono-cecil/c/fIOqSZQvA1o. I suspect I may need to use IGenericParameterProvider and IGenericContext somehow.

Best regards,
Manuel.

---

The full method is ThrowExceptionForIoErrno listed below, which is defined in the Interop class in mono's mscorlib.

	.method private hidebysig static 
		void ThrowExceptionForIoErrno (
			valuetype Interop/ErrorInfo errorInfo,
			string path,
			bool isDirectory,
			class System.Func`2<valuetype Interop/ErrorInfo, valuetype Interop/ErrorInfo> errorRewriter
		) cil managed 
	{
		// Method begins at RVA 0x2058
		// Header size: 1
		// Code size: 21 (0x15)
		.maxstack 8

		IL_0000: ldarg.3
		IL_0001: brfalse.s IL_000c

		IL_0003: ldarg.3
		IL_0004: ldarg.0
		IL_0005: callvirt instance !1 class System.Func`2<valuetype Interop/ErrorInfo, valuetype Interop/ErrorInfo>::Invoke(!0)
		IL_000a: starg.s errorInfo

		IL_000c: ldarg.0
		IL_000d: ldarg.1
		IL_000e: ldarg.2
		IL_000f: call class System.Exception Interop::GetExceptionForIoErrno(valuetype Interop/ErrorInfo, string, bool)
		IL_0014: throw
	} // end of method Interop::ThrowExceptionForIoErrno

[m-carrasco]

I think I've might have found an answer: https://groups.google.com/g/mono-cecil/c/m_hv7mHXpCI

I'll update if I solved my issue.

[m-carrasco]

I wrote this piece of code which seems to work for a wide range of cases. Any suggestion is welcomed!

The first call should be done like this: ResolveParametersFromReturnedType(method.ReturnType, method)

        public static TypeReference ResolveParametersFromReturnedType(TypeReference type, MethodReference method){

            if (type is GenericParameter genericParameter){
                return ResolveGenericParameterAsArgument(genericParameter, method);
            }

            // Composite types (recursion)

            if (type is ArrayType arrayType){
                TypeReference element = arrayType.ElementType;
                TypeReference resolved = ResolveParametersFromReturnedType(element, method);
                return resolved == element ? type : new ArrayType(resolved, arrayType.Rank);
            }

            if (type is ByReferenceType byReferenceType){
                TypeReference element = byReferenceType.ElementType;
                TypeReference resolved = ResolveParametersFromReturnedType(element, method);
                return resolved == element ? type : new ByReferenceType(resolved);
            }

            if (type is OptionalModifierType optType){
                TypeReference element = optType.ElementType;
                TypeReference resolved = ResolveParametersFromReturnedType(element, method);
                return resolved == element ? type : new OptionalModifierType(optType.ModifierType, resolved);
            }

            if (type is RequiredModifierType reqType){
                TypeReference element = reqType.ElementType;
                TypeReference resolved = ResolveParametersFromReturnedType(element, method);
                return resolved == element ? type : new RequiredModifierType(reqType.ModifierType, resolved);
            }

            if (type is GenericInstanceType genericInstance){
                var resolvedGas = genericInstance.GenericArguments.Select(ga => ResolveParametersFromReturnedType(ga, method)).ToList();

                bool changed = false;
                for (int i=0; i < resolvedGas.Count; i++){
                    if (resolvedGas[i] != genericInstance.GenericArguments[i]){
                        changed = true;
                        break;
                    }
                }

                if (changed){
                    GenericInstanceType res = new GenericInstanceType(genericInstance.ElementType);
                    foreach (var ga in resolvedGas){
                        res.GenericArguments.Add(ga);
                    }    
                    return res;
                }

                return type;
            }

            return type;
        }

        private static TypeReference ResolveGenericParameterAsArgument(GenericParameter genericParameter, MethodReference methodReference){
            // https://groups.google.com/g/mono-cecil/c/m_hv7mHXpCI
            
            IGenericInstance genericInstance = null;
            switch (genericParameter.Type){
                case GenericParameterType.Type:
                    {
                        genericInstance = (GenericInstanceType)methodReference.DeclaringType;
                        break;
                    }
                case GenericParameterType.Method:
                    {
                        genericInstance = (GenericInstanceMethod)methodReference;
                        break;
                    }
                default:
                    throw new NotSupportedException("Something wrong happened");
            }

            return genericInstance.GenericArguments[genericParameter.Position];
        }