How to get MethodInfo of interface method, having implementing MethodInfo of class method?

Question

I have a MethodInfo of an interface method and Type of a class that implements the interface. I want to find the MethodInfo of the class method that implements the interface method.

The simple method.GetBaseDefinition() does not work with interface methods. Lookup by name won't work either, because when implementing interface method explicitly it can have any name (yes, not in C#).

So what is the correct way of doing that that covers all the possibilities?

Answer 1

OK, I found a way, using GetInterfaceMap.

var map = targetType.GetInterfaceMap(interfaceMethod.DeclaringType);
var index = Array.IndexOf(map.InterfaceMethods, interfaceMethod);

if (index == -1)
{
    //this should literally be impossible
}

return map.TargetMethods[index];

Answer 2

Here's an extension method!

public static MethodInfo GetImplementedMethod(this Type targetType, MethodInfo interfaceMethod)
{
    if (targetType is null) throw new ArgumentNullException(nameof(targetType));
    if (interfaceMethod is null) throw new ArgumentNullException(nameof(interfaceMethod));

    var map = targetType.GetInterfaceMap(interfaceMethod.DeclaringType);
    var index = Array.IndexOf(map.InterfaceMethods, interfaceMethod);
    if (index < 0) return null;

    return map.TargetMethods[index];
    
}

Answer 3

I use this.

var interfacemethodParameterTypes = interfaceMethodInfo.GetParameters().Select(p => p.ParameterType).ToArray();

var map = targetType.GetInterfaceMap(interfaceMethodInfo.DeclaringType);

return map.TargetType.GetMethod(interfaceMethodInfo.Name, interfacemethodParameterTypes);