I can't get parameter names from valuetuple via reflection in c# 7.0

Question

I want to Map a ValueTuple to a class using reflection. Documentation says that there is a Attribute attached to ValueTuple with parameters names (others than Item1, Item2, etc...) but I can't see any Attribute.

Disassembly shows nothing.

What's happens?

Example:

public static T ToStruct<T, T1,T2>(this ValueTuple<T1,T2> tuple) where T : struct

Via reflection can't get Item1, Item2 names to match with T fields via reflection.

Answer 1

You should have the TupleElementNames attribute on the method created by the compiler.

See this code:

public class C {
    public (int a, int b) M() {

        return (1, 2);
    }
}

Which compiles to:

[return: TupleElementNames(new string[] {
    "a",
    "b"
})]
public ValueTuple<int, int> M()
{
    return new ValueTuple<int, int>(1, 2);
}

You can get that attribute using this code:

Type t = typeof(C);
MethodInfo method = t.GetMethod(nameof(C.M));
var attr = method.ReturnParameter.GetCustomAttribute<TupleElementNamesAttribute>();

string[] names = attr.TransformNames;