Question

I’m trying to write an extension method that will give me the MemberInfo representing a member of a given type, using lambda expressions. Ideally, I’d like to be able to write

var info = MyType.GetMember(m => m.MyProperty);

or, also acceptable

var info = typeof(MyType).GetMember(m => m.MyProperty);

or even

var info = typeof(MyType).GetMember((MyType m) => m.MyProperty);

I have a generic method signature that works, but requires me to specify all the type parameters, and I’d very much like C# to infer them. As far as I can see, if I just find the right way to specify the extension method signature, there should be enough information in (at least the last one of) the code snippets to infer everything – but according to the compiler, there isn’t.

I’ve read an old blog post on static extension methods but I haven’t been able to find anything on it more recent than that. If that came true, I’d be able to write

public static MemberInfo GetMember<TType, TReturnType>(static TType, Expression<Func<TType, TReturnType>> member)

which would solve my problem. But as I said, I seem to be stuck with instance extensions, in which case

public static MemberInfo GetMember<TType, TReturnType>(this Type t, Expression<Func<TType, TReturnType>> member)

just isn’t good enough for the compiler to infer type members.

Answer 1

How about this:

public static MemberInfo GetMember<TType, TReturnType>
    (this TType ignored,
     Expression<Func<TType, TReturnType>> expression)

which you’d then call like this:

default(MyType).GetMember(m => m.MyProperty)

It’s a bit icky, but using default(MyType) is a simple and efficient way of getting an expression of type MyType, which is what you want for type inference.