Question

This may sound stupid but I cannot get the MethodInfo of Queryable.Join(...). I want to get it because of How to use a Type variable in a generic method call (C#)

It has 2 available method signatures and I want to get the one without IEqualityComparer, so I need to specify Type[] in GetMethod.

I wrote something like

MethodInfo joinMethod = typeof( Queryable ).GetMethod( "Join", new Type[] { typeof(IEnumerable<>), typeof(Expression<Func<>>), typeof(Expression<Func<>>), typeof(Expression<Func<>>)});

but it doesn’t work. I’m unable to specify the types in generics above, because they are passed as Type from outside (and this is why I need this reflection).

Can anyone tell me how? Thanks!

Answer 1

Working with generics and reflection can be a bit tedious. Your best bet (to keep things simple) is to use GetMethods and filter by what you are looking for.

//Overly simplified
MethodInfo joinMethod = typeof(Queryable)
            .GetMethods(BindingFlags.Static | BindingFlags.Public)
            .Where(m => m.Name == "Join" && m.GetParameters().Length == 5)
            .First();

Given that, the MethodInfo is not invokable at this point. You need to make a generic version of it by using joinMethod.MakeGenericMethod(/*type array*/). In your case, you would need to use 4 types: TOuter, TInner, TKey, TResult.

var genericJoinMethod = joinMethod.MakeGenericMethod(new Type[]{your types here});

Now you can use genericJoinMethod as you’d expect to.

As far as I know, that is the only way to do it if you don’t know the types at compile-time.

EDIT:

Given your comment, I think it should look something like this:

MethodInfo joinMethod = typeof(Queryable)
            .GetMethods(BindingFlags.Static | BindingFlags.Public)
            .Where(m => m.Name == "Join" && m.GetParameters().Length == 5)
            .First();
var genericJoinMethod = joinMethod.MakeGenericMethod(typeof(TType), typeof(TType), JoinKeyType, typeof(TType));
result = genericJoinMethod.Invoke( result, new object[] { result, items, OuterKeySelector, InnerKeySelector, ResultSelector } );