Question

I want to select the right generic method via reflection and then call it.

Usually this is quite easy. For example

var method = typeof(MyType).GetMethod("TheMethod");
var typedMethod = method.MakeGenericMethod(theTypeToInstantiate);

However the issue start when there are different generic overloads of the method. For example the static-methods in the System.Linq.Queryable-class. There are two definitions of the ‘Where’-method

static IQueryable<T> Where(this IQueryable<T> source, Expression<Func<T,bool>> predicate)
static IQueryable<T> Where(this IQueryable<T> source, Expression<Func<T,int,bool>> predicate)

This meand that GetMethod doesn’t work, because it cannot destiguish the two. Therefore I want to select the right one.

So far I often just took the first or second method, depending on my need. Like this:

var method = typeof (Queryable).GetMethods().First(m => m.Name == "Where");
var typedMethod = method.MakeGenericMethod(theTypeToInstantiate);

However I’m not happy with this, because I make a huge assumption that the first method is the right one. I rather want to find the right method by the argument type. But I couldn’t figure out how.

I tried it with passing the ‘types’, but it didn’t work.

        var method = typeof (Queryable).GetMethod(
            "Where", BindingFlags.Static,
            null,
            new Type[] {typeof (IQueryable<T>), typeof (Expression<Func<T, bool>>)},
            null);

So has anyone an idea how I can find the ‘right’ generic method via reflection. For example the right version of the ‘Where’-method on the Queryable-class?

Answer 1

It can be done, but it’s not pretty!

For example, to get the first overload of Where mentioned in your question you could do this:

var where1 = typeof(Queryable).GetMethods()
                 .Where(x => x.Name == "Where")
                 .Select(x => new { M = x, P = x.GetParameters() })
                 .Where(x => x.P.Length == 2
                             && x.P[0].ParameterType.IsGenericType
                             && x.P[0].ParameterType.GetGenericTypeDefinition() == typeof(IQueryable<>)
                             && x.P[1].ParameterType.IsGenericType
                             && x.P[1].ParameterType.GetGenericTypeDefinition() == typeof(Expression<>))
                 .Select(x => new { x.M, A = x.P[1].ParameterType.GetGenericArguments() })
                 .Where(x => x.A[0].IsGenericType
                             && x.A[0].GetGenericTypeDefinition() == typeof(Func<,>))
                 .Select(x => new { x.M, A = x.A[0].GetGenericArguments() })
                 .Where(x => x.A[0].IsGenericParameter
                             && x.A[1] == typeof(bool))
                 .Select(x => x.M)
                 .SingleOrDefault();

Or if you wanted the second overload:

var where2 = typeof(Queryable).GetMethods()
                 .Where(x => x.Name == "Where")
                 .Select(x => new { M = x, P = x.GetParameters() })
                 .Where(x => x.P.Length == 2
                             && x.P[0].ParameterType.IsGenericType
                             && x.P[0].ParameterType.GetGenericTypeDefinition() == typeof(IQueryable<>)
                             && x.P[1].ParameterType.IsGenericType
                             && x.P[1].ParameterType.GetGenericTypeDefinition() == typeof(Expression<>))
                 .Select(x => new { x.M, A = x.P[1].ParameterType.GetGenericArguments() })
                 .Where(x => x.A[0].IsGenericType
                             && x.A[0].GetGenericTypeDefinition() == typeof(Func<,,>))
                 .Select(x => new { x.M, A = x.A[0].GetGenericArguments() })
                 .Where(x => x.A[0].IsGenericParameter
                             && x.A[1] == typeof(int)
                             && x.A[2] == typeof(bool))
                 .Select(x => x.M)
                 .SingleOrDefault();