Damir's Corner.

Call a Generic Extension Method Using Reflection

Reflection is a great tool for calling methods on objects when their types are not known at compile time. Unfortunately the Type.GetMethod method doesn't work with generic methods therefore we seem to be stuck with finding the right method by iterating through all the methods returned by Type.GetMethods:

public static MethodInfo GetMethodWithLinq(this Type staticType, string methodName,
    params Type[] paramTypes)
{
    var methods = from method in staticType.GetMethods()
                  where method.Name == methodName
                        && method.GetParameters()
                                 .Select(parameter => parameter.ParameterType)
                                 .Select(type => type.IsGenericType ?
                                     type.GetGenericTypeDefinition() : type)
                                 .SequenceEqual(paramTypes)
                  select method;
    try
    {
        return methods.SingleOrDefault();
    }
    catch (InvalidOperationException)
    {
        throw new AmbiguousMatchException();
    }
}

Searching for alternatives I stumbled upon a solution using expression trees posted by Neil Whitaker on StackOverflow:

private static MethodInfo GetMethodWithExpressions(Type innerType)
{
    var method = typeof(Program).GetMethod("GetSingleOrDefaultMethod",
                                    BindingFlags.Static | BindingFlags.NonPublic,
                                    null, Type.EmptyTypes, null)
                                .MakeGenericMethod(innerType);
    return method.Invoke(null, new object[] { }) as MethodInfo;
}

private static MethodInfo GetSingleOrDefaultMethod<TSource>()
{
    Expression<Func<TSource, bool>> fakePredicate = i => true;
    Expression<Func<IQueryable<TSource>, TSource>> lambda =
        l => l.SingleOrDefault(fakePredicate);

    return (lambda.Body as MethodCallExpression).Method;
}

Sure, it is more difficult to comprehend and a lot less flexible, requiring a helper method building the expression tree for every method to be returned as MethodInfo. It got me wondering though, which of the two solutions actually performs better. Let's try them out:

static void Main(string[] args)
{
    MethodInfo method;
    Type innerType = typeof(string);

    int count = 10000;
    Stopwatch stopwatch = new Stopwatch();

    stopwatch.Restart();
    for (int i = 0; i < count; i++)
    {
        method = typeof(Queryable).GetMethodWithLinq("SingleOrDefault",
                                       typeof(IQueryable<>), typeof(Expression<>))
                                  .MakeGenericMethod(innerType);
    }
    stopwatch.Stop();
    Console.WriteLine(String.Format("Method 1: {0}", stopwatch.ElapsedMilliseconds));

    stopwatch.Restart();
    for (int i = 0; i < count; i++)
    {
        method = GetMethodWithExpressions(innerType);
    }
    stopwatch.Stop();
    Console.WriteLine(String.Format("Method 2: {0}", stopwatch.ElapsedMilliseconds));

    Console.ReadLine();
}

And the results?

Method 1: 224
Method 2: 396

Not only is the Type.GetMethods based approach simpler, it also consistently performs a lot better. We'll just stick with it until support for generic methods is added to Type.GetMethod.