Get the actual type of a generic object parameter

Question

No doubt elements of this question have been asked before, but I\'m having trouble finding an answer. (Disclaimer: this is related, but separate from a recent question I as

Answer 1

This should give you a callable method (I'll test it in a little while). The boxing/unboxing it incurs is much faster than the security checks required for the Reflection API invocation (which happens to also require boxing).

private static Action<MethodInfo, object> BuildAccessor(Type valueType)
{
    MethodInfo genericMethod = null; // <-- fill this in
    MethodInfo method = genericMethod.MakeGenericMethod(new Type[] { valueType });
    ParameterExpression methodInfo = Expression.Parameter(typeof(MethodInfo), "methodInfo");
    ParameterExpression obj = Expression.Parameter(typeof(object), "obj");

    Expression<Action<MethodInfo, object>> expr =
        Expression.Lambda<Action<MethodInfo, object>>(
            Expression.Call(method, methodInfo, Expression.Convert(obj, valueType)),
            methodInfo,
            obj);

    return expr.Compile();
}

Answer 2

You're in luck. I actually had to do something very similar a few weeks ago.

For a detailed explanation see the above blog post, but basically the general idea is to reflect the type and manually invoke the method with an explicit set of parameters.

typeof(MyClass).GetMethod("Foo").MakeGenericMethod(new[] { param.GetType() }).Invoke(null, new[] { param });

It's not very type safe, but it does exactly what you're looking for.

class Program
{

    static void Main(string[] args)
    {
        object str = "Hello World";
        object num = 5;
        object obj = new object();

        Console.WriteLine("var\tvalue\t\tFoo() Type\tCallFoo() Type");
        Console.WriteLine("-------------------------------------------------------");
        Console.WriteLine("{0}\t{1}\t{2}\t{3}", "str", str, MyClass.Foo(str), MyClass.CallFoo(str));
        Console.WriteLine("{0}\t{1}\t\t{2}\t{3}", "num", num, MyClass.Foo(num), MyClass.CallFoo(num));
        Console.WriteLine("{0}\t{1}\t{2}\t{3}", "obj", obj, MyClass.Foo(obj), MyClass.CallFoo(obj));
    }

}

class MyClass
{
    public static Type Foo<T>(T param)
    {
        return typeof(T);
    }

    public static Type CallFoo(object param)
    {
        return (Type)typeof(MyClass).GetMethod("Foo").MakeGenericMethod(new[] { param.GetType() }).Invoke(null, new[] { param });
    }

}

Output

   var     value           Foo() Type      CallFoo() Type
   -------------------------------------------------------
   str     Hello World     System.Object   System.String
   num     5               System.Object   System.Int32
   obj     System.Object   System.Object   System.Object