获取通用对象参数的实际类型

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 asked).

I have a method like this:

public static void Method<T>(MethodInfo m, T value)
{
  Type memberType = m.GetValueType();

  if (memberType.IsAssignableFrom(typeof(List<T>))
  {
    object memberValue = Activator.CreateInstance(memberType);
    ((List<T>)memberValue).Add(value);
  }
}

This works fine when I call it like this:

string s = "blah";
Method(memberInfo, s);

However, I need to call this method using a generic type, so I'm calling it like this:

Type valueType = someType;
object passValue = someMethod.MakeGenericMethod(new Type[] { valueType }).Invoke(this, new object[] { });
/* Call my original method */
Method(memberInfo, passValue );

Now, intellisense knows that 'value' in Method<T> is whatever type valueType is (say 'FooObject'). But 'T' is object, which means that a List<FooObject> is not assignable from a List<T> (ie a List<object>).

I've tried using Convert.ChangeType on the variable ('passValue') beforehand but that wasn't any more useful.

As there is no way to cast a variable to the Type of a type variable, how do I get around this?

Is the best solution to somehow not rely on IsAssignableFrom and do a looser type check of whether this will work? The problem with this is that I'm not sure I'll be able to cast the memberValue properly unless 'T' is truly the element type of memberValue.

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