CODEWITHSUNDEEP
c#.netreflection
mnield
mnield

Reputation: 1869

Cast using Reflection in C#

I've created a generic function as below (just a s a proof) that will take a List<T> collection and reverse it, returning a new List<T> as its output.

public static List<T> ReverseList<T>(List<T> sourceList)
{
    T[] outputArray = new T[sourceList.Count];
    sourceList.CopyTo(outputArray);
    return outputArray.Reverse().ToList();
}

The purpose of the proof is that I only know what T is at runtime. I am therefore using reflection to call the above method as follows:

List<int> myList = new List<int>() { 1, 2, 3, 4, 5 }; // As an example, but could be any type for T

MethodInfo myMethod = this.GetType().GetMethod("ReverseList");
MethodInfo resultMethod = myMethod.MakeGenericMethod(new Type[] { typeof(int) });
object result = resultMethod.Invoke(null, new object[] { myList });

There are two problems here:

  1. In the second line, rather than supplying typeof(int), I would like suppliy somthign akin to myList.GetType().GetGenericArguments()[0].GetType() in order to make things more flexible because I do not know T until runtime. Doing this results in a runtime error when the Invoke runs as follows: "Object of type 'System.Collections.Generic.List'1[System.Int32]' cannot be converted to type 'System.Collections.Generic.List'1[System.RuntimeType]'."
  2. The result of the Invoke() method returns an object. When debugging, I can see that the object is of type List, but attempting to use it tells me that I have an invalid cast. I assume that I need to use reflection to box the result in to the correct type (i.e. in this example, the equivalent of (result as List<int>).

Does anyone have any pointers that could help me resolve this? Apologies if this is not to clear, I can probably provide more detail if asked.

TIA

Upvotes: 4

Views: 3706

Answers (3)

BrokenGlass
BrokenGlass

Reputation: 161012

The result of the Invoke() method returns an object. When debugging, I can see that the object is of type List, but attempting to use it tells me that I have an invalid cast. I assume that I need to use reflection to box the result in to the correct type (i.e. in this example, the equivalent of (result as List).

The only workaround for this is I can think of is to pass an empty list as the second parameter of the method and to populate that list - the reference returned by Invoke() will always be only of type object, but inside the generic method you do have access to the type itself:

List<int> reverseList = new List<int>();
resultMethod.Invoke(null, new object[] { myList, reverseList });

...

public static void ReverseList<T>(List<T> sourceList, List<T> resultList)
{
    T[] outputArray = new T[sourceList.Count];
    sourceList.CopyTo(outputArray);
    resultList.AddRange(outputArray.Reverse());
}

Upvotes: 1

Ben Voigt
Ben Voigt

Reputation: 283921

You've got one GetType() too many. Happens to everyone.

myList.GetType().GetGenericArguments()[0] IS a System.Type -- the one you're looking for.

myList.GetType().GetGenericArguments()[0].GetType() is a System.Type describing System.Type (well, actually the concrete subclass System.RuntimeType).

Also, your ReverseList function is serious overkill. It does an extra copy just to avoid calling List.Reverse. There's a better way to circumvent that:

public static List<T> ReverseList<T>(List<T> sourceList)
{
    return Enumerable.Reverse(sourceList).ToList();
}

or

public static List<T> ReverseList<T>(List<T> sourceList)
{
    var result = new List<T>(sourceList);
    result.Reverse();
    return result;
}

or

public static List<T> ReverseList<T>(List<T> sourceList)
{
    var result = new List<T>();
    result.Capacity = sourceList.Count;
    int i = sourceList.Count;
    while (i > 0)
        result.Add(sourceList[--i]);
    return result;
}

Upvotes: 5

Marc Gravell
Marc Gravell

Reputation: 1064204

To access it as a List<T>, yes you'd need to find T using reflection (probably over the interfaces, for example typeof(IList<>), and use more reflection and MakeGenericMethod etc. In all honesty, it isn't worth it: you would do better to check for the non-generic IList:

var list = result as IList;
if (list != null)
{
    // loop over list etc
}

Generics ad reflection are not good friends.

Note in 4.0 there are also some tricks you can do here with dynamic and generics.

Upvotes: 3

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