Get actual type of T in a generic List<T>

Question

How do I get the actual type of T in a generic List at run time using reflection?

Answer 1

You can use Type.GetGenericArguments to return the type T in a List<T>.

For example, this will return the Type for any List<T> passed as an argument:

Type GetListType(object list)
{
    Type type = list.GetType();
    if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(List<>))
        return type.GetGenericArguments()[0];
    else
        throw new ArgumentException("list is not a List<T>", "list");
}

Answer 2

It depends on what exactly you’re asking:

• While writing code inside a generic type Blah<T>, how do I get the reflection type T?

  Answer: typeof(T)

• I have an object which contains a List<T> for some type T. How do I retrieve the type T via reflection?

  Short answer: myList.GetType().GetGenericArguments()[0]

  Long answer:

  var objectType = myList.GetType();
  if (!objectType.IsGenericType() ||
      objectType.GetGenericTypeDefinition() != typeof(List<>))
  {
      throw new InvalidOperationException(
          "Object is not of type List<T> for any T");
  }
  var elementType = objectType.GetGenericArguments()[0];