Why I cannot create a List<> using myObject?

Question

I created a class dynamically (found it here) but I don't understand why I cannot use it to create a list?

var myType = CompileResultType();
var myObject = Activator.CreateInstance(myType);
var myList = new List<myObject>();

// Error: The type or namespace name 'myObject' 
// could not be found (are you missing a using directive 
// or an assembly reference?)

How should I do it?

Answer 1

They're called generic type parameters, because they expect types, not instances. So you need to know the actual type or create the list object with reflection as well.

Type listType = typeof(List<>);
Type dynamicClassType = listType.MakeGenericType(myObject.GetType());
object myList = Activator.CreateInstance(dynamicClassType);

However, you're unable to use the instance in any meaningful way since it's an object.

• You could cast it to an IList as suggested in Avner Shahar-Kashtan's answer and use the nongeneric methods.

  IList myList = (IList)Activator.CreateInstance(dynamicClassType);
  

• You could call the methods of the list-instance through reflection as well.

  // roughly
  MethodInfo addMethod = myList.GetType().GetMethod("Add");
  addMethod.Invoke(myList, objectToAdd);
  

• Or do as suggested in Cuong Le's answer and use dynamic as the type of the list-instance.

  dynamic myList = Activator.CreateInstance(dynamicClassType);
  

Answer 2

In order to construct a generic type dynamically, you'll have to use Reflection, and specifically the MakeGenericType method. Something like this:

Type baseListType = typeof(List<>); // get the generic List<>
Type myType = CompileResultType; // same as in yours.
Type myListType = baseListType.MakeGenericType(myType); // Get the List<MyType>
IList myList = (IList)Activator.CreateInstance(myListType); // Instantiate.

Notice that I defined myList as IList, since we don't have a compile-time type for it. Luckily, we have convenient base classes and interfaces like IList, IEnumerable and several others that List<T> implements, which we can use.