Adding items to List<T> using reflection

Question

I was trying to add items to IList through reflection, but while calling the "Add" method an error was thrown "object ref. not set". While debugging I came to know that the GetMethod("Add") was returning a NULL reference.

Type objTyp = typeof(MyObject); //HardCoded TypeName for demo purpose
var IListRef = typeof (List<>);
Type[] IListParam = {objTyp};          
object Result = IListRef.MakeGenericType(IListParam);

MyObject objTemp = new MyObject(); 
Result.GetType().GetMethod("Add").Invoke(Result, new[] {objTemp });

Please help.

Answer 1

You're trying to find an Add method in Type , not in List<MyObject> - and then you're trying to invoke it on a Type .

MakeGenericType returns a type, not an instance of that type. If you want to create an instance, Activator.CreateInstance is usually the way to go. Try this:

Type objTyp = typeof(MyObject); //HardCoded TypeName for demo purpose
var IListRef = typeof (List<>);
Type[] IListParam = {objTyp};          
object Result = Activator.CreateInstance(IListRef.MakeGenericType(IListParam));

MyObject objTemp = new MyObject(); 
Result.GetType().GetMethod("Add").Invoke(Result, new[] {objTemp });

(I would also suggest that you start following conventions for variable names, but that's a separate matter.)

Answer 2

    private static void Test()
    {
        IList<Guid> list = CreateList<Guid>();
        Guid objTemp = Guid.NewGuid();
        list.Add(objTemp);
    }

    private static List<TItem> CreateList<TItem>()
    {
        Type listType = GetGenericListType<TItem>();
        List<TItem> list = (List<TItem>)Activator.CreateInstance(listType);
        return list;
    }

    private static Type GetGenericListType<TItem>()
    {
        Type objTyp = typeof(TItem);
        var defaultListType = typeof(List<>);
        Type[] itemTypes = { objTyp };
        Type listType = defaultListType.MakeGenericType(itemTypes);
        return listType;
    }

IList.Add(object item); => you can use Add method in IList interface instead of Reflection.

Answer 3

You have only created a generic type, you haven't created an instance of the type. You have a list type, but you don't have a list.

The Result variabled contains a Type object, so Result.Gettype() returns the same as typeof(Type) . You are trying to find an Add method in the Type class, not your list class.

Could you not use generics instead of reflection, eg:

public static List<T> CreateListAndAddEmpty<T>() where T : new() {
  List<T> list = new List<T>();
  list.Add(new T());
  return list;
}