Indeed you would not be able to write the last line.
But you probably don't want to create the object, just for the sake or creating it. You probably want to call some method on your newly created instance.
You'll then need something like an interface :
public interface ITask
{
void Process(object o);
}
public class Task<T> : ITask
{
void ITask.Process(object o)
{
if(o is T) // Just to be sure, and maybe throw an exception
Process(o as T);
}
public void Process(T o) { }
}
and call it with :
Type d1 = Type.GetType("TaskA"); //or "TaskB"
Type[] typeArgs = { typeof(Item) };
Type makeme = d1.MakeGenericType(typeArgs);
ITask task = Activator.CreateInstance(makeme) as ITask;
// This can be Item, or any type derived from Item
task.Process(new Item());
In any case, you won't be statically cast to a type you don't know beforehand ("makeme" in this case). ITask allows you to get to your target type.
If this is not what you want, you'll probably need to be a bit more specific in what you are trying to achieve with this.