Question

I have the following object:

public class ItemChange<T> where T : MyBase
{
public DateTime When { get; set; }
public string Who { get; set; }
public T NewState;
public T OldState;
}

and i am trying to cast and instance of ItemChange<T> to ItemChange<MyBase> but i am getting a cast exception.

Unable to cast object of type . . .

WHat is the proper way to cast a generic type to its base calss (assuming the where constraint above)

Answer 1

You can’t do this directly since classes in C# do not support co-variance. You can do it with interfaces however:

public interface IItemChange<out T> where T : MyBase
{
    DateTime When { get; set; }
    string Who { get; set; }
    T NewState { get; }
    T OldState { get; }
}

public class ItemChange<T> : IItemChange<T> where T : MyBase
{
    public DateTime When { get; set; }
    public string Who { get; set; }
    public T NewState { get; set; }
    public T OldState { get; set; }
}

Then you can do:

IItemChange<MyBase> base = new ItemChange<MySubclass>();

for some subclass MySubclass of MyBase.