Question

public interface IMyControl<in T> where T : ICoreEntity
{
    void SetEntity(T dataObject);
}

public class MyControl : UserControl, IMyControl<DataObject>   // DataObject implements ICoreEntity
{
    void SetEntity(T dataObject);
}

All fine so far, but why does this create null?

var control = LoadControl("~/Controls/MyControl.ascx"); // assume this line works
IMyControl<ICoreEntity> myControl = control;

myControl is now null…

Answer 1

You cannot have dataObject as parameter for this to work. Methods could only return it.

public interface ICoreEntity { }
public class DataObject: ICoreEntity { }

public interface IMyControl<out T> where T : ICoreEntity
{
    T GetEntity();
}

public class MyControl : IMyControl<DataObject>   // DataObject implements ICoreEntity
{
    public DataObject GetEntity()
    {
        throw new NotImplementedException();
    }
}

Now you can:

MyControl control = new MyControl();
IMyControl<ICoreEntity> myControl = control;