接口作为泛型类型 - 方法如何返回子接口类型的值？

Question

I have project in C# that represents table using two different types of row. SimpleRow and BetterRow. SimpleRow implements interface ISimpleRow and BetterRow implements IBetterRow where ISimpleRow is superclass of IBetterRow (SimpleRow cannot be superclass of BetterRow directly because that design is a little more complicated then I describe here. All row's interfaces and classes have generic TCell (not important here).

Now I have SimpleTable<TRow> and BetterTable<TRow>: SimpleTable<TRow> . It looks like:

public interface ISimpleRow<TCell> {...}
public interface IBetterRow<TCell> : ISimpleRow<TCell> {...}
public class SimpleRow<TCell> : ISimpleRow<TCell> {...}
public class BetterRow<TCell> : SimpleRow<TCell>, IBetterRow<TCell> {...}

public class SimpleTable<TRow> where TRow : SimpleRow<ICell> {
...
   public virtual TRow getRow() {
      return new SimpleRow<ICell>();
   }
}
public class BetterTable<TRow> : SimpleTable<TRow> where TRow : SimpleRow<ICell> {
   ...
   public override TRow getRow() {
      return new BetterRow<ICell>();
   }
}

There is my problem (line "return BetterRow..."):

Cannot implicitly convert type BetterRow to TRow

And also when I am trying to create an instance eg new BetterTable<BetterRow<ICell>>()

The type BetterRow cannot be used as type parametr TRow in the generic type or method BetterRow. There is no implicit reference conversion from BetterRow to SimpleRow

Any idea how to solve this? My method getRow() actually can return instance of one of two classes that implemented it's interface. There are 4 classes where to implements SimpleRow and two implements BetterRow. Method getRow returns one of them (SimpleRowA or SimpleRowB if TRow is ISimpleRow or BetterRowA or BetterB if TRow is IBetterRow>). But it is not important here, I guess. I just wanted to mention it for case and to make you sure I cannot change return type of getRow method.

Answer 1

You can't do exactly that case SimpleRow<ICell>() is not guaranteed to be any type of TRow caller can provide. Though one option is to add new() constraint if it is suitable for you:

public class SimpleTable<TRow> where TRow : SimpleRow<ICell>, new()
{

   public virtual TRow getRow()
   {
        return new TRow();
   }
}
public class BetterTable<TRow> : SimpleTable<TRow> where TRow : SimpleRow<ICell>, new()
{

   public override TRow getRow()
    {
        return new TRow();
    }
}

But in general it seems that you need to think about changing approach in general and without having full api it is hard to argue about that.