通用继承类的替代方法，用于将继承的属性约束为其基类型本身的子级

Question

I want to restrain a member of an inherited class to a specific type. I know I could do it with generic class, but it seems more complicated than it should be. So, here's an exemple.

public class Shape
{
    public Texture texture { get; set; }
}

public class Texture { ... }
public class SpecficTexture : Texture { ... }

public class SpecificShape : Shape
{
    public SpecificShape()
    {
        texture = new SpecficTexture();
    }
}

Essentially, in my case I need to implement the class SpecificShape. It's all good but the texture property inside SpecificShape is not known as a specific texture. From the SpecificShape, it need to be known as such. Obviously, I could have the following classes instead.

public class Shape2<T> where T : Texture
{
    public T texture { get; set; }
}

public class SpecifShape2 : Shape2<SpecficTexture> { ... }

I see two solutions to my problem. Either I go to my team to switch to make Shape a generic class or I cast texture to SpecificTexture whenever I need it. Is there any other way?

I don't like the idea of casting, it doesn't seem elegant to me, and I'm worried about performance issue if I had to do it multiple times. At the same time, I'm not sure it's justified to change back what was already implemented.

Answer 1

You can make the texture property virtual in the Shape , override it in SpecificShape and limit the casting to one specific place:

public class Shape
{
    public virtual Texture texture { get; set; }
}

public class SpecificShape : Shape
{
    public SpecificTexture specificTexture { get; set; }

    public override Texture texture
    {
        get { return specificTexture; }
        set
        {
            if (value is SpecificTexture specificValue) //this is the only place we have to have a cast
            {
                specificTexture = specificValue;
            }
            else
            {
                //handle error, throw exception, Debug.Assert, or something
            }
        }
    }

    public SpecificShape()
    {
        specificTexture = new SpecificTexture();
    }
}

Whenever you need to use SpecificTexture in your SpecificShape class, you can simply use the specificTexture property instead of texture .

Furthermore, if the parent Shape class doesn't really need the texture property setter (perhaps, the texture property is only set once in the constructor?), then just get rid of the setter, and you can avoid any casts.