我试图列出模板化类的列表,将基类传递给模板。但是,似乎不允许这样做。有没有办法避免这种限制,或者更适当地重组我的代码?
这是一个抽象的例子:
using System;
using System.Collections.Generic;
namespace TempInherit
{
abstract class Shape{}
class Triangle : Shape{}
class Square : Shape{}
class ShapeHolder<T>{}
class MainClass
{
public static void Main(string[] args)
{
// list of base class, add subclass - works
List<Shape> shapes = new List<Shape>();
shapes.Add(new Triangle());
shapes.Add(new Square());
// list of holders of base class, add holders of subclass - fails
List<ShapeHolder<Shape>> shapeHolders = new List<ShapeHolder<Shape>>();
shapeHolders.Add(new ShapeHolder<Triangle>());
shapeHolders.Add(new ShapeHolder<Square>());
}
}
}
产生:
错误CS1502:最佳匹配的重载方法
`System.Collections.Generic.List> .Add(TempInherit.ShapeHolder)'
有一些无效的参数(CS1502)(TempInherit)
错误CS1503:要键入的参数
#1' cannot convert
TempInherit.ShapeHolder'表达式`TempInherit.ShapeHolder'(CS1503)(TempInherit)
最佳答案
协方差问题:
您可以创建接口IShapeHolder<out T>
,因为接口上的通用参数可以协变(但不能在类上)
这样的东西
public class Shape
{
}
public class Triangle : Shape
{
}
public class Square : Shape
{
}
//T generic parameter is covariant (out keyword)
public interface IShapeHolder<out T> where T : Shape
{
}
public class ShapeHolder<T> : IShapeHolder<T> where T: Shape
{
}
然后,
var shapes = new List<Shape>();
shapes.Add(new Triangle());
shapes.Add(new Square());
// list of holders of base class, add holders of subclass - fails no more
var shapeHolders = new List<IShapeHolder<Shape>>();
shapeHolders.Add(new ShapeHolder<Triangle>());
shapeHolders.Add(new ShapeHolder<Square>());