c# - C# 和基类到派生类转换

Question

我想将基类实例列表中的信息投影到派生类实例列表中，但我一直遇到强制转换异常。这是我正在尝试做的一个例子......我如何让它工作？

以下代码在http://ideone.com/CaXQS上，如果有帮助的话……在此先感谢！

using System;
using System.Collections.Generic;

namespace AddingReversedNumbers
{
        public class MyDerivedClass : MyBaseClass, IMyInterface
        {
                public int InterfaceProperty { get; set; }
                public int DerivedClassProperty { get; set; }
                public List<int> DerivedClassList { get; set; }
        }

        public class MyBaseClass
        {
                public int BaseClassProperty { get; set; }
        }

        public interface IMyInterface
        {
                int InterfaceProperty { get; set; }
        }

        class Program
        {
                static void Main()
                {
                        //// This code works just fine.
                        //var derivedList = new List<MyDerivedClass>();
                        //derivedList.Add(new MyDerivedClass { BaseClassProperty = 10, DerivedClassProperty = 20, InterfaceProperty = 30 });
                        //derivedList.Add(new MyDerivedClass { BaseClassProperty = 20, DerivedClassProperty = 40, InterfaceProperty = 60 });
                        //var baseList = derivedList.ConvertAll(x => (MyBaseClass)x);

                        // This code breaks when ConvertAll() is called.
                        var baseList = new List<MyBaseClass>();
                        baseList.Add(new MyBaseClass{ BaseClassProperty = 10 });
                        baseList.Add(new MyBaseClass{ BaseClassProperty = 20 });
                        var derivedList = baseList.ConvertAll(x => (MyDerivedClass)x);
                }
        }
}

Answer 1

在您的ConvertAll代码中，您只是在投射。您不能将基类实例强制转换为派生类实例。例如，你希望演员object做FileStream什么？它会引用哪个文件？

如果要在投影中创建新对象ConvertAll，只需执行以下操作：

var derivedList = baseList.ConvertAll
      (x => new MyDerivedClass { BaseClassProperty = x.BaseClassProperty });