虽然可以从基类/接口继承，但为什么不能声明List<>呢 使用相同的类/接口?
interface A
{ }
class B : A
{ }
class C : B
{ }
class Test
{
static void Main(string[] args)
{
A a = new C(); // OK
List<A> listOfA = new List<C>(); // compiler Error
}
}
有别的办法吗?
当前回答
对于你的问题，有几种原生c#的可能性:
动态 数组 IReadOnlyList, IEnumerable 正确使用List<>。 它们都工作得很好!不需要任何复杂的编程!
下面是它们各自的例子:
1. 动态:最通用的解决方案
运行时类型检查 您放弃了编译器错误检查支持，所以要小心处理!如果您尝试添加错误类型的元素，则只会得到一个运行时错误! 您甚至可以分配不相关类的集合。
简单写动态listOfA = new List<C>();listOfA = new List<C>();
首先是所有示例的接口和类定义:
using System;
using System.Collections.Generic;
using System.Linq;
interface IAnimal
{
public string Name { get; }
}
class Bear : IAnimal
{
public string BearName = "aBear";
public string Name => BearName;
}
class Cat : IAnimal
{
public string CatName = "aCat";
public string Name => CatName;
}
// Dog has no base class/interface; it isn't related to the other classes
class Dog
{
public string DogName = "aDog";
public string Name => DogName;
}
下面是使用dynamic的例子
public class AssignDerivedClass
{
public static void TestDynamicListAndArray()
{
dynamic any = new List<Bear>() // List of derived
{
new Bear() { BearName = "Bear-1" },
new Bear() { BearName = "Bear-2" }
};
//any[0].CatName = "NewCat"; // => Microsoft.CSharp.RuntimeBinder.RuntimeBinderException
Console.WriteLine($"Bear names: {any[0].BearName}, {Name(any[1])}");
any = new Cat[] // Array of derived
{
new Cat() { CatName = "Cat-3" },
new Cat() { CatName = "Cat-4" }
};
Console.WriteLine($"Cat names: {any[0].CatName}, {any[1].Name}");
any = new List<Dog>() // List of non-related class
{
new Dog() { DogName = "Dog-5" },
new Dog() { DogName = "Dog-6" }
};
Console.WriteLine($"Dog names: {any[0].DogName}, {Name(any[1])}");
any = new List<IAnimal>() // List of interface
// any = new IAnimal[] // Array of interface works the same
{
new Bear() { BearName = "Bear-7" },
new Cat() { CatName = "Cat-8" }
};
Console.WriteLine($"Animal names: {any[0].BearName}, {any[1].CatName}");
any[0].BearName = "NewBear";
Console.WriteLine($"Animal names: {Name(any[0])}, {any[1].Name}");
}
private static string Name(dynamic anymal)
{
return anymal switch
{
Bear bear => bear.BearName,
Cat cat => cat.CatName,
Dog dog => dog.DogName,
_ => "No known Animal"
};
}
// Bear names: Bear-1, Bear-2
// Cat names: Cat-3, Cat-4
// Dog names: Dog-5, Dog-6
// Animal names: Bear-7, Cat-8
// Animal names: NewBear, Cat-8
}
2. Array:创建一个Bear[]数组，保证所有数组元素都引用Bear的实例。
You can exchange elements, but you can't remove or add new elements. Trying to set a wrong type yields a runtime error. public static void TestArray() { Bear[] bears = { new Bear(), null }; IAnimal[] bearAnimals = bears; //bearAnimals[1] = new Cat(); // System.ArrayTypeMismatchException bearAnimals[1] = new Bear() { BearName = "Bear-1" }; Console.WriteLine($"Bear names: {bearAnimals[0].Name}, {bears[1].BearName}"); } // Result => Bear names: aBear, Bear-1
3.IReadOnlyList IEnumerable:
Assign your List<C> to an IEnumerable<A> or IReadOnlyList<A> Neither of them can be changed at runtime, i.e. you can't Add or Remove elements. Why should the compiler allow assigning your List<C> to a List<A> instead of IReadOnlyList<A> when adding an element will lead to an error anyway? public static void TestIEnumerableAndIReadonlyList() { var cats = new List<Cat>() { new Cat() { CatName = "Cat-3" }, new Cat() { CatName = "Cat-4" } }; IEnumerable<IAnimal> iEnumerable = cats; Console.WriteLine($"Cat names: {(iEnumerable.ElementAt(0) as Cat).CatName}, " + Name(iEnumerable.Last())); IReadOnlyList<IAnimal> iROList = cats; Console.WriteLine($"Cat names: {iROList[0].Name}, {Name(iROList[1])}"); //iROList.Add(new Cat()); // compiler error CS61: no definition for 'Add' } // Result: // Cat names: Cat-3, Cat-4 // Cat names: Cat-3, Cat-4
4. 正确使用List<>: List<A> listOfA = new List<A>()
Define a List of your interface Assign instances of one derived class only - you didn't want to store other classes anyway, did you? public static void TestListOfInterface() { var bears = new List<IAnimal>() { new Bear() { BearName = "Bear-1" }, new Cat() { CatName = "Cat-3" }, }; bears.Add(new Bear() { BearName = "Bear-2" }); string bearNames = string.Join(", ", bears.Select(animal => animal.Name)); Console.WriteLine($"Bear names: {bearNames}"); string bearInfo0 = VerifyBear(bears[0]); string bearInfo1 = VerifyBear(bears[1]); Console.WriteLine($"One animal is {bearInfo0}, the other one is {bearInfo1}"); string VerifyBear(IAnimal bear) => (bear as Bear)?.BearName ?? "disguised as a bear!!!"; } // Bear names: Bear-1, Cat-3, Bear-2 // One animal is Bear-1, the other one is disguised as a bear!!!
其他回答
你也可以使用System.Runtime.CompilerServices.Unsafe NuGet包来创建同一个列表的引用:
using System.Runtime.CompilerServices;
...
class Tool { }
class Hammer : Tool { }
...
var hammers = new List<Hammer>();
...
var tools = Unsafe.As<List<Tool>>(hammers);
对于上面的示例，您可以使用tools变量访问列表中现有的Hammer实例。将Tool实例添加到列表中会抛出ArrayTypeMismatchException异常，因为tools引用了与hammers相同的变量。
可能会迟到。
转换为数组也可以完成这项工作。
main()
{
List<Camel> camels = new List<Camel>();
Reproducton(camels.ToArray());
}
public void Reproducton(Animal[] animals)
{
foreach(var animal in animals.ToList())
{
var baby = animal.Reproduce();
}
}
至于为什么它不起作用，理解协方差和逆变可能会有所帮助。
只是为了说明为什么这不能工作，这里是对您提供的代码的更改:
void DoesThisWork()
{
List<C> DerivedList = new List<C>();
List<A> BaseList = DerivedList;
BaseList.Add(new B());
C FirstItem = DerivedList.First();
}
这可行吗?列表中的第一个项是类型“B”，但派生列表项的类型是C。
现在，假设我们真的只想创建一个泛型函数，它对实现了a的某个类型的列表进行操作，但我们不关心它是什么类型:
void ThisWorks<T>(List<T> GenericList) where T:A
{
}
void Test()
{
ThisWorks(new List<B>());
ThisWorks(new List<C>());
}
对于你的问题，有几种原生c#的可能性:
动态 数组 IReadOnlyList, IEnumerable 正确使用List<>。 它们都工作得很好!不需要任何复杂的编程!
下面是它们各自的例子:
1. 动态:最通用的解决方案
运行时类型检查 您放弃了编译器错误检查支持，所以要小心处理!如果您尝试添加错误类型的元素，则只会得到一个运行时错误! 您甚至可以分配不相关类的集合。
简单写动态listOfA = new List<C>();listOfA = new List<C>();
首先是所有示例的接口和类定义:
using System;
using System.Collections.Generic;
using System.Linq;
interface IAnimal
{
public string Name { get; }
}
class Bear : IAnimal
{
public string BearName = "aBear";
public string Name => BearName;
}
class Cat : IAnimal
{
public string CatName = "aCat";
public string Name => CatName;
}
// Dog has no base class/interface; it isn't related to the other classes
class Dog
{
public string DogName = "aDog";
public string Name => DogName;
}
下面是使用dynamic的例子
public class AssignDerivedClass
{
public static void TestDynamicListAndArray()
{
dynamic any = new List<Bear>() // List of derived
{
new Bear() { BearName = "Bear-1" },
new Bear() { BearName = "Bear-2" }
};
//any[0].CatName = "NewCat"; // => Microsoft.CSharp.RuntimeBinder.RuntimeBinderException
Console.WriteLine($"Bear names: {any[0].BearName}, {Name(any[1])}");
any = new Cat[] // Array of derived
{
new Cat() { CatName = "Cat-3" },
new Cat() { CatName = "Cat-4" }
};
Console.WriteLine($"Cat names: {any[0].CatName}, {any[1].Name}");
any = new List<Dog>() // List of non-related class
{
new Dog() { DogName = "Dog-5" },
new Dog() { DogName = "Dog-6" }
};
Console.WriteLine($"Dog names: {any[0].DogName}, {Name(any[1])}");
any = new List<IAnimal>() // List of interface
// any = new IAnimal[] // Array of interface works the same
{
new Bear() { BearName = "Bear-7" },
new Cat() { CatName = "Cat-8" }
};
Console.WriteLine($"Animal names: {any[0].BearName}, {any[1].CatName}");
any[0].BearName = "NewBear";
Console.WriteLine($"Animal names: {Name(any[0])}, {any[1].Name}");
}
private static string Name(dynamic anymal)
{
return anymal switch
{
Bear bear => bear.BearName,
Cat cat => cat.CatName,
Dog dog => dog.DogName,
_ => "No known Animal"
};
}
// Bear names: Bear-1, Bear-2
// Cat names: Cat-3, Cat-4
// Dog names: Dog-5, Dog-6
// Animal names: Bear-7, Cat-8
// Animal names: NewBear, Cat-8
}
2. Array:创建一个Bear[]数组，保证所有数组元素都引用Bear的实例。
You can exchange elements, but you can't remove or add new elements. Trying to set a wrong type yields a runtime error. public static void TestArray() { Bear[] bears = { new Bear(), null }; IAnimal[] bearAnimals = bears; //bearAnimals[1] = new Cat(); // System.ArrayTypeMismatchException bearAnimals[1] = new Bear() { BearName = "Bear-1" }; Console.WriteLine($"Bear names: {bearAnimals[0].Name}, {bears[1].BearName}"); } // Result => Bear names: aBear, Bear-1
3.IReadOnlyList IEnumerable:
Assign your List<C> to an IEnumerable<A> or IReadOnlyList<A> Neither of them can be changed at runtime, i.e. you can't Add or Remove elements. Why should the compiler allow assigning your List<C> to a List<A> instead of IReadOnlyList<A> when adding an element will lead to an error anyway? public static void TestIEnumerableAndIReadonlyList() { var cats = new List<Cat>() { new Cat() { CatName = "Cat-3" }, new Cat() { CatName = "Cat-4" } }; IEnumerable<IAnimal> iEnumerable = cats; Console.WriteLine($"Cat names: {(iEnumerable.ElementAt(0) as Cat).CatName}, " + Name(iEnumerable.Last())); IReadOnlyList<IAnimal> iROList = cats; Console.WriteLine($"Cat names: {iROList[0].Name}, {Name(iROList[1])}"); //iROList.Add(new Cat()); // compiler error CS61: no definition for 'Add' } // Result: // Cat names: Cat-3, Cat-4 // Cat names: Cat-3, Cat-4
4. 正确使用List<>: List<A> listOfA = new List<A>()
Define a List of your interface Assign instances of one derived class only - you didn't want to store other classes anyway, did you? public static void TestListOfInterface() { var bears = new List<IAnimal>() { new Bear() { BearName = "Bear-1" }, new Cat() { CatName = "Cat-3" }, }; bears.Add(new Bear() { BearName = "Bear-2" }); string bearNames = string.Join(", ", bears.Select(animal => animal.Name)); Console.WriteLine($"Bear names: {bearNames}"); string bearInfo0 = VerifyBear(bears[0]); string bearInfo1 = VerifyBear(bears[1]); Console.WriteLine($"One animal is {bearInfo0}, the other one is {bearInfo1}"); string VerifyBear(IAnimal bear) => (bear as Bear)?.BearName ?? "disguised as a bear!!!"; } // Bear names: Bear-1, Cat-3, Bear-2 // One animal is Bear-1, the other one is disguised as a bear!!!
因为c#目前不允许这种类型的继承转换。
