无法通过引用转换、装箱转换、拆箱转换、包装转换或空类型转换来转换类型 [英] Cannot convert type via a reference conversion, boxing conversion, unboxing conversion, wrapping conversion, or null type conversion

问题描述

在 C# 中，如果我有一个参数类型为接口的函数的参数，如何传入实现该接口的对象.

In C#, if I have a parameter for a function where the parameter type is of an interface, how do a pass in an object that implements the interface.

这是一个例子:

函数的参数如下:

List<ICustomRequired>

我已有的列表如下:

List<CustomObject> exampleList

CustomObject 继承自 ICustomRequired 接口

将 exampleList 作为参数传递的正确语法是什么?

What is the correct syntax to pass the exampleList as a parameter?

这就是我想完成上述任务的方式:

This is how I thought to do the above task:

exampleList as List<ICustomRequired>

但是我收到以下错误:

无法通过引用转换、装箱转换、拆箱转换、包装转换或空类型转换

Cannot convert type via a reference conversion, boxing conversion, unboxing conversion, wrapping conversion, or null type conversion

谢谢

推荐答案

您不能将一种类型的 List 强制转换为另一种类型的 List.

You cannot cast a List of one type to a List of a different type.

如果你考虑一下，你会很高兴你不能.想象一下，如果可能的话，您可能会造成什么样的破坏:

And if you think about it, you would be glad that you can't. Imagine the havoc you could cause if it was possible:

 interface ICustomRequired
 {
 }

 class ImplementationOne : ICustomRequired
 {
 }

 class ImplementationTwo: ICustomRequired
 {
 }

 var listOne = new List<ImplementationOne>();
 var castReference = listOne as List<ICustomRequired>();
 // Because you did a cast, the two instances would point
 // to the same in-memory object

 // Now I can do this....
 castReference.Add(new ImplementationTwo());

 // listOne was constructed as a list of ImplementationOne objects,
 // but I just managed to insert an object of a different type

但是请注意，这行代码是合法的:

Note, however, that this line of code is legal:

 exampleList as IEnumerable<ICustomRequired>;

这是安全的，因为 IEnumerable 没有为您提供任何添加新对象的方法.

This would be safe, because IEnumerable does not provide you with any means to add new objects.

IEnumerable 实际上定义为IEnumerable，表示类型参数为协变.

IEnumerable<T> is actually defined as IEnumerable<out t>, which means the type parameter is Covariant.

能不能把函数的参数改成IEnumerable?

Are you able to change the parameter of the function to IEnumerable<ICustomRequired>?

否则您唯一的选择就是创建一个新列表.

Otherwise your only option will be to create a new List.

var newList = (exampleList as IEnumerable<ICustomRequired>).ToList();

或

var newList = exampleList.Cast<ICustomRequired>().ToList();

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