自IEnumerable&LT铸造;对象>到了IEnumerable<串GT; [英] Casting from IEnumerable<Object> to IEnumerable<string>

问题描述

最近我发现在C＃中一个非常令人惊讶的行为。
我有这需要一种方法的IEnumerable<对象> 作为参数，我经过
的IEnumerable<串> ，但它是不可能的。
虽然在C＃中一切都可以上溯造型比为什么这是不可能的对象吗？
这对我来说完全混乱。
请人清除我这个问题。

Recently I found a very surprising behavior in c#. I had a method which takes IEnumerable<Object> as a parameter and i was passing IEnumerable<string> but it's not possible. While in c# everything can be upcast to Object than why this is not possible? It's totally confusing for me. Please someone clear me on this issue.

推荐答案

此的技术术语为泛型在C＃3.0不变和更早版本。从C＃4.0起，中投的作品。

The technical term for this is that generics are invariant in C# 3.0 and earlier. From C#4.0 onward, the cast works.

什么不变的手段是，有两个泛型类型之间没有任何关系，只是因为他们的泛型类型的参数相关（即是子或对方的超类型）

What invariant means is that there is no relationship between two generic types just because their generic type parameters are related (i.e. are sub- or supertypes of each other).

在你的榜样，有一个的IEnumerable℃之间没有打字关系;对象> ; 和的IEnumerable<串GT; ，只因为字符串是对象的子类型。他们只是考虑两个完全不相干的类型，如字符串和一个int（他们仍然都是对象的亚型，但一切）

In your example, there is no typing relationship between an IEnumerable<object> and an IEnumerable<string>, just because string is a subtype of object. They're just considered two completely unrelated types, like a string and an int (they still both are subtypes of object, but everything is)

有一些变通方法，并例外这个问题你已经碰上了。

There are a few workarounds and exceptions for this issue you've run into.

首先，你可以单独投每个字符串对象，如果你使用.NET 3.0，你可以做到这一点使用演员LT; T>（）扩展方法。否则，你可以使用foreach和结果放入你想要的静态类型的新变量。

First, you can cast each string individually to object, if you're using .NET 3.0 you can do that using the Cast<T>() extension method. Otherwise, you can use a foreach and put the result into a new variable of the static type you want.

二，数组是引用类型的异常，即通过在字符串[]类型的方法acccepting对象[]类型应该工作。

Second, arrays are an exception for reference type, i.e. passing in a string[] type to a method acccepting object[] types should work.

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