什么是使用的IEqualityComparer&LT推荐的最佳做法; T&GT ;? [英] What's the recommended best practice for using IEqualityComparer<T>?

问题描述

我正在寻找现实世界中的最佳实践，别人怎么可能有复杂的领域实施的解决方案。

I'm looking for real world best practices, how other people might have implemented solutions with complex domains.

推荐答案

任何时候你可以考虑使用的IEqualityComparer< T> ，停下来想，如果这个类可以作出实施 IEquatable< T> 来代替。如果产品应该总是通过ID进行比较，只是把它定义为等同这样，所以你可以使用默认的比较。

Any time you consider using an IEqualityComparer<T>, pause to think if the class could be made to implement IEquatable<T> instead. If a Product should always be compared by ID, just define it to be equated as such so you can use the default comparer.

不过，还是有一些原因，你可能想自定义比较：

That said, there are still a few of reasons you might want a custom comparer:

1. 如果有多种方式实例一类可被认为是相等的。这样做的最好的例子是一个字符串，为此，该框架提供的 StringComparer 。


2. 如果该类以这样的方式定义的，你不能把它定义为 IEquatable< T> 。这将包括由编译器（特别是匿名类型，默认情况下使用属性明智的比较）产生别人和类定义的类。

1. If there are multiple ways instances of a class could be considered equal. The best example of this is a string, for which the framework provides six different comparers in StringComparer.
2. If the class is defined in such a way that you can't define it as IEquatable<T>. This would include classes defined by others and classes generated by the compiler (specifically anonymous types, which use a property-wise comparison by default).

如果你决定你需要一个比较器，你当然可以用广义的比较器（见DMenT的答案），但如果你需要重新使用逻辑应该封装在一个专用的类。你甚至可以通过从通用基继承声明它：

If you do decide you need a comparer, you can certainly use a generalized comparer (see DMenT's answer), but if you need to reuse that logic you should encapsulate it in a dedicated class. You could even declare it by inheriting from the generic base:

class ProductByIdComparer : GenericEqualityComparer<ShopByProduct>
{
    public ProductByIdComparer()
        : base((x, y) => x.ProductId == y.ProductId, z => z.ProductId)
    { }
}

至于使用，您应该利用comparers的可能时。例如，而不是调用 ToLower将（）在作为字典的键每个字符串（其逻辑将在您的应用程序，简直是俯拾即是），你应该申报字典使用不区分大小写 StringComparer 。这同样适用于LINQ的运营商接受一个比较器。但同样，始终考虑如果equatable行为应该是内在的类，而不是外部定义的。

As far as use, you should take advantage of comparers when possible. For example, rather than calling ToLower() on every string used as a dictionary key (logic for which will be strewn across your app), you should declare the dictionary to use a case-insensitive StringComparer. The same goes for the LINQ operators that accept a comparer. But again, always consider if the equatable behavior that should be intrinsic to the class rather than defined externally.

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