如何使用 IEqualityComparer [英] How to use the IEqualityComparer

问题描述

我的数据库中有一些具有相同编号的铃铛.我想在没有重复的情况下获得所有这些.我创建了一个比较类来完成这项工作，但是该函数的执行会导致该函数出现很大的延迟，从 0.6 秒到 3.2 秒！

I have some bells in my database with the same number. I want to get all of them without duplication. I created a compare class to do this work, but the execution of the function causes a big delay from the function without distinct, from 0.6 sec to 3.2 sec!

我做得对还是必须使用其他方法?

Am I doing it right or do I have to use another method?

reg.AddRange(
    (from a in this.dataContext.reglements
     join b in this.dataContext.Clients on a.Id_client equals b.Id
     where a.date_v <= datefin && a.date_v >= datedeb
     where a.Id_client == b.Id
     orderby a.date_v descending 
     select new Class_reglement
     {
         nom  = b.Nom,
         code = b.code,
         Numf = a.Numf,
     })
    .AsEnumerable()
    .Distinct(new Compare())
    .ToList());

class Compare : IEqualityComparer<Class_reglement>
{
    public bool Equals(Class_reglement x, Class_reglement y)
    {
        if (x.Numf == y.Numf)
        {
            return true;
        }
        else { return false; }
    }
    public int GetHashCode(Class_reglement codeh)
    {
        return 0;
    }
}

推荐答案

您的 GetHashCode 实现始终返回相同的值.Distinct 依靠良好的哈希函数来高效工作，因为它在内部构建了一个哈希表.

Your GetHashCode implementation always returns the same value. Distinct relies on a good hash function to work efficiently because it internally builds a hash table.

在实现类的接口时，阅读文档，了解您应该实施哪个合同.¹

在您的代码中，解决方案是将 GetHashCode 转发到 Class_reglement.Numf.GetHashCode 并在那里适当地实现它.

In your code, the solution is to forward GetHashCode to Class_reglement.Numf.GetHashCode and implement it appropriately there.

除此之外，您的 Equals 方法充满了不必要的代码.它可以重写如下(语义相同，代码的 1/4，更具可读性):

Apart from that, your Equals method is full of unnecessary code. It could be rewritten as follows (same semantics, ¼ of the code, more readable):

public bool Equals(Class_reglement x, Class_reglement y)
{
    return x.Numf == y.Numf;
}

最后，ToList 调用是不必要且耗时的:AddRange 接受任何 IEnumerable 因此转换为 List 不是必需的.AsEnumerable 在这里也是多余的，因为在 AddRange 中处理结果无论如何都会导致这种情况.

Lastly, the ToList call is unnecessary and time-consuming: AddRange accepts any IEnumerable so conversion to a List isn’t required. AsEnumerable is also redundant here since processing the result in AddRange will cause this anyway.

¹在不知道实际作用的情况下编写代码被称为货物崇拜编程.这是一种令人惊讶的普遍做法.它从根本上不起作用.

¹ Writing code without knowing what it actually does is called cargo cult programming. It’s a surprisingly widespread practice. It fundamentally doesn’t work.

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