为什么 .Except() 和 Intersect() 不能使用 LINQ 在这里工作？

发布于 2024-09-12 18:01:13 字数 818 浏览 4 评论 0原文

我有以下代码，但似乎不起作用：

上下文： 我有两个对象列表：
* listOne 有 100 条记录
* listTwo 有 70 条记录，

其中许多记录具有相同的 Id 属性（在两个列表中）；

 var listOneOnlyItems = listOne.Except(listTwo, new ItemComparer ());

比较器

public class ItemComparer : IEqualityComparer<Item>
{
    public bool Equals(Item x, Item y)
    {
        if (x.Id == y.Id)
            return true;

        return false;
    }

    public int GetHashCode(Item obj)
    {
        return obj.GetHashCode();
    }
}

这是我运行此代码并查看结果后的

listOneOnlyItems

，仍然有 100 条记录（应该只有 30 条）。谁能帮助我吗？

此外，运行

    IEnumerable<Item> sharedItems = listOne.Intersect(listTwo, new ItemComparer());

在sharedItems集合中返回零结果

原文

i have the following code which doesnt seem to be working:

Context:
I have two lists of objects:
* listOne has 100 records
* listTwo has 70 records

many of them have the same Id property (in both lists);

 var listOneOnlyItems = listOne.Except(listTwo, new ItemComparer ());

here is the comparer

public class ItemComparer : IEqualityComparer<Item>
{
    public bool Equals(Item x, Item y)
    {
        if (x.Id == y.Id)
            return true;

        return false;
    }

    public int GetHashCode(Item obj)
    {
        return obj.GetHashCode();
    }
}

after i run this code and look into the results

listOneOnlyItems

still has 100 records (should only have 30). Can anyone help me?

also, running

    IEnumerable<Item> sharedItems = listOne.Intersect(listTwo, new ItemComparer());

returns zero reesults in the sharedItems collection

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∝单色的世界 2024-09-19 18:01:13

public int GetHashCode(Item obj)
{
    return obj.Id.GetHashCode();
}

至少值得检查一下——IIRC GetHashCode() 在相等性之前首先被测试，如果它们没有相同的哈希值，则不会费心检查相等性。我不确定 obj.GetHashCode() 会发生什么——这取决于您在 Item 类上实现的内容。

public int GetHashCode(Item obj)
{
    return obj.Id.GetHashCode();
}

Worth a check at least -- IIRC GetHashCode() is tested first before equality, and if they don't have the same hash it won't bother checking equality. I'm not sure what to expect from obj.GetHashCode() -- it depends on what you've implemented on the Item class.

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沉鱼一梦 2024-09-19 18:01:13

考虑让 GetHashCode() 返回 obj.Id.GetHashCode()

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时常饿 2024-09-19 18:01:13

此代码工作正常：

static void TestLinqExcept()
{
    var seqA = Enumerable.Range(1, 10);
    var seqB = Enumerable.Range(1, 7);
    var seqAexceptB = seqA.Except(seqB, new IntComparer());
    foreach (var x in seqAexceptB)
    {
        Console.WriteLine(x);
    }
}

class IntComparer: EqualityComparer<int>
{
    public override bool Equals(int x, int y)
    {
        return x == y;
    }

    public override int GetHashCode(int x)
    {
        return x;
    }
}

您需要将“覆盖”关键字添加到 EqualityComparer 方法中。（我认为没有将“覆盖”作为隐式是 C# 设计者的一个错误）。

This code works fine:

static void TestLinqExcept()
{
    var seqA = Enumerable.Range(1, 10);
    var seqB = Enumerable.Range(1, 7);
    var seqAexceptB = seqA.Except(seqB, new IntComparer());
    foreach (var x in seqAexceptB)
    {
        Console.WriteLine(x);
    }
}

class IntComparer: EqualityComparer<int>
{
    public override bool Equals(int x, int y)
    {
        return x == y;
    }

    public override int GetHashCode(int x)
    {
        return x;
    }
}

You need to add 'override' keywords to your EqualityComparer methods. (I think not having 'override' as implicit was a mistake on the part of the C# designers).

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