c# - LINQ 按自定义类型分组不起作用

Question

我有通过 HTTPWebRequest 从 Web 服务接收的数据.在我使用 NewtonSoft.Deserialize 解析它之后到自定义类型(具有公共(public)字符串属性的简单类)中，我想使用 LINQ 操作此数据- 更具体地说，我想对数据进行分组。

我的问题是，如果我按单个字符串属性分组，分组工作正常

from x in myList
group x by x.myStr into grp
select grp;

因为我想按更多列分组，所以我返回了一个自定义类型

new MyType { a = ..., b = ... }

但是该组不工作。我想原因一定是编译器不知道如何比较这些对象——所以如果这个类型实现了 IEqualityComparer<MyType>它会解决它。

但是不，它仍然没有相应地分组，它创建了几个具有完全相同字符串值的键。

我分组的自定义类型类似于

public class MyType
{
    public string a;
    public string b;
    public string c;
}

关于我缺少什么的任何想法？

下面是上述场景的具体示例:

//The type that models the data returned from the web service
public class MyClass
{
    public string a { get; set; }

    public string b { get; set; }

    public string c { get; set; }

    public DateTime d { get; set; }

    public DateTime e { get; set; }
}

// the type by which I want to group my data
public class MyGroup : IEquatable<MyGroup>, IEqualityComparer<MyGroup>
{
    public string f1 { get; set; }

    public DateTime d1 { get; set; }

    public DateTime d2 { get; set; }

    public bool Equals(MyGroup other)
    {
        return string.Compare(this.f1, other.f1) == 0;
    }

    public bool Equals(MyGroup x, MyGroup y)
    {
        return string.Compare(x.f1, y.f1) == 0;
    }

    public int GetHashCode(MyGroup obj)
    {
        return obj.GetHashCode();
    }
}    
    List<MyClass> l = new List<MyClass>();
    l.Add(new MyClass { a = "aaa", b = "bbb", c = "ccc", d = DateTime.ParseExact("20081405", "yyyyddMM", Thread.CurrentThread.CurrentCulture), e = DateTime.ParseExact("20140101", "yyyyddMM", Thread.CurrentThread.CurrentCulture) });
    l.Add(new MyClass { a = "aaaa", b = "bbb", c = "ccc", d = DateTime.ParseExact("20090105", "yyyyddMM", Thread.CurrentThread.CurrentCulture), e = DateTime.ParseExact("20140201", "yyyyddMM", Thread.CurrentThread.CurrentCulture) });
    l.Add(new MyClass { a = "aa", b = "bbbb", c = "cccc", d = DateTime.ParseExact("20081405", "yyyyddMM", Thread.CurrentThread.CurrentCulture), e = DateTime.ParseExact("20140201", "yyyyddMM", Thread.CurrentThread.CurrentCulture) });
    l.Add(new MyClass { a = "aaa", b = "bbbbb", c = "ccc", d = DateTime.ParseExact("20121111", "yyyyddMM", Thread.CurrentThread.CurrentCulture), e = DateTime.ParseExact("20140101", "yyyyddMM", Thread.CurrentThread.CurrentCulture) });
    l.Add(new MyClass { a = "aaaaa", b = "bbb", c = "ccc", d = DateTime.ParseExact("20081405", "yyyyddMM", Thread.CurrentThread.CurrentCulture), e = DateTime.ParseExact("20140101", "yyyyddMM", Thread.CurrentThread.CurrentCulture) });
    l.Add(new MyClass { a = "aaaa", b = "bbbbb", c = "ccc", d = DateTime.ParseExact("20121111", "yyyyddMM", Thread.CurrentThread.CurrentCulture), e = DateTime.ParseExact("20140101", "yyyyddMM", Thread.CurrentThread.CurrentCulture) });
    l.Add(new MyClass { a = "aaaa", b = "bbbb", c = "cccccc", d = DateTime.ParseExact("20081405", "yyyyddMM", Thread.CurrentThread.CurrentCulture), e = DateTime.ParseExact("20140201", "yyyyddMM", Thread.CurrentThread.CurrentCulture) });
    l.Add(new MyClass { a = "aaaaa", b = "bbb", c = "cccc", d = DateTime.ParseExact("20090105", "yyyyddMM", Thread.CurrentThread.CurrentCulture), e = DateTime.ParseExact("20140301", "yyyyddMM", Thread.CurrentThread.CurrentCulture) });
    l.Add(new MyClass { a = "aaa", b = "bbb", c = "cccc", d = DateTime.ParseExact("20081405", "yyyyddMM", Thread.CurrentThread.CurrentCulture), e = DateTime.ParseExact("20140201", "yyyyddMM", Thread.CurrentThread.CurrentCulture) });

    //The following does not really group
    //IEnumerable<IGrouping<MyGroup, MyClass>> r = from x in l
    IEnumerable<IGrouping<string, MyClass>> r = from x in l
                                                //group x by new MyGroup { f1 = x.a /*, d1 = x.d, d2 = x.e*/ } into grp
                                                orderby x.a
                                                group x by x.a into grp
                                                select grp;

    //foreach (IGrouping<MyGroup, MyClass> g in r)
    foreach (IGrouping<string, MyClass> g in r)
    {
        //Console.WriteLine(g.Key.f1);

        Console.WriteLine(g.Key);
    }

Answer 1

I thought the reason must be the compiler does not know how to compare these objects - so if this type implements IEqualityComparer<MyType> it will solve it.

实际上，要在 Linq 函数中使用自定义“相等”检查，您需要实现 IEquatable<T> . IEquatable<T>用于将对象的实例与同一类型的另一个对象进行比较 - 而 IEqualityProvider<T>旨在由外部类实现以比较任意两个 T s(和/或有多种方法来确定“平等”)。

请注意，您还应该实现 Object.Equals和 Object.GetHashCode - IEquatable<T>只允许您以类型安全的方式进行比较。

Why the need for overriding Object's Equals and GetHashCode?

确保用于比较两个对象的任何方法(Object.Equals(object)、静态Object.Equals(object, object等)是一致的。任何时候你覆盖 Equals ，你还应该覆盖 GetHashCode确保对象可以正确存储在基于散列的集合中，例如 Dictionary或 HashSet .

What does it mean IEquitable only compares in a type-safe manner?

使用 IEquatable<T> 时，您要比较的对象保证是T (或 T 的子类型)，而 Object.Equals ，您不知道其他对象的类型，必须先检查它的类型。

例如:

// IEquatable<T>.Equals()
public bool Equals(MyGroup other)
{
    return string.Compare(this.f1, other.f1) == 0;
}

对比

// Object.Equals()
public bool Equals(object other)
{
    // need to check the type of the passed in object
    MyGroup grp = other as MyGroup;

    // other is not a MyGroup
    if(grp == null return false);        

    return string.Compare(this.f1, grp.f1) == 0;

    // you could also use
    //    return this.Equals(grp);
    // as a shortcut to reuse the same "equality" logic
}