LINQ按自定义类型分组不起作用

Question

我有通过网络服务收到的数据HTTPWebRequest.在我使用NewtonSoft.Deserialize自定义类型(具有公共字符串属性的简单类)解析它之后,我想使用LINQ- 更具体地说,我想对数据进行分组来操纵这些数据.

我的问题是,如果我按单个字符串属性进行分组,则分组工作正常

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

由于我想按更多列分组,我将返回一个自定义类型

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

然而,该小组没有工作.我认为原因必须是编译器不知道如何比较这些对象 - 所以如果这种类型实现IEqualityComparer<MyType>它将解决它.

但不,它仍然没有相应的分组,它创建了几个具有完全相同的字符串值的键.

我正在分组的自定义类型就像

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

我错过了什么想法？

这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

我认为原因一定是编译器不知道如何比较这些对象-因此，如果此类型实现IEqualityComparer<MyType>，它将解决它。

实际上，要在Linq函数中使用自定义的“相等”检查，您需要实现IEquatable<T>。 IEquatable<T>用于将一个对象的实例与相同类型的另一个对象进行比较-同时IEqualityProvider<T>旨在由外部类实现以比较两个任意Ts（和/或具有确定“相等”的多种方法）。

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

为什么需要覆盖Object的Equals和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。而且Equals，无论何时重写，都应该重写GetHashCode以确保对象可以正确存储在基于哈希的集合中，例如a Dictionary或HashSet。

IEquitable仅以类型安全的方式进行比较意味着什么？

使用时IEquatable<T>，你比较的对象保证是一个T（或亚型T），而使用Object.Equals，你不知道其他对象的类型，必须首先检查它的类型。

例如：

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

与

// 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
}