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。在使用NewtonSoft.Deserialize将其解析为自定义类型(具有公共字符串属性的简单类)之后，我想使用LINQ操作该数据-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，则分组工作正常

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

因为我想按更多列分组，所以我返回一个自定义类型

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

组不工作。我认为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>，它将解决它。

但是，它仍然没有相应地分组，并且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;
}

你知道我错过了什么吗?

下面是上述场景的一个具体例子:

//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-所以如果这个类型实现了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>用于比较一个对象的实例与另一个相同类型的对象——而IEqualityProvider<T>是由一个外部类实现的，用于比较两个任意的T(和/或有多个方法来确定"相等")。

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

为什么需要重写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。任何时候重写Equals，也应该重写GetHashCode，以确保对象可以正确地存储在基于散列的集合中，如Dictionary或HashSet。

公平只以类型安全的方式比较是什么意思?

当使用IEquatable<T>时，您比较的对象保证是T(或T的子类型)，而使用Object.Equals时，您不知道另一个对象的类型，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;
}

和

// 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
}

Answer 2

你知道我错过了什么吗?

类似:

public class MyType : IEquatable<MyType>
{
  public string a;
  public string b;
  public string c;
  public bool Equals(MyType other)
  {
    if (other == null)
      return false;
    if (GetType() != other.GetType()) // can be omitted if you mark the CLASS as sealed
      return false;
    return a == other.a && b == other.b && c == other.c;
  }
  public override bool Equals(object obj)
  {
    return Equals(obj as MyType);
  }
  public override int GetHashCode()
  {
    int hash = 0;
    if (a != null)
      hash ^= a.GetHashCode();
    if (b != null)
      hash ^= b.GetHashCode();
    if (c != null)
      hash ^= c.GetHashCode();
    return hash;
  }
}

补充:注意上面的MyType是可变的，如果a, b和c中的一个字段被重新分配，哈希码就会改变。如果实例在Dictionary<MyType, whatever>, HashSet<MyType>等中保存时发生重新赋值，则会出现问题。

---

或者，您可以按照DavidG的回答中建议的匿名类型"group by"，或者"group by"Tuple.Create(.. , .. , ..)。

Answer 3

使用匿名类型进行分组。单个值本身似乎是字符串(即已经内置比较器的简单类型)，因此除了返回之外，您不需要自定义类型。这样就不用担心IEqualityComparer了。

from x in myList
group x by new { x.myStr, x.otherStr, x.AnotherStr } into grp
select new MyTpe 
{
    a = grp.Key.myStr, 
    b = grp.Key.otherStr, 
    c = grp.Key.AnotherStr
};