使用Default EqualityComparer进行Linq GroupBy的关键比较

Question

我正在尝试使用显式键类型对某些对象执行Linq GroupBy.我没有传递IEqualityComparer给GroupBy,所以根据文档:

默认的相等比较器Default用于比较键.

它解释了EqualityComparer<T>.Default这样的属性:

该Default属性检查type是否T实现 System.IEquatable<T>泛型接口,如果是EqualityComparer<T>,则返回 使用该实现的类型.

在下面的代码中,我正在对一组Fred对象进行分组.它们有一个名为的密钥类型FredKey,它实现了IEquatable<FredKey>.

这应该足以使分组工作,但分组不起作用.在下面的最后一行我应该有2组,但我没有,我只有3组包含3个输入项.

为什么分组不起作用？

class Fred
{
    public string A;
    public string B;
    public FredKey Key
    {
        get { return new FredKey() { A = this.A }; }
    }
}

class FredKey : IEquatable<FredKey>
{
    public string A;
    public bool Equals(FredKey other)
    {
        return A == other.A;
    }
}

class Program
{
    static void Main(string[] args)
    {
        var f = new Fred[]
        {
            new Fred {A = "hello", B = "frog"},
            new Fred {A = "jim", B = "jog"},
            new Fred {A = "hello", B = "bog"},
        };

        var groups = f.GroupBy(x => x.Key);
        Debug.Assert(groups.Count() == 2);  // <--- fails
    }
}

Answer 1

来自MSDN

如果实现IEquatable,还应该覆盖Object :: Equals(Object)和GetHashCode()的基类实现,以使它们的行为与IEquatable :: Equals方法的行为一致.如果您重写Object :: Equals(Object),则在对类的静态Equals(System.Object,System.Object)方法的调用中也会调用重写的实现.这确保了Equals()方法的所有调用都返回一致的结果.

将此添加到FredKey,它应该工作

public override int GetHashCode()
    {
        return A.GetHashCode();
    }

Answer 2

这是一个完整的小提琴示例.注意:该示例与问题的示例略有不同.

以下实现IEquatable可以作为TKeyin GroupBy.请注意,它包括GetHashCode和Equals.

public class CustomKey : IEquatable<CustomKey>
{
    public string A { get; set; }
    public string B { get; set; }

    public bool Equals(CustomKey other)
    {
        return other.A == A && other.B == B;
    }

    public override int GetHashCode()
    {
        return string.Format("{0}{1}", A, B).GetHashCode();
    }
}

public class Custom
{
    public string A { get; set; }
    public string B { get; set; }
    public string C { get; set; }
}

public static void Main()
{
    var c = new Custom[]
       {
           new Custom {A = "hello", B = "frog" },
           new Custom {A = "jim", B = "jog" },
           new Custom {A = "hello", B = "frog" },
       };

    var groups = c.GroupBy(x => new CustomKey { A = x.A, B = x.B } );
       Console.WriteLine(groups.Count() == 2);  
}