c# - 如何为lambda表达式中的匿名变量编写IEqualityComparer？

Question

我想知道有一种方法可以在 lambda 表达式中为匿名变量实现 IEqualityComparer，或者在任何情况下，我都需要编写类来将匿名变量转换为特定类，并创建一个我需要实现 IEqualtyComparer 的类？

我编写了创建笛卡尔(Decart 的)产生式的代码:我定义了 Decart 类。

public class Decart
{
    public int X;
    public int Y;
}

我为 Decart 类定义了 IEqualtityComparer

public class Ext : IEqualityComparer<Decart>
{

    public bool Equals(Decart x, Decart y)
    {
        if ((x.X == y.X && x.Y == y.Y) ||
            x.X == y.Y && x.Y == y.X)
            return true;

        return false;
    }

    public int GetHashCode(Decart obj)
    {
        return obj.X + obj.Y;
    }
}

我运行这段代码:

static void Main(string[] args)
{
    Ext ext = new Ext();
    IEnumerable<int> input = Enumerable.Range(1, 3);
        var secondResult = input
        .SelectMany(x => input.Select(y => new Decart{ X = x, Y = y }))
        .Distinct(new Ext());
    Console.WriteLine(new string('-', 50));

    foreach (var x in secondResult)
    {
        Console.WriteLine(string.Format("{0} {1}", x.X, x.Y));
    }
    //output:
    //1 1
    //1 2
    //1 3
    //2 2
    //2 3
    //3 3
}

我想在不为匿名变量定义一个类的情况下运行下一个代码，一个用于实现 IEqualityComparer 的类。

    var thirdResult = input
        .SelectMany(x => input
            .SelectMany(y => input
                .Select(z => new { x, y, z })))
                    .Distinct( ???? );

//in this case output need to be like this:
//1 1 1
//1 2 1
//1 3 1
//2 1 2
//2 2 2
//2 3 2
//3 1 3
//3 2 3
//3 3 3

怎么做？

Answer 1

可以声明IEqualityComparer<T>将委托(delegate)作为 GetHashCode 的实现的实现和 Equals接口(interface)的方法。然后你可以传入匿名方法作为实现。

public static IEqualityComparer<T> CreateEqualityComparer<T>(T ignore, Func<T, int> getHashCode, Func<T, T, bool> equals) => new DelegatedEqualityComparer<T>(getHashCode, equals);

public class DelegatedEqualityComparer<T> : EqualityComparer<T> {
    private Func<T, int> getHashCode;
    private Func<T, T, bool> equals;
    public DelegatedEqualityComparer(Func<T, int> getHashCode, Func<T, T, bool> equals) {
        if(getHashCode==null) throw new ArgumentNullException(nameof(getHashCode));
        if(equals==null) throw new ArgumentNullException(nameof(equals));
        this.getHashCode=getHashCode;
        this.equals=equals;
    }
    public override int GetHashCode(T x) => getHashCode(x);
    public override bool Equals(T x, T y) => equals(x, y);
}

然后你像这样使用它:

var equalityComparer = CreateEqualityComparer(true ? null : new { x = 0, y = 0 }, a => a.x+a.y, (a, b) => (a.x==b.x&&a.y==b.y)||(a.x==b.y&&a.y==b.x));
var result = input
    .SelectMany(x => input
        .Select(y => new { x, y }))
    .Distinct(equalityComparer);

true ? null : new { x = 0, y = 0 }的含义:

需要 CreateEqualityComparer ( T ignore ) 的第一个参数以允许编译器推断类型 T ，因为您无法拼写匿名类型的名称。 true三元运算符的条件使编译器始终选择左分支 null , 但由于三元运算符的两个分支必须返回相同的类型，则 new { x = 0, y = 0 }让编译器隐式转换 null给定的匿名类型。

此外，规范中的相关注释:

7.6.10.6 Anonymous object creation expressions
Within the same program, two anonymous object initializers that specify a sequence of properties of the same names and compile-time types in the same order will produce instances of the same anonymous type.