使用委托的相等比较器对LINQ的鲜明的（） [英] Use a delegate for the equality comparer for LINQ's Distinct()

问题描述

我有一个使用我自己的自定义比较，这样的LINQ鲜明（）语​​句：

I have a LINQ Distinct() statement that uses my own custom comparer, like this:

class MyComparer<T> : IEqualityComparer<T> where T : MyType
{
    public bool Equals(T x, T y)
    {
        return x.Id.Equals(y.Id);
    }

    public int GetHashCode(T obj)
    {
        return obj.Id.GetHashCode();
    }
}

...

var distincts = bundle.GetAllThings.Distinct(new MyComparer<MySubType>());

这是所有罚款和花花公子和作品，因为我想要的。出于好奇，我需要确定自己的Comparer，或者我可以与代理人代替它呢？我想我应该可以做这样的事情：

This is all fine and dandy and works as I want. Out of curiosity, do I need to define my own Comparer, or can I replace it with a delegate? I thought I should be able to do something like this:

var distincts = bundle.GetAllThings.Distinct((a,b) => a.Id == b.Id);

但是，这并不编译。是否有一个巧妙的方法？

But this doesn't compile. Is there a neat trick?

推荐答案

分明需要一个的IEqualityComparer作为第二个参数，所以你需要的IEqualityComparer。这不是太难做一个通用的一个，将采取委托，虽然。当然，这有可能已经被一些地方，如MoreLINQ实施了其他的答案中的一个建议。

Distinct takes an IEqualityComparer as the second argument, so you will need an IEqualityComparer. It's not too hard to make a generic one that will take a delegate, though. Of course, this has probably already been implemented in some places, such as MoreLINQ suggested in one of the other answers.

您可以实现它是这样的：

You could implement it something like this:

public static class Compare
{
    public static IEnumerable<T> DistinctBy<T, TIdentity>(this IEnumerable<T> source, Func<T, TIdentity> identitySelector)
    {
        return source.Distinct(Compare.By(identitySelector));
    }

    public static IEqualityComparer<TSource> By<TSource, TIdentity>(Func<TSource, TIdentity> identitySelector)
    {
        return new DelegateComparer<TSource, TIdentity>(identitySelector);
    }

    private class DelegateComparer<T, TIdentity> : IEqualityComparer<T>
    {
        private readonly Func<T, TIdentity> identitySelector;

        public DelegateComparer(Func<T, TIdentity> identitySelector)
        {
            this.identitySelector = identitySelector;
        }

        public bool Equals(T x, T y)
        {
            return Equals(identitySelector(x), identitySelector(y));
        }

        public int GetHashCode(T obj)
        {
            return identitySelector(obj).GetHashCode();
        }
    }
}

它给你的语法：

source.DistinctBy(a => a.Id);

或者，如果你觉得它更清晰是这样的：

Or, if you feel it's clearer this way:

source.Distinct(Compare.By(a => a.Id));

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