为什么没有LINQ的方法由predicate返回不同​​的值？ [英] Why is there no Linq method to return distinct values by a predicate?

问题描述

我想在列表中的不同的值，而不是由标准相等比较。

I want to get the distinct values in a list, but not by the standard equality comparison.

我想要做的是这样的：

return myList.Distinct( (x, y) => x.Url == y.Url );

我不能，有一个在LINQ的没有扩展方法，将做到这一点 - 只有一个接受一个的IEqualityComparer

我可以围绕它破解这个：

I can hack around it with this:

return myList.GroupBy( x => x.Url ).Select( g => g.First() );

但似乎混乱。它也并不完全做同样的事情 - 因为我有一个关键的我只能在这里使用它

But that seems messy. It also doesn't quite do the same thing - I can only use it here because I have a single key.

我也可以添加自己的：

public static IEnumerable<T> Distinct<T>( 
    this IEnumerable<T> input, Func<T,T,bool> compare )
{
    //write my own here
}

不过，这似乎相当喜欢写东西，应该在那里摆在首位。

But that does seem rather like writing something that should be there in the first place.

谁知道为什么这种方法是不是有？

Anyone know why this method isn't there?

我缺少的东西吗？

推荐答案

这很烦人，肯定。这也是我的MoreLINQ项目的一部分，我必须在某种程度上给予一些关注，:)还有许多其他的操作作用在投影的时候，却在返回原来这是有意义的 - MaxBy和MinBy映入脑海

It's annoying, certainly. It's also part of my "MoreLINQ" project which I must pay some attention to at some point :) There are plenty of other operations which make sense when acting on a projection, but returning the original - MaxBy and MinBy spring to mind.

正如你所说的，它很容易写的 - 尽管我preFER名DistinctBy来匹配排序依据等等。这里是我的实现，如果你有兴趣：

As you say, it's easy to write - although I prefer the name "DistinctBy" to match OrderBy etc. Here's my implementation if you're interested:

    public static IEnumerable<TSource> DistinctBy<TSource, TKey>
        (this IEnumerable<TSource> source,
         Func<TSource, TKey> keySelector)
    {
        return source.DistinctBy(keySelector,
                                 EqualityComparer<TKey>.Default);
    }

    public static IEnumerable<TSource> DistinctBy<TSource, TKey>
        (this IEnumerable<TSource> source,
         Func<TSource, TKey> keySelector,
         IEqualityComparer<TKey> comparer)
    {
        if (source == null)
        {
            throw new ArgumentNullException("source");
        }
        if (keySelector == null)
        {
            throw new ArgumentNullException("keySelector");
        }
        if (comparer == null)
        {
            throw new ArgumentNullException("comparer");
        }
        return DistinctByImpl(source, keySelector, comparer);
    }

    private static IEnumerable<TSource> DistinctByImpl<TSource, TKey>
        (IEnumerable<TSource> source,
         Func<TSource, TKey> keySelector,
         IEqualityComparer<TKey> comparer)
    {
        HashSet<TKey> knownKeys = new HashSet<TKey>(comparer);
        foreach (TSource element in source)
        {
            if (knownKeys.Add(keySelector(element)))
            {
                yield return element;
            }
        }
    }

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