blocks|key|1632588|text|当然，这很烦人。这也是我的"MoreLINQ“项目的一部分，在某种程度上我必须加以注意:)在对投影进行操作时，还有许多其他操作是有意义的，但将原始的-+MaxBy和MinBy+spring返回到脑海中。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1632589|正如你所说，它很容易写-尽管我更喜欢名字"DistinctBy“来匹配OrderBy等。如果你感兴趣，这里是我的实现：|1632590|++++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;
++++++++++++}
++++++++}
++++}|code-block|syntax|javascript|1632591|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

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.

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:

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

 public static IEnumerable&lt;TSource&gt; DistinctBy&lt;TSource, TKey&gt;
 (this IEnumerable&lt;TSource&gt; source,
 Func&lt;TSource, TKey&gt; keySelector,
 IEqualityComparer&lt;TKey&gt; 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&lt;TSource&gt; DistinctByImpl&lt;TSource, TKey&gt;
 (IEnumerable&lt;TSource&gt; source,
 Func&lt;TSource, TKey&gt; keySelector,
 IEqualityComparer&lt;TKey&gt; comparer)
 {
 HashSet&lt;TKey&gt; knownKeys = new HashSet&lt;TKey&gt;(comparer);
 foreach (TSource element in source)
 {
 if (knownKeys.Add(keySelector(element)))
 {
 yield return element;
 }
 }
 }
</code></pre>

blocks|key|1417933|text|乔恩，你的解决方案很不错。不过，有一个小小的变化。我不认为我们需要EqualityComparer.Default在那里。这是我的解决方案(当然，起点是Jon+Skeet的解决方案)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1417934|++++public+static+IEnumerable<T>+DistinctBy<T,+TKey>(this+IEnumerable<T>+source,+Func<T,+TKey>+keySelector)
++++{
++++++++//TODO+All+arg+checks
++++++++HashSet<TKey>+keys+=+new+HashSet<TKey>();
++++++++foreach+(T+item+in+source)
++++++++{
++++++++++++TKey+key+=+keySelector(item);
++++++++++++if+(!keys.Contains(key))
++++++++++++{
++++++++++++++++keys.Add(key);
++++++++++++++++yield+return+item;
++++++++++++}
++++++++}
++++}|code-block|syntax|javascript|1417935|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Jon, your solution is pretty good. One minor change though. I don't think we need EqualityComparer.Default in there. Here is my solution (ofcourse the starting point was Jon Skeet's solution)

<pre><code> public static IEnumerable&lt;T&gt; DistinctBy&lt;T, TKey&gt;(this IEnumerable&lt;T&gt; source, Func&lt;T, TKey&gt; keySelector)
 {
 //TODO All arg checks
 HashSet&lt;TKey&gt; keys = new HashSet&lt;TKey&gt;();
 foreach (T item in source)
 {
 TKey key = keySelector(item);
 if (!keys.Contains(key))
 {
 keys.Add(key);
 yield return item;
 }
 }
 }
</code></pre>

blocks|key|1417988|text|使用AmyB的answer，我编写了一个小的DistinctBy扩展方法，以允许传递谓词：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1417989|///+<summary>
///+Distinct+method+that+accepts+a+perdicate
///+</summary>
///+<typeparam+name="TSource">The+type+of+the+t+source.</typeparam>
///+<typeparam+name="TKey">The+type+of+the+t+key.</typeparam>
///+<param+name="source">The+source.</param>
///+<param+name="predicate">The+predicate.</param>
///+<returns>IEnumerable&lt;TSource&gt;.</returns>
///+<exception+cref="System.ArgumentNullException">source</exception>
public+static+IEnumerable<TSource>+DistinctBy<TSource,+TKey>
++++(this+IEnumerable<TSource>+source,
+++++Func<TSource,+TKey>+predicate)
{
++++if+(source+==+null)
++++++++throw+new+ArgumentNullException("source");

++++return+source
++++++++.GroupBy(predicate)
++++++++.Select(x+=>+x.First());
}|code-block|syntax|javascript|1417990|现在，您可以传递一个谓词来对列表进行分组：|1417991|var+distinct+=+myList.DistinctBy(x+=>+x.Id);|1417992|或按多个属性分组：|1417993|var+distinct+=+myList.DistinctBy(x+=>+new+{+x.Id,+x.Title+});|1417994|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/520315/1835379^0|M|A|7|6|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@$9|11|A|12|B|C]]|D|@$9|13|A|14|1|15]]|E|$]]|$1|F|3|G|5|H|7|16|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|17|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|18|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|19|8|@]|D|@]|E|$]]|$1|Q|3|R|5|H|7|1A|8|@]|D|@]|E|$I|J]]|$1|S|3|-4|5|6|7|1B|8|@]|D|@]|E|$]]]|T|$U|$5|V|W|X|E|$Y|Z]]]]

Using AmyB's <a href="https://stackoverflow.com/a/520315/1835379">answer</a>, I've written a small <code>DistinctBy</code> extension method, to allow a predicate to be passed:

<pre><code>/// &lt;summary&gt;
/// Distinct method that accepts a perdicate
/// &lt;/summary&gt;
/// &lt;typeparam name="TSource"&gt;The type of the t source.&lt;/typeparam&gt;
/// &lt;typeparam name="TKey"&gt;The type of the t key.&lt;/typeparam&gt;
/// &lt;param name="source"&gt;The source.&lt;/param&gt;
/// &lt;param name="predicate"&gt;The predicate.&lt;/param&gt;
/// &lt;returns&gt;IEnumerable&amp;lt;TSource&amp;gt;.&lt;/returns&gt;
/// &lt;exception cref="System.ArgumentNullException"&gt;source&lt;/exception&gt;
public static IEnumerable&lt;TSource&gt; DistinctBy&lt;TSource, TKey&gt;
 (this IEnumerable&lt;TSource&gt; source,
 Func&lt;TSource, TKey&gt; predicate)
{
 if (source == null)
 throw new ArgumentNullException("source");

 return source
 .GroupBy(predicate)
 .Select(x =&gt; x.First());
}
</code></pre>

You can now pass a predicate to group the list by:

<pre><code>var distinct = myList.DistinctBy(x =&gt; x.Id);
</code></pre>

Or group by multiple properties:

<pre><code>var distinct = myList.DistinctBy(x =&gt; new { x.Id, x.Title });
</code></pre>

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

What I want to do is something like this:

<pre><code>return myList.Distinct( (x, y) =&gt; x.Url == y.Url );
</code></pre>

I can't, there's no extension method in Linq that will do this - just one that takes an <code>IEqualityComparer</code>.

I can hack around it with this:

<pre><code>return myList.GroupBy( x =&gt; x.Url ).Select( g =&gt; g.First() );
</code></pre>

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.

I could also add my own:

<pre><code>public static IEnumerable&lt;T&gt; Distinct&lt;T&gt;( 
 this IEnumerable&lt;T&gt; input, Func&lt;T,T,bool&gt; compare )
{
 //write my own here
}
</code></pre>

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

Anyone know why this method isn't there?

Am I missing something?

Why is there no Linq method to return distinct values by a predicate?

我想要在列表中获得不同的值，而不是通过标准的相等比较。我想做的是这样的：return myList.Distinct( (x, y) => x.Url == y.Url );我不能，Linq中没有可以做到这一点的扩展方法--只有一个接受IEqualityComparer的方法。我可以用这个来解决这个问题：return ...

问为什么没有Linq方法通过谓词返回不同的值？
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