blocks|key|664401|text|如果您需要消除列表之间的重复项，我建议您使用带有HashSet的简单嵌套循环。它将SelectMany和Distinct操作组合到集合插入逻辑中，并且应该更快，因为HashSet的查找时间为O(1)。Internally+()实际上可能使用类似的东西，但这完全省略了单个列表的构造。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|664402|var+set+=+new+HashSet<T>();
foreach+(var+list+in+listOfLists)
{
++++foreach+(var+item+in+list)
++++{
++++++++set.Add(item);
++++}
}
var+result+=+set.Count;|code-block|syntax|javascript|664403|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|$]]

I would recommend a simple, nested loop with a HashSet if you need to eliminate duplicates between lists. It combines the SelectMany and Distinct operations into the set insertion logic and should be faster since the HashSet has O(1) lookup time. Internally Distinct() may actually use something similar, but this omits the construction of the single list entirely.

<pre><code>var set = new HashSet&lt;T&gt;();
foreach (var list in listOfLists)
{
 foreach (var item in list)
 {
 set.Add(item);
 }
}
var result = set.Count;
</code></pre>

blocks|key|881552|text|通过使用LINQ，我认为你的代码很好，只需要做一点改动，不需要.ToList()，只需像下面这样调用Count()扩展即可。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|881553|int+result+=+listOfLists.SelectMany(list+=>+list).Distinct().Count();|code-block|syntax|javascript|881554|entityMap^0|V|9|1E|7|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|L|$]]

By using LINQ, I think your code is good with a bit changes that no need for <code>.ToList()</code>, just call <code>Count()</code> extension as the following:

<pre><code>int result = listOfLists.SelectMany(list =&gt; list).Distinct().Count();
</code></pre>

blocks|key|664337|text|要对列表中所有列表中的所有元素进行计数，可以使用聚合运算符：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|664338|int+count+=+listOfLists.Sum(l+=>+l.Distinct().Count());|code-block|syntax|javascript|664339|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|$]]

To count all the elements in all the lists in the list, you could use the aggregating operators:

<pre><code>int count = listOfLists.Sum(l =&gt; l.Distinct().Count());
</code></pre>

blocks|key|664568|text|我想有机会回答这个问题，只是为了强调我们什么时候应该使用linq，什么时候是经典的。不幸的是，今天的人们不太关心性能，因为我们习惯于在非常强大的计算机上工作。无论如何，只要尝试下面的代码，你就会发现Linq比经典的for版本慢100多倍。只有当需要编写的表达式非常复杂，并且希望使其更具可读性时，才应该使用Linq。我没有花时间在下面的解决方案上，因为我想把重点放在性能上|type|unstyled|depth|inlineStyleRanges|entityRanges|data|664569|public+static+void+Main(string+[]+arg)
{
++++//create+the+list
++++List<List<string>>+listOfList+=+new+List<List<string>>()
++++++++++++++++++++++++++++++++++++++{
++++++++++++++++++++++++++++++++++++++++++new+List<string>()
++++++++++++++++++++++++++++++++++++++++++++++{
++++++++++++++++++++++++++++++++++++++++++++++++++"1.1","2.2"
++++++++++++++++++++++++++++++++++++++++++++++}
++++++++++++++++++++++++++++++++++++++,
+++++++++++++++++++++++++++++++++++++++new+List<string>()
++++++++++++++++++++++++++++++++++++++++++++++{
++++++++++++++++++++++++++++++++++++++++++++++++++"2.1","2.2","2.3"
++++++++++++++++++++++++++++++++++++++++++++++}
++++++++++++++++++++++++++++++++++++++};
++++//stopwatch+using+Linq
++++Stopwatch+stopwatch=new+Stopwatch();
++++stopwatch.Start();

++++int+totalUsingLinq+=+listOfList.Sum(x+=>+x.Count);

++++stopwatch.Stop();
++++Console.WriteLine("Using+Linq:{0}",stopwatch.Elapsed);+//00005713

++++int+totalUsingFor+=+0;
++++//stopwatch+using+classic+for+
++++stopwatch.Reset();
++++stopwatch.Start();
++++totalUsingFor+=+0;
++++for(int+i=0;i<listOfList.Count;i%2B%2B)
++++{
+++++++var+mainItem+=+listOfList[i];
++++++++if(mainItem!=null)
++++++++{
++++++++++++totalUsingFor+%2B=+mainItem.Count;
++++++++}
++++}
++++stopwatch.Stop();
++++Console.WriteLine("Using+for:{0}",+stopwatch.Elapsed);+//0000010

}|code-block|syntax|javascript|664570|使用for的不同版本(仅举个例子)。在这种情况下，我创建了一个非常“瓶颈”的函数，该函数执行distinct操作，而且速度更快。|664571|+public+class+Program
++++{
++++++public+static+void+Main(string[]+arg)
++++++++{
++++++++++++//create+the+list
++++++++++++List<List<string>>+listOfList+=+new+List<List<string>>()
++++++++++++++++++++++++++++++++++++++{
++++++++++++++++++++++++++++++++++++++++++new+List<string>()
++++++++++++++++++++++++++++++++++++++++++++++{
++++++++++++++++++++++++++++++++++++++++++++++++++"1.1","2.2","1.1","1.1","2.2","1.1","1.1","2.2","1.1","1.1"
++++++++++++++++++++++++++++++++++++++++++++++}
++++++++++++++++++++++++++++++++++++++,
+++++++++++++++++++++++++++++++++++++++new+List<string>()
++++++++++++++++++++++++++++++++++++++++++++++{
++++++++++++++++++++++++++++++++++++++++++++++++++"2.1","2.2","2.3","2.3","1.1","2.2","1.1","1.1","2.2","1.1","1.1","2.2","1.1","1.1","2.2","1.1","1.1","2.2","1.1"
++++++++++++++++++++++++++++++++++++++++++++++}
++++++++++++++++++++++++++++++++++++++};
++++++++++++//stopwatch+using+Linq
++++++++++++Stopwatch+stopwatch+=+new+Stopwatch();
++++++++++++stopwatch.Start();

++++++++++++int+totalUsingLinq+=+listOfList.Sum(l+=>+l.Distinct().Count());


++++++++++++stopwatch.Stop();
++++++++++++Console.WriteLine("Using+Linq:{0}",+stopwatch.Elapsed);+//000012150++++
++++++++++++int+totalUsingFor+=+0;
++++++++++++//stopwatch+using+classic+for+
++++++++++++stopwatch.Reset();
++++++++++++stopwatch.Start();
++++++++++++totalUsingFor+=+0;
++++++++++++for+(int+i+=+0;+i+<+listOfList.Count;+i%2B%2B)
++++++++++++{
++++++++++++++++var+mainItem+=+listOfList[i];
++++++++++++++++if+(mainItem+!=+null)
++++++++++++++++{
++++++++++++++++++++for(int+y=0;y<mainItem.Count;y%2B%2B)
++++++++++++++++++++{
++++++++++++++++++++++if(mainItem[y]!=null)
++++++++++++++++++++++{
++++++++++++++++++++++++++totalUsingFor%2B%2B;
++++++++++++++++++++++++++NullDuplicateItems(y,+ref+mainItem);
++++++++++++++++++++++}+++
++++++++++++++++++++}
++++++++++++++++}
++++++++++++}
++++++++++++stopwatch.Stop();
++++++++++++Console.WriteLine("Using+for:{0}",+stopwatch.Elapsed);+//0009440
++++++++}

++++++++public+static+void+NullDuplicateItems(int+index,ref+List<string+>+list)
++++++++{
++++++++++++var+item+=+list[index];
++++++++++++for(int+i=index%2B1;i<list.Count;i%2B%2B)
++++++++++++{
++++++++++++++++if(list[i]==item)
++++++++++++++++{
++++++++++++++++++++list[i]+=+null;
++++++++++++++++}
++++++++++++}
++++++++}

++++}|664572|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

I'd like to get the chance to answer this question just to highlight when we should use linq and when a classic for.
Unfortunately today people doesn’t care a lot about performance as we got use to work on very powerful computer. Anyway just try the code below and you will discover that Linq is more then 100 times slower than the classic for version. You should use Linq only when the expression you need to write is really complex and you want make it more readable.
I didn't spend time to the solution shoed below as I'd like to focus on the performance

<pre><code>public static void Main(string [] arg)
{
 //create the list
 List&lt;List&lt;string&gt;&gt; listOfList = new List&lt;List&lt;string&gt;&gt;()
 {
 new List&lt;string&gt;()
 {
 "1.1","2.2"
 }
 ,
 new List&lt;string&gt;()
 {
 "2.1","2.2","2.3"
 }
 };
 //stopwatch using Linq
 Stopwatch stopwatch=new Stopwatch();
 stopwatch.Start();

 int totalUsingLinq = listOfList.Sum(x =&gt; x.Count);

 stopwatch.Stop();
 Console.WriteLine("Using Linq:{0}",stopwatch.Elapsed); //00005713

 int totalUsingFor = 0;
 //stopwatch using classic for 
 stopwatch.Reset();
 stopwatch.Start();
 totalUsingFor = 0;
 for(int i=0;i&lt;listOfList.Count;i++)
 {
 var mainItem = listOfList[i];
 if(mainItem!=null)
 {
 totalUsingFor += mainItem.Count;
 }
 }
 stopwatch.Stop();
 Console.WriteLine("Using for:{0}", stopwatch.Elapsed); //0000010

}
</code></pre>

distinct version using for (just for example).
In this case I have create a very "bottleneck" function that does the distinct and it is still faster. 

<pre><code> public class Program
 {
 public static void Main(string[] arg)
 {
 //create the list
 List&lt;List&lt;string&gt;&gt; listOfList = new List&lt;List&lt;string&gt;&gt;()
 {
 new List&lt;string&gt;()
 {
 "1.1","2.2","1.1","1.1","2.2","1.1","1.1","2.2","1.1","1.1"
 }
 ,
 new List&lt;string&gt;()
 {
 "2.1","2.2","2.3","2.3","1.1","2.2","1.1","1.1","2.2","1.1","1.1","2.2","1.1","1.1","2.2","1.1","1.1","2.2","1.1"
 }
 };
 //stopwatch using Linq
 Stopwatch stopwatch = new Stopwatch();
 stopwatch.Start();

 int totalUsingLinq = listOfList.Sum(l =&gt; l.Distinct().Count());


 stopwatch.Stop();
 Console.WriteLine("Using Linq:{0}", stopwatch.Elapsed); //000012150 
 int totalUsingFor = 0;
 //stopwatch using classic for 
 stopwatch.Reset();
 stopwatch.Start();
 totalUsingFor = 0;
 for (int i = 0; i &lt; listOfList.Count; i++)
 {
 var mainItem = listOfList[i];
 if (mainItem != null)
 {
 for(int y=0;y&lt;mainItem.Count;y++)
 {
 if(mainItem[y]!=null)
 {
 totalUsingFor++;
 NullDuplicateItems(y, ref mainItem);
 } 
 }
 }
 }
 stopwatch.Stop();
 Console.WriteLine("Using for:{0}", stopwatch.Elapsed); //0009440
 }

 public static void NullDuplicateItems(int index,ref List&lt;string &gt; list)
 {
 var item = list[index];
 for(int i=index+1;i&lt;list.Count;i++)
 {
 if(list[i]==item)
 {
 list[i] = null;
 }
 }
 }

 }
</code></pre>

I have a <code>List&lt;List&lt;T&gt;&gt;</code>.
How can I count all the elements in this as if it was a single <code>List&lt;T&gt;</code> in the fastest way?

So far I have used 

<pre><code>List&lt;int&gt; result = listOfLists
 .SelectMany(list =&gt; list)
 .Distinct()
 .ToList().Count;
</code></pre>

but this actually creates a list and then counts the element which is not a very good idea.

Count number of element in List<List<T>>

我有一台List<List<T>>。我如何才能以最快的方式计算出其中的所有元素，就好像它是一个List<T>一样？到目前为止，我已经使用了List<int> result = listOfLists .SelectMany(list => list) .Distinct() .ToList().Count;但这实际上创建了一个列表，然后计算元素，这不是一个很好的想法。

问计算List<List<T>>中的元素个数
EN

回答 4

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