linq之Capacity（转载）

出处：博客园

作者：mumuliang

连接：http://www.cnblogs.com/mumuliang/p/3914425.html

Capacity

在.NET中List的容量应该只是受到硬件限制。

属性Capacity的真正含义，是创建List时给它预分配的容量。

一旦项的数量超过了当前的Capacity，Capacity会以

Capacity = (Capacity==0)?A_VALUE_YOU_CANNOT_SURE:(Capacity*2)

的方式增长。

当使用默认构造函数创建一个List时，Capacity就是0。（或者你非要用一个0作为capacity参数调用构造）

所以当Capacity是0时，我们为List添加了第一个项目后，Capacity是多少呢？

对，是A_VALUE_YOU_CANNOT_SURE。

不过，在.NET2.0和.NET4.5中，这个值看上去都是4。

            System.Diagnostics.Debug.WriteLine(string.Format("default capacity is : {0}", new List<string>().Capacity));int initCapacity = 0;List<string> l = new List<string>(initCapacity);System.Diagnostics.Debug.WriteLine(string.Format("constructe a list with capacity: {0}", l.Capacity));for (int u = 0; u < 65; ++u){System.Diagnostics.Debug.WriteLine(string.Format("Count: {0}, Capacity: {1}", l.Count, l.Capacity));//l.TrimExcess();//System.Diagnostics.Debug.WriteLine(string.Format("    TrimExcess() -> Count: {0}, Capacity: {1}", l.Count, l.Capacity));l.Add("1");}

输出

default capacity is : 0
constructe a list with capacity: 0
Count: 0, Capacity: 0
Count: 1, Capacity: 4
Count: 2, Capacity: 4
Count: 3, Capacity: 4
Count: 4, Capacity: 4
Count: 5, Capacity: 8
Count: 6, Capacity: 8
Count: 7, Capacity: 8
Count: 8, Capacity: 8
Count: 9, Capacity: 16
Count: 10, Capacity: 16
Count: 11, Capacity: 16
Count: 12, Capacity: 16
Count: 13, Capacity: 16
Count: 14, Capacity: 16
Count: 15, Capacity: 16
Count: 16, Capacity: 16
Count: 17, Capacity: 32
Count: 18, Capacity: 32
Count: 19, Capacity: 32
Count: 20, Capacity: 32
Count: 21, Capacity: 32
Count: 22, Capacity: 32
Count: 23, Capacity: 32
Count: 24, Capacity: 32
Count: 25, Capacity: 32
Count: 26, Capacity: 32
Count: 27, Capacity: 32
Count: 28, Capacity: 32
Count: 29, Capacity: 32
Count: 30, Capacity: 32
Count: 31, Capacity: 32
Count: 32, Capacity: 32
Count: 33, Capacity: 64
Count: 34, Capacity: 64
Count: 35, Capacity: 64
Count: 36, Capacity: 64
Count: 37, Capacity: 64
Count: 38, Capacity: 64
Count: 39, Capacity: 64
Count: 40, Capacity: 64
Count: 41, Capacity: 64
Count: 42, Capacity: 64
Count: 43, Capacity: 64
Count: 44, Capacity: 64
Count: 45, Capacity: 64
Count: 46, Capacity: 64
Count: 47, Capacity: 64
Count: 48, Capacity: 64
Count: 49, Capacity: 64
Count: 50, Capacity: 64
Count: 51, Capacity: 64
Count: 52, Capacity: 64
Count: 53, Capacity: 64
Count: 54, Capacity: 64
Count: 55, Capacity: 64
Count: 56, Capacity: 64
Count: 57, Capacity: 64
Count: 58, Capacity: 64
Count: 59, Capacity: 64
Count: 60, Capacity: 64
Count: 61, Capacity: 64
Count: 62, Capacity: 64
Count: 63, Capacity: 64
Count: 64, Capacity: 64

View Code

TrimExcess()

TrimExcess()的意思就是，如果Capacity大于Count，清掉哪些没有使用的空间。基本上可以理解成让Capacity等于Count，只是基本上，不能信赖这一点。.NET毕竟还要考虑很多性能问题。TrimExcess method does nothing if the list is at more than 90 percent of capacity——这就是.NET的方式。实际上把上面那段代码中的注释放开，就会看到确实存在Capacity和Count不等的情况（然而，并不是90%哟）。Capacity不够以后的增长规则，仍然是*2，并不会因为被Trim过就只是加1。

放开注释后的输出

default capacity is : 0
constructe a list with capacity: 0
Count: 0, Capacity: 0TrimExcess() -> Count: 0, Capacity: 0
Count: 1, Capacity: 4TrimExcess() -> Count: 1, Capacity: 1
Count: 2, Capacity: 2TrimExcess() -> Count: 2, Capacity: 2
Count: 3, Capacity: 4TrimExcess() -> Count: 3, Capacity: 4
Count: 4, Capacity: 4TrimExcess() -> Count: 4, Capacity: 4
Count: 5, Capacity: 8TrimExcess() -> Count: 5, Capacity: 5
Count: 6, Capacity: 10TrimExcess() -> Count: 6, Capacity: 6
Count: 7, Capacity: 12TrimExcess() -> Count: 7, Capacity: 7
Count: 8, Capacity: 14TrimExcess() -> Count: 8, Capacity: 8
Count: 9, Capacity: 16TrimExcess() -> Count: 9, Capacity: 9
Count: 10, Capacity: 18TrimExcess() -> Count: 10, Capacity: 10
Count: 11, Capacity: 20TrimExcess() -> Count: 11, Capacity: 11
Count: 12, Capacity: 22TrimExcess() -> Count: 12, Capacity: 12
Count: 13, Capacity: 24TrimExcess() -> Count: 13, Capacity: 13
Count: 14, Capacity: 26TrimExcess() -> Count: 14, Capacity: 14
Count: 15, Capacity: 28TrimExcess() -> Count: 15, Capacity: 15
Count: 16, Capacity: 30TrimExcess() -> Count: 16, Capacity: 16
Count: 17, Capacity: 32TrimExcess() -> Count: 17, Capacity: 17
Count: 18, Capacity: 34TrimExcess() -> Count: 18, Capacity: 18
Count: 19, Capacity: 36TrimExcess() -> Count: 19, Capacity: 19
Count: 20, Capacity: 38TrimExcess() -> Count: 20, Capacity: 20
Count: 21, Capacity: 40TrimExcess() -> Count: 21, Capacity: 21
Count: 22, Capacity: 42TrimExcess() -> Count: 22, Capacity: 22
Count: 23, Capacity: 44TrimExcess() -> Count: 23, Capacity: 23
Count: 24, Capacity: 46TrimExcess() -> Count: 24, Capacity: 24
Count: 25, Capacity: 48TrimExcess() -> Count: 25, Capacity: 25
Count: 26, Capacity: 50TrimExcess() -> Count: 26, Capacity: 26
Count: 27, Capacity: 52TrimExcess() -> Count: 27, Capacity: 27
Count: 28, Capacity: 54TrimExcess() -> Count: 28, Capacity: 28
Count: 29, Capacity: 56TrimExcess() -> Count: 29, Capacity: 29
Count: 30, Capacity: 58TrimExcess() -> Count: 30, Capacity: 30
Count: 31, Capacity: 60TrimExcess() -> Count: 31, Capacity: 31
Count: 32, Capacity: 62TrimExcess() -> Count: 32, Capacity: 32
Count: 33, Capacity: 64TrimExcess() -> Count: 33, Capacity: 33
Count: 34, Capacity: 66TrimExcess() -> Count: 34, Capacity: 34
Count: 35, Capacity: 68TrimExcess() -> Count: 35, Capacity: 35
Count: 36, Capacity: 70TrimExcess() -> Count: 36, Capacity: 36
Count: 37, Capacity: 72TrimExcess() -> Count: 37, Capacity: 37
Count: 38, Capacity: 74TrimExcess() -> Count: 38, Capacity: 38
Count: 39, Capacity: 76TrimExcess() -> Count: 39, Capacity: 39
Count: 40, Capacity: 78TrimExcess() -> Count: 40, Capacity: 40
Count: 41, Capacity: 80TrimExcess() -> Count: 41, Capacity: 41
Count: 42, Capacity: 82TrimExcess() -> Count: 42, Capacity: 42
Count: 43, Capacity: 84TrimExcess() -> Count: 43, Capacity: 43
Count: 44, Capacity: 86TrimExcess() -> Count: 44, Capacity: 44
Count: 45, Capacity: 88TrimExcess() -> Count: 45, Capacity: 45
Count: 46, Capacity: 90TrimExcess() -> Count: 46, Capacity: 46
Count: 47, Capacity: 92TrimExcess() -> Count: 47, Capacity: 47
Count: 48, Capacity: 94TrimExcess() -> Count: 48, Capacity: 48
Count: 49, Capacity: 96TrimExcess() -> Count: 49, Capacity: 49
Count: 50, Capacity: 98TrimExcess() -> Count: 50, Capacity: 50
Count: 51, Capacity: 100TrimExcess() -> Count: 51, Capacity: 51
Count: 52, Capacity: 102TrimExcess() -> Count: 52, Capacity: 52
Count: 53, Capacity: 104TrimExcess() -> Count: 53, Capacity: 53
Count: 54, Capacity: 106TrimExcess() -> Count: 54, Capacity: 54
Count: 55, Capacity: 108TrimExcess() -> Count: 55, Capacity: 55
Count: 56, Capacity: 110TrimExcess() -> Count: 56, Capacity: 56
Count: 57, Capacity: 112TrimExcess() -> Count: 57, Capacity: 57
Count: 58, Capacity: 114TrimExcess() -> Count: 58, Capacity: 58
Count: 59, Capacity: 116TrimExcess() -> Count: 59, Capacity: 59
Count: 60, Capacity: 118TrimExcess() -> Count: 60, Capacity: 60
Count: 61, Capacity: 120TrimExcess() -> Count: 61, Capacity: 61
Count: 62, Capacity: 122TrimExcess() -> Count: 62, Capacity: 62
Count: 63, Capacity: 124TrimExcess() -> Count: 63, Capacity: 63
Count: 64, Capacity: 126TrimExcess() -> Count: 64, Capacity: 64

View Code

发现并不是90%以后

System.Diagnostics.Debug.WriteLine(string.Format("default capacity is : {0}", new List<string>().Capacity));int initCapacity = 100;
List<string> l = new List<string>(initCapacity);
System.Diagnostics.Debug.WriteLine(string.Format("constructe a list with capacity: {0}", l.Capacity));int initCount = 100;
for (int u = 0; u < initCount; ++u)
{l.Add(u.ToString());
}for (int u = 0; u < initCount; ++u)
{System.Diagnostics.Debug.WriteLine(string.Format("Count: {0}, Capacity: {1}", l.Count, l.Capacity));l.TrimExcess();System.Diagnostics.Debug.WriteLine(string.Format("    TrimExcess() -> Count: {0}, Capacity: {1}", l.Count, l.Capacity));l.Remove(u.ToString());
}

View Code

得到

default capacity is : 0
constructe a list with capacity: 100
Count: 100, Capacity: 100TrimExcess() -> Count: 100, Capacity: 100
Count: 99, Capacity: 100TrimExcess() -> Count: 99, Capacity: 100
Count: 98, Capacity: 100TrimExcess() -> Count: 98, Capacity: 100
Count: 97, Capacity: 100TrimExcess() -> Count: 97, Capacity: 100
Count: 96, Capacity: 100TrimExcess() -> Count: 96, Capacity: 100
Count: 95, Capacity: 100TrimExcess() -> Count: 95, Capacity: 100
Count: 94, Capacity: 100TrimExcess() -> Count: 94, Capacity: 100
Count: 93, Capacity: 100TrimExcess() -> Count: 93, Capacity: 100
Count: 92, Capacity: 100TrimExcess() -> Count: 92, Capacity: 100
Count: 91, Capacity: 100TrimExcess() -> Count: 91, Capacity: 100
Count: 90, Capacity: 100TrimExcess() -> Count: 90, Capacity: 100
Count: 89, Capacity: 100TrimExcess() -> Count: 89, Capacity: 89
Count: 88, Capacity: 89TrimExcess() -> Count: 88, Capacity: 89
Count: 87, Capacity: 89TrimExcess() -> Count: 87, Capacity: 89
Count: 86, Capacity: 89TrimExcess() -> Count: 86, Capacity: 89
Count: 85, Capacity: 89TrimExcess() -> Count: 85, Capacity: 89
Count: 84, Capacity: 89TrimExcess() -> Count: 84, Capacity: 89
Count: 83, Capacity: 89TrimExcess() -> Count: 83, Capacity: 89
Count: 82, Capacity: 89TrimExcess() -> Count: 82, Capacity: 89
Count: 81, Capacity: 89TrimExcess() -> Count: 81, Capacity: 89
Count: 80, Capacity: 89TrimExcess() -> Count: 80, Capacity: 89
Count: 79, Capacity: 89TrimExcess() -> Count: 79, Capacity: 79
Count: 78, Capacity: 79TrimExcess() -> Count: 78, Capacity: 79
Count: 77, Capacity: 79TrimExcess() -> Count: 77, Capacity: 79
Count: 76, Capacity: 79TrimExcess() -> Count: 76, Capacity: 79
Count: 75, Capacity: 79TrimExcess() -> Count: 75, Capacity: 79
Count: 74, Capacity: 79TrimExcess() -> Count: 74, Capacity: 79
Count: 73, Capacity: 79TrimExcess() -> Count: 73, Capacity: 79
Count: 72, Capacity: 79TrimExcess() -> Count: 72, Capacity: 79
Count: 71, Capacity: 79TrimExcess() -> Count: 71, Capacity: 79
Count: 70, Capacity: 79TrimExcess() -> Count: 70, Capacity: 70
Count: 69, Capacity: 70TrimExcess() -> Count: 69, Capacity: 70
Count: 68, Capacity: 70TrimExcess() -> Count: 68, Capacity: 70
Count: 67, Capacity: 70TrimExcess() -> Count: 67, Capacity: 70
Count: 66, Capacity: 70TrimExcess() -> Count: 66, Capacity: 70
Count: 65, Capacity: 70TrimExcess() -> Count: 65, Capacity: 70
Count: 64, Capacity: 70TrimExcess() -> Count: 64, Capacity: 70
Count: 63, Capacity: 70TrimExcess() -> Count: 63, Capacity: 70
Count: 62, Capacity: 70TrimExcess() -> Count: 62, Capacity: 62
Count: 61, Capacity: 62TrimExcess() -> Count: 61, Capacity: 62
Count: 60, Capacity: 62TrimExcess() -> Count: 60, Capacity: 62
Count: 59, Capacity: 62TrimExcess() -> Count: 59, Capacity: 62
Count: 58, Capacity: 62TrimExcess() -> Count: 58, Capacity: 62
Count: 57, Capacity: 62TrimExcess() -> Count: 57, Capacity: 62
Count: 56, Capacity: 62TrimExcess() -> Count: 56, Capacity: 62
Count: 55, Capacity: 62TrimExcess() -> Count: 55, Capacity: 62
Count: 54, Capacity: 62TrimExcess() -> Count: 54, Capacity: 54
Count: 53, Capacity: 54TrimExcess() -> Count: 53, Capacity: 54
Count: 52, Capacity: 54TrimExcess() -> Count: 52, Capacity: 54
Count: 51, Capacity: 54TrimExcess() -> Count: 51, Capacity: 54
Count: 50, Capacity: 54TrimExcess() -> Count: 50, Capacity: 54
Count: 49, Capacity: 54TrimExcess() -> Count: 49, Capacity: 54
Count: 48, Capacity: 54TrimExcess() -> Count: 48, Capacity: 54
Count: 47, Capacity: 54TrimExcess() -> Count: 47, Capacity: 47
Count: 46, Capacity: 47TrimExcess() -> Count: 46, Capacity: 47
Count: 45, Capacity: 47TrimExcess() -> Count: 45, Capacity: 47
Count: 44, Capacity: 47TrimExcess() -> Count: 44, Capacity: 47
Count: 43, Capacity: 47TrimExcess() -> Count: 43, Capacity: 47
Count: 42, Capacity: 47TrimExcess() -> Count: 42, Capacity: 47
Count: 41, Capacity: 47TrimExcess() -> Count: 41, Capacity: 41
Count: 40, Capacity: 41TrimExcess() -> Count: 40, Capacity: 41
Count: 39, Capacity: 41TrimExcess() -> Count: 39, Capacity: 41
Count: 38, Capacity: 41TrimExcess() -> Count: 38, Capacity: 41
Count: 37, Capacity: 41TrimExcess() -> Count: 37, Capacity: 41
Count: 36, Capacity: 41TrimExcess() -> Count: 36, Capacity: 41
Count: 35, Capacity: 41TrimExcess() -> Count: 35, Capacity: 35
Count: 34, Capacity: 35TrimExcess() -> Count: 34, Capacity: 35
Count: 33, Capacity: 35TrimExcess() -> Count: 33, Capacity: 35
Count: 32, Capacity: 35TrimExcess() -> Count: 32, Capacity: 35
Count: 31, Capacity: 35TrimExcess() -> Count: 31, Capacity: 35
Count: 30, Capacity: 35TrimExcess() -> Count: 30, Capacity: 30
Count: 29, Capacity: 30TrimExcess() -> Count: 29, Capacity: 30
Count: 28, Capacity: 30TrimExcess() -> Count: 28, Capacity: 30
Count: 27, Capacity: 30TrimExcess() -> Count: 27, Capacity: 30
Count: 26, Capacity: 30TrimExcess() -> Count: 26, Capacity: 26
Count: 25, Capacity: 26TrimExcess() -> Count: 25, Capacity: 26
Count: 24, Capacity: 26TrimExcess() -> Count: 24, Capacity: 26
Count: 23, Capacity: 26TrimExcess() -> Count: 23, Capacity: 26
Count: 22, Capacity: 26TrimExcess() -> Count: 22, Capacity: 22
Count: 21, Capacity: 22TrimExcess() -> Count: 21, Capacity: 22
Count: 20, Capacity: 22TrimExcess() -> Count: 20, Capacity: 22
Count: 19, Capacity: 22TrimExcess() -> Count: 19, Capacity: 22
Count: 18, Capacity: 22TrimExcess() -> Count: 18, Capacity: 18
Count: 17, Capacity: 18TrimExcess() -> Count: 17, Capacity: 18
Count: 16, Capacity: 18TrimExcess() -> Count: 16, Capacity: 18
Count: 15, Capacity: 18TrimExcess() -> Count: 15, Capacity: 15
Count: 14, Capacity: 15TrimExcess() -> Count: 14, Capacity: 15
Count: 13, Capacity: 15TrimExcess() -> Count: 13, Capacity: 15
Count: 12, Capacity: 15TrimExcess() -> Count: 12, Capacity: 12
Count: 11, Capacity: 12TrimExcess() -> Count: 11, Capacity: 12
Count: 10, Capacity: 12TrimExcess() -> Count: 10, Capacity: 12
Count: 9, Capacity: 12TrimExcess() -> Count: 9, Capacity: 9
Count: 8, Capacity: 9TrimExcess() -> Count: 8, Capacity: 9
Count: 7, Capacity: 9TrimExcess() -> Count: 7, Capacity: 7
Count: 6, Capacity: 7TrimExcess() -> Count: 6, Capacity: 7
Count: 5, Capacity: 7TrimExcess() -> Count: 5, Capacity: 5
Count: 4, Capacity: 5TrimExcess() -> Count: 4, Capacity: 5
Count: 3, Capacity: 5TrimExcess() -> Count: 3, Capacity: 3
Count: 2, Capacity: 3TrimExcess() -> Count: 2, Capacity: 3
Count: 1, Capacity: 3TrimExcess() -> Count: 1, Capacity: 1

View Code

貌似条件是酱紫的

if( count<(int)Math.Floor((double)capacity*0.9) )

do trim

转载于:https://www.cnblogs.com/paulhe/p/4638657.html

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