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List的Capacity

Posted on 2014-08-15 11:31  mumuliang  阅读(...)  评论(...编辑  收藏

  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: 0
    TrimExcess() -> Count: 0, Capacity: 0
Count: 1, Capacity: 4
    TrimExcess() -> Count: 1, Capacity: 1
Count: 2, Capacity: 2
    TrimExcess() -> Count: 2, Capacity: 2
Count: 3, Capacity: 4
    TrimExcess() -> Count: 3, Capacity: 4
Count: 4, Capacity: 4
    TrimExcess() -> Count: 4, Capacity: 4
Count: 5, Capacity: 8
    TrimExcess() -> Count: 5, Capacity: 5
Count: 6, Capacity: 10
    TrimExcess() -> Count: 6, Capacity: 6
Count: 7, Capacity: 12
    TrimExcess() -> Count: 7, Capacity: 7
Count: 8, Capacity: 14
    TrimExcess() -> Count: 8, Capacity: 8
Count: 9, Capacity: 16
    TrimExcess() -> Count: 9, Capacity: 9
Count: 10, Capacity: 18
    TrimExcess() -> Count: 10, Capacity: 10
Count: 11, Capacity: 20
    TrimExcess() -> Count: 11, Capacity: 11
Count: 12, Capacity: 22
    TrimExcess() -> Count: 12, Capacity: 12
Count: 13, Capacity: 24
    TrimExcess() -> Count: 13, Capacity: 13
Count: 14, Capacity: 26
    TrimExcess() -> Count: 14, Capacity: 14
Count: 15, Capacity: 28
    TrimExcess() -> Count: 15, Capacity: 15
Count: 16, Capacity: 30
    TrimExcess() -> Count: 16, Capacity: 16
Count: 17, Capacity: 32
    TrimExcess() -> Count: 17, Capacity: 17
Count: 18, Capacity: 34
    TrimExcess() -> Count: 18, Capacity: 18
Count: 19, Capacity: 36
    TrimExcess() -> Count: 19, Capacity: 19
Count: 20, Capacity: 38
    TrimExcess() -> Count: 20, Capacity: 20
Count: 21, Capacity: 40
    TrimExcess() -> Count: 21, Capacity: 21
Count: 22, Capacity: 42
    TrimExcess() -> Count: 22, Capacity: 22
Count: 23, Capacity: 44
    TrimExcess() -> Count: 23, Capacity: 23
Count: 24, Capacity: 46
    TrimExcess() -> Count: 24, Capacity: 24
Count: 25, Capacity: 48
    TrimExcess() -> Count: 25, Capacity: 25
Count: 26, Capacity: 50
    TrimExcess() -> Count: 26, Capacity: 26
Count: 27, Capacity: 52
    TrimExcess() -> Count: 27, Capacity: 27
Count: 28, Capacity: 54
    TrimExcess() -> Count: 28, Capacity: 28
Count: 29, Capacity: 56
    TrimExcess() -> Count: 29, Capacity: 29
Count: 30, Capacity: 58
    TrimExcess() -> Count: 30, Capacity: 30
Count: 31, Capacity: 60
    TrimExcess() -> Count: 31, Capacity: 31
Count: 32, Capacity: 62
    TrimExcess() -> Count: 32, Capacity: 32
Count: 33, Capacity: 64
    TrimExcess() -> Count: 33, Capacity: 33
Count: 34, Capacity: 66
    TrimExcess() -> Count: 34, Capacity: 34
Count: 35, Capacity: 68
    TrimExcess() -> Count: 35, Capacity: 35
Count: 36, Capacity: 70
    TrimExcess() -> Count: 36, Capacity: 36
Count: 37, Capacity: 72
    TrimExcess() -> Count: 37, Capacity: 37
Count: 38, Capacity: 74
    TrimExcess() -> Count: 38, Capacity: 38
Count: 39, Capacity: 76
    TrimExcess() -> Count: 39, Capacity: 39
Count: 40, Capacity: 78
    TrimExcess() -> Count: 40, Capacity: 40
Count: 41, Capacity: 80
    TrimExcess() -> Count: 41, Capacity: 41
Count: 42, Capacity: 82
    TrimExcess() -> Count: 42, Capacity: 42
Count: 43, Capacity: 84
    TrimExcess() -> Count: 43, Capacity: 43
Count: 44, Capacity: 86
    TrimExcess() -> Count: 44, Capacity: 44
Count: 45, Capacity: 88
    TrimExcess() -> Count: 45, Capacity: 45
Count: 46, Capacity: 90
    TrimExcess() -> Count: 46, Capacity: 46
Count: 47, Capacity: 92
    TrimExcess() -> Count: 47, Capacity: 47
Count: 48, Capacity: 94
    TrimExcess() -> Count: 48, Capacity: 48
Count: 49, Capacity: 96
    TrimExcess() -> Count: 49, Capacity: 49
Count: 50, Capacity: 98
    TrimExcess() -> Count: 50, Capacity: 50
Count: 51, Capacity: 100
    TrimExcess() -> Count: 51, Capacity: 51
Count: 52, Capacity: 102
    TrimExcess() -> Count: 52, Capacity: 52
Count: 53, Capacity: 104
    TrimExcess() -> Count: 53, Capacity: 53
Count: 54, Capacity: 106
    TrimExcess() -> Count: 54, Capacity: 54
Count: 55, Capacity: 108
    TrimExcess() -> Count: 55, Capacity: 55
Count: 56, Capacity: 110
    TrimExcess() -> Count: 56, Capacity: 56
Count: 57, Capacity: 112
    TrimExcess() -> Count: 57, Capacity: 57
Count: 58, Capacity: 114
    TrimExcess() -> Count: 58, Capacity: 58
Count: 59, Capacity: 116
    TrimExcess() -> Count: 59, Capacity: 59
Count: 60, Capacity: 118
    TrimExcess() -> Count: 60, Capacity: 60
Count: 61, Capacity: 120
    TrimExcess() -> Count: 61, Capacity: 61
Count: 62, Capacity: 122
    TrimExcess() -> Count: 62, Capacity: 62
Count: 63, Capacity: 124
    TrimExcess() -> Count: 63, Capacity: 63
Count: 64, Capacity: 126
    TrimExcess() -> 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: 100
    TrimExcess() -> Count: 100, Capacity: 100
Count: 99, Capacity: 100
    TrimExcess() -> Count: 99, Capacity: 100
Count: 98, Capacity: 100
    TrimExcess() -> Count: 98, Capacity: 100
Count: 97, Capacity: 100
    TrimExcess() -> Count: 97, Capacity: 100
Count: 96, Capacity: 100
    TrimExcess() -> Count: 96, Capacity: 100
Count: 95, Capacity: 100
    TrimExcess() -> Count: 95, Capacity: 100
Count: 94, Capacity: 100
    TrimExcess() -> Count: 94, Capacity: 100
Count: 93, Capacity: 100
    TrimExcess() -> Count: 93, Capacity: 100
Count: 92, Capacity: 100
    TrimExcess() -> Count: 92, Capacity: 100
Count: 91, Capacity: 100
    TrimExcess() -> Count: 91, Capacity: 100
Count: 90, Capacity: 100
    TrimExcess() -> Count: 90, Capacity: 100
Count: 89, Capacity: 100
    TrimExcess() -> Count: 89, Capacity: 89
Count: 88, Capacity: 89
    TrimExcess() -> Count: 88, Capacity: 89
Count: 87, Capacity: 89
    TrimExcess() -> Count: 87, Capacity: 89
Count: 86, Capacity: 89
    TrimExcess() -> Count: 86, Capacity: 89
Count: 85, Capacity: 89
    TrimExcess() -> Count: 85, Capacity: 89
Count: 84, Capacity: 89
    TrimExcess() -> Count: 84, Capacity: 89
Count: 83, Capacity: 89
    TrimExcess() -> Count: 83, Capacity: 89
Count: 82, Capacity: 89
    TrimExcess() -> Count: 82, Capacity: 89
Count: 81, Capacity: 89
    TrimExcess() -> Count: 81, Capacity: 89
Count: 80, Capacity: 89
    TrimExcess() -> Count: 80, Capacity: 89
Count: 79, Capacity: 89
    TrimExcess() -> Count: 79, Capacity: 79
Count: 78, Capacity: 79
    TrimExcess() -> Count: 78, Capacity: 79
Count: 77, Capacity: 79
    TrimExcess() -> Count: 77, Capacity: 79
Count: 76, Capacity: 79
    TrimExcess() -> Count: 76, Capacity: 79
Count: 75, Capacity: 79
    TrimExcess() -> Count: 75, Capacity: 79
Count: 74, Capacity: 79
    TrimExcess() -> Count: 74, Capacity: 79
Count: 73, Capacity: 79
    TrimExcess() -> Count: 73, Capacity: 79
Count: 72, Capacity: 79
    TrimExcess() -> Count: 72, Capacity: 79
Count: 71, Capacity: 79
    TrimExcess() -> Count: 71, Capacity: 79
Count: 70, Capacity: 79
    TrimExcess() -> Count: 70, Capacity: 70
Count: 69, Capacity: 70
    TrimExcess() -> Count: 69, Capacity: 70
Count: 68, Capacity: 70
    TrimExcess() -> Count: 68, Capacity: 70
Count: 67, Capacity: 70
    TrimExcess() -> Count: 67, Capacity: 70
Count: 66, Capacity: 70
    TrimExcess() -> Count: 66, Capacity: 70
Count: 65, Capacity: 70
    TrimExcess() -> Count: 65, Capacity: 70
Count: 64, Capacity: 70
    TrimExcess() -> Count: 64, Capacity: 70
Count: 63, Capacity: 70
    TrimExcess() -> Count: 63, Capacity: 70
Count: 62, Capacity: 70
    TrimExcess() -> Count: 62, Capacity: 62
Count: 61, Capacity: 62
    TrimExcess() -> Count: 61, Capacity: 62
Count: 60, Capacity: 62
    TrimExcess() -> Count: 60, Capacity: 62
Count: 59, Capacity: 62
    TrimExcess() -> Count: 59, Capacity: 62
Count: 58, Capacity: 62
    TrimExcess() -> Count: 58, Capacity: 62
Count: 57, Capacity: 62
    TrimExcess() -> Count: 57, Capacity: 62
Count: 56, Capacity: 62
    TrimExcess() -> Count: 56, Capacity: 62
Count: 55, Capacity: 62
    TrimExcess() -> Count: 55, Capacity: 62
Count: 54, Capacity: 62
    TrimExcess() -> Count: 54, Capacity: 54
Count: 53, Capacity: 54
    TrimExcess() -> Count: 53, Capacity: 54
Count: 52, Capacity: 54
    TrimExcess() -> Count: 52, Capacity: 54
Count: 51, Capacity: 54
    TrimExcess() -> Count: 51, Capacity: 54
Count: 50, Capacity: 54
    TrimExcess() -> Count: 50, Capacity: 54
Count: 49, Capacity: 54
    TrimExcess() -> Count: 49, Capacity: 54
Count: 48, Capacity: 54
    TrimExcess() -> Count: 48, Capacity: 54
Count: 47, Capacity: 54
    TrimExcess() -> Count: 47, Capacity: 47
Count: 46, Capacity: 47
    TrimExcess() -> Count: 46, Capacity: 47
Count: 45, Capacity: 47
    TrimExcess() -> Count: 45, Capacity: 47
Count: 44, Capacity: 47
    TrimExcess() -> Count: 44, Capacity: 47
Count: 43, Capacity: 47
    TrimExcess() -> Count: 43, Capacity: 47
Count: 42, Capacity: 47
    TrimExcess() -> Count: 42, Capacity: 47
Count: 41, Capacity: 47
    TrimExcess() -> Count: 41, Capacity: 41
Count: 40, Capacity: 41
    TrimExcess() -> Count: 40, Capacity: 41
Count: 39, Capacity: 41
    TrimExcess() -> Count: 39, Capacity: 41
Count: 38, Capacity: 41
    TrimExcess() -> Count: 38, Capacity: 41
Count: 37, Capacity: 41
    TrimExcess() -> Count: 37, Capacity: 41
Count: 36, Capacity: 41
    TrimExcess() -> Count: 36, Capacity: 41
Count: 35, Capacity: 41
    TrimExcess() -> Count: 35, Capacity: 35
Count: 34, Capacity: 35
    TrimExcess() -> Count: 34, Capacity: 35
Count: 33, Capacity: 35
    TrimExcess() -> Count: 33, Capacity: 35
Count: 32, Capacity: 35
    TrimExcess() -> Count: 32, Capacity: 35
Count: 31, Capacity: 35
    TrimExcess() -> Count: 31, Capacity: 35
Count: 30, Capacity: 35
    TrimExcess() -> Count: 30, Capacity: 30
Count: 29, Capacity: 30
    TrimExcess() -> Count: 29, Capacity: 30
Count: 28, Capacity: 30
    TrimExcess() -> Count: 28, Capacity: 30
Count: 27, Capacity: 30
    TrimExcess() -> Count: 27, Capacity: 30
Count: 26, Capacity: 30
    TrimExcess() -> Count: 26, Capacity: 26
Count: 25, Capacity: 26
    TrimExcess() -> Count: 25, Capacity: 26
Count: 24, Capacity: 26
    TrimExcess() -> Count: 24, Capacity: 26
Count: 23, Capacity: 26
    TrimExcess() -> Count: 23, Capacity: 26
Count: 22, Capacity: 26
    TrimExcess() -> Count: 22, Capacity: 22
Count: 21, Capacity: 22
    TrimExcess() -> Count: 21, Capacity: 22
Count: 20, Capacity: 22
    TrimExcess() -> Count: 20, Capacity: 22
Count: 19, Capacity: 22
    TrimExcess() -> Count: 19, Capacity: 22
Count: 18, Capacity: 22
    TrimExcess() -> Count: 18, Capacity: 18
Count: 17, Capacity: 18
    TrimExcess() -> Count: 17, Capacity: 18
Count: 16, Capacity: 18
    TrimExcess() -> Count: 16, Capacity: 18
Count: 15, Capacity: 18
    TrimExcess() -> Count: 15, Capacity: 15
Count: 14, Capacity: 15
    TrimExcess() -> Count: 14, Capacity: 15
Count: 13, Capacity: 15
    TrimExcess() -> Count: 13, Capacity: 15
Count: 12, Capacity: 15
    TrimExcess() -> Count: 12, Capacity: 12
Count: 11, Capacity: 12
    TrimExcess() -> Count: 11, Capacity: 12
Count: 10, Capacity: 12
    TrimExcess() -> Count: 10, Capacity: 12
Count: 9, Capacity: 12
    TrimExcess() -> Count: 9, Capacity: 9
Count: 8, Capacity: 9
    TrimExcess() -> Count: 8, Capacity: 9
Count: 7, Capacity: 9
    TrimExcess() -> Count: 7, Capacity: 7
Count: 6, Capacity: 7
    TrimExcess() -> Count: 6, Capacity: 7
Count: 5, Capacity: 7
    TrimExcess() -> Count: 5, Capacity: 5
Count: 4, Capacity: 5
    TrimExcess() -> Count: 4, Capacity: 5
Count: 3, Capacity: 5
    TrimExcess() -> Count: 3, Capacity: 3
Count: 2, Capacity: 3
    TrimExcess() -> Count: 2, Capacity: 3
Count: 1, Capacity: 3
    TrimExcess() -> 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

 

我也不知道我要干嘛