List的Capacity

  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