运行时和编译时的常量的声明
// Compile time constant:
public const int Millennium = 2000;
// Runtime constant:
public static readonly int ThisYear = 2004;
区别：
编译时常量只能用来声明原始类型变量 如 ： int double float string and so on
而运行时常量可以声明任何常量
好处：
在改变常量值的时候，用运行时常量声明的程序集 只需重新编译自己 而不需编译所有引用该程序集的应用程序就可改变程序集的版本，而编译时的常量则需编译所有的应用及相关程序集。
用编译时变量的场景，如：程序的版本控制
1 private const int Version1_0 = 0x0100;
2
3 private const int Version1_1 = 0x0101;
4
5 private const int Version1_2 = 0x0102;
6
7 // major release:
8
9 private const int Version2_0 = 0x0200;
10
11 // check for the current version:
12
13 private static readonly int CurrentVersion =
14
15 Version2_0;
16
17 You use the runtime version to store the current version in each saved file:
18
19 // Read from persistent storage, check
20
21 // stored version against compile-time constant:
22
23 protected MyType(SerializationInfo info,
24
25 StreamingContext cntxt)
26
27 {
28
29 int storedVersion = info.GetInt32("VERSION");
30
31 switch (storedVersion)
32
33 {
34
35 case Version2_0:
36
37 readVersion2(info, cntxt);
38
39 break;
40
41 case Version1_1:
42
43 readVersion1Dot1(info, cntxt);
44
45 break;
46
47 // etc.
48
49 }
50
51 }
52
53 // Write the current version:
54
55 [SecurityPermissionAttribute(SecurityAction.Demand,
56
57 SerializationFormatter = true)]
58
59 void ISerializable.GetObjectData(SerializationInfo inf,
60
61 StreamingContext cxt)
62
63 {
64
65 // use runtime constant for current version:
66
67 inf.AddValue("VERSION", CurrentVersion);
68
69 // write remaining elements...
70
71 }
2
3 private const int Version1_1 = 0x0101;
4
5 private const int Version1_2 = 0x0102;
6
7 // major release:
8
9 private const int Version2_0 = 0x0200;
10
11 // check for the current version:
12
13 private static readonly int CurrentVersion =
14
15 Version2_0;
16
17 You use the runtime version to store the current version in each saved file:
18
19 // Read from persistent storage, check
20
21 // stored version against compile-time constant:
22
23 protected MyType(SerializationInfo info,
24
25 StreamingContext cntxt)
26
27 {
28
29 int storedVersion = info.GetInt32("VERSION");
30
31 switch (storedVersion)
32
33 {
34
35 case Version2_0:
36
37 readVersion2(info, cntxt);
38
39 break;
40
41 case Version1_1:
42
43 readVersion1Dot1(info, cntxt);
44
45 break;
46
47 // etc.
48
49 }
50
51 }
52
53 // Write the current version:
54
55 [SecurityPermissionAttribute(SecurityAction.Demand,
56
57 SerializationFormatter = true)]
58
59 void ISerializable.GetObjectData(SerializationInfo inf,
60
61 StreamingContext cxt)
62
63 {
64
65 // use runtime constant for current version:
66
67 inf.AddValue("VERSION", CurrentVersion);
68
69 // write remaining elements...
70
71 }
总结：在属性参数和枚举参数中使用const ，其他情况请使用static readonly声明常量