C#语言知识点整理 - 常量
| 关键字 | const | readonly |
| 名称 | 静态常量 编译时常量(compile-time constants) | 动态常量 运行时常量(runtime constants) |
| 初始化、赋值时机 | const 字段只能在该字段的声明中初始化。 | readonly 字段可以在声明或构造函数中初始化。 |
| 工作原理 | const为编译时常量，程序编译时将所有常量引用替换为相应值。 | readonly为运行时常量，程序运行时赋值，赋值完成后便无法更改 |
| 可维护性 | const是编译时替换，防止跨程序集调用问题，所有相关程序集都得编译。 | readonly常量更新后，所有引用该常量的地方均能得到更新。 |
| 使用场景 | 取值恒定不变，范围较小。对程序性能要求较高 | 取值类型范围较大，可以是运行时的常量。如： public readonly DateTime D = DateTime.MinValue; 性能略有损耗。 |
初始化、赋值时机 示例说明:
1: using System;
2: using System.Collections.Generic;
3: using System.Linq;
4: using System.Text;
5: 6: namespace CSharp.Contant_const
7: {8: class Class1
9: {10: const int _x=2013;
11: 12: Class1(int x)
13: {14: //_x = x;//赋值号左边必须是变量、属性或索引器
15: } 16: 17: void Method()
18: {19: //_x = 2013;//赋值号左边必须是变量、属性或索引器
20: } 21: 22: static void Main(string[] args)
23: {24: //Class1 instance1 = new Class1(2013);
25: //instance1._x = 2014;//无法使用实例引用来访问成员
26: 27: //Class1._x = 2014;//赋值号左边必须是变量、属性或索引器
28: 29: Console.WriteLine("x:{0}",Class1._x);
30: } 31: } 32: } 33: 1: using System;
2: using System.Collections.Generic;
3: using System.Linq;
4: using System.Text;
5: 6: namespace CSharp.Constant_readonly
7: {8: class Class1
9: {10: readonly int _x;
11: Class1(int x)
12: { 13: _x = x; 14: }15: void Method()
16: {17: //_x = 2013;//无法对只读的字段赋值(构造函数或变量初始值指定项中除外)
18: } 19: 20: static void Main(string[] args)
21: {22: Class1 instance1 = new Class1(2013);
23: //instance1._x = 2014;//无法对只读的字段赋值(构造函数或变量初始值指定项中除外)
24: } 25: } 26: } 27: 工作原理示例说明:
ConstTest:
1: using System;
2: using System.Collections.Generic;
3: using System.Linq;
4: using System.Text;
5: 6: namespace CSharp.Contant_const
7: {8: class ConstTest
9: {10: private const int b = a * 3;
11: private const int a = 2;
12: static void Main(string[] args)
13: {14: Console.WriteLine("a={0}\nb={1}",a,b);
15: } 16: } 17: }运行结果：
ReadonlyTest：
1: using System;
2: using System.Collections.Generic;
3: using System.Linq;
4: using System.Text;
5: 6: namespace CSharp.Constant_readonly
7: {8: class ReadonlyTest
9: {10: private static readonly int b = a * 3;
11: private static readonly int a = 2;
12: static void Main(string[] args)
13: {14: Console.WriteLine("a={0}\nb={1}", a, b);
15: } 16: } 17: }运行结果：
分析两种结果：
1) 启动"ILDASM"
2) 对比两种结果
ConstantTest：
.field private static literal int32 a = int32(0x00000002)
.field private static literal int32 b = int32(0x00000006)
.method private hidebysig static void Main(string[] args) cil managed
{
.entrypoint
// 代码大小 25 (0x19)
.maxstack 8
IL_0000: nop
IL_0001: ldstr "a={0}\nb={1}"
IL_0006: ldc.i4.2
IL_0007: box [mscorlib]System.Int32
IL_000c: ldc.i4.6
IL_000d: box [mscorlib]System.Int32
IL_0012: call void [mscorlib]System.Console::WriteLine(string,
object,
object)
IL_0017: nop
IL_0018: ret
} // end of method ConstTest::Main
ReadonlyTest:
.field private static initonly int32 a
.field private static initonly int32 b
.method private hidebysig static void Main(string[] args) cil managed
{
.entrypoint
// 代码大小 33 (0x21)
.maxstack 8
IL_0000: nop
IL_0001: ldstr "a={0}\nb={1}"
IL_0006: ldsfld int32 CSharp.Constant_readonly.ReadonlyTest::a
IL_000b: box [mscorlib]System.Int32
IL_0010: ldsfld int32 CSharp.Constant_readonly.ReadonlyTest::b
IL_0015: box [mscorlib]System.Int32
IL_001a: call void [mscorlib]System.Console::WriteLine(string,
object,
object)
IL_001f: nop
IL_0020: ret
} // end of method ReadonlyTest::Main