C# 中 const 和 readonly 关键字的区别和用法

前言

今天我们一起来讲讲 C# 中 const 和 readonly 关键字的区别和用法。

const 和 readonly 关键字区别

基本介绍

• const（常量）： 在C#中用于声明编译时常量，其值在编译时就必须确定，并且在程序生命周期内不可更改。
• readonly（只读字段）： 在C#中用于声明运行时常量，其值可以在声明时或构造函数中初始化，之后不可更改（可通过反射强制修改）。

const 和 readonly 异同点

const 和 readonly 关键字使用

const 使用

    public enum UserRole
    {
        Admin,
        User,
        Guest
    }

    public class ConstAndReadonlyExercise
    {
        // const 初始化
        public const int MaxCount = 999;
        public const UserRole CurrentUserRole = UserRole.Admin;
    }

编译后 IL 代码：

• literal 关键字：标记为字面量，值直接嵌入调用处的 IL。

  .field public static literal int32 MaxCount = int32(999) // 0x000003e7

  .field public static literal valuetype 'HelloDotNetGuide.CSharp语法.UserRole' CurrentUserRole = int32(0) // 0x00000000

readonly 使用

 // readonly 初始化
 public readonly string _applicationName = "HelloDotNetGuide";

 public ConstAndReadonlyExercise()
 {
     _applicationName = "HelloDotNetGuide_V2";
 }

 // 懒汉式单例模式示例
 private static ConstAndReadonlyExercise? _instance;
 private static readonly object _lockObj = new object();

 public static ConstAndReadonlyExercise Instance
 {
     get
     {
         if (_instance == null)
         {
             lock (_lockObj)
             {
                 _instance ??= new ConstAndReadonlyExercise();
             }
         }
         return _instance;
     }
 }

 /// <summary>
 /// 反射修改 readonly 字段的值
 /// </summary>
 public static void UpdateApplicationNameValue()
 {
     var instance = new ConstAndReadonlyExercise();
     Console.WriteLine($"初始值: {instance._applicationName}");
     // 输出: 初始值: HelloDotNetGuide_V2

     var field = instance.GetType().GetField("_applicationName");
     field.SetValue(instance, "HelloDotNetGuide_V3");

     Console.WriteLine($"修改后: {instance._applicationName}");
     // 输出: 修改后: HelloDotNetGuide_V3
 }

编译后 IL 代码：

• initonly 关键字：标志被 CLR 识别为仅构造函数可写约束。

  .field public initonly string _applicationName

  .field private static class 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise' _instance
    .custom instance void [System.Runtime]System.Runtime.CompilerServices.NullableAttribute::.ctor(unsigned int8)
      = (01 00 02 00 00 ) // .....
      // unsigned int8(2) // 0x02

  .field private static initonly object _lockObj

  .method public hidebysig specialname rtspecialname instance void
    .ctor() cil managed
  {
    .maxstack 8

    // [25 9 - 25 70]
    IL_0000: ldarg.0      // this
    IL_0001: ldstr        "HelloDotNetGuide"
    IL_0006: stfld        string 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise'::_applicationName

    // [29 9 - 29 42]
    IL_000b: ldarg.0      // this
    IL_000c: call         instance void [System.Runtime]System.Object::.ctor()
    IL_0011: nop

    // [30 9 - 30 10]
    IL_0012: nop

    // [31 13 - 31 54]
    IL_0013: ldarg.0      // this
    IL_0014: ldstr        "HelloDotNetGuide_V2"
    IL_0019: stfld        string 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise'::_applicationName

    // [32 9 - 32 10]
    IL_001e: ret

  } // end of method ConstAndReadonlyExercise::.ctor

  .method public hidebysig static specialname class 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise'
    get_Instance() cil managed
  {
    .maxstack 2
    .locals init (
      [0] bool V_0,
      [1] object V_1,
      [2] bool V_2,
      [3] class 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise' V_3
    )

    // [37 13 - 37 14]
    IL_0000: nop

    // [38 17 - 38 39]
    IL_0001: ldsfld       class 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise''HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise'::_instance
    IL_0006: ldnull
    IL_0007: ceq
    IL_0009: stloc.0      // V_0

    IL_000a: ldloc.0      // V_0
    IL_000b: brfalse.s    IL_0040

    // [39 17 - 39 18]
    IL_000d: nop

    // [40 21 - 40 36]
    IL_000e: ldsfld       object 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise'::_lockObj
    IL_0013: stloc.1      // V_1
    IL_0014: ldc.i4.0
    IL_0015: stloc.2      // V_2
    .try
    {
      IL_0016: ldloc.1      // V_1
      IL_0017: ldloca.s     V_2
      IL_0019: call         void [System.Threading]System.Threading.Monitor::Enter(object, bool&)
      IL_001e: nop

      // [41 21 - 41 22]
      IL_001f: nop

      // [42 25 - 42 70]
      IL_0020: ldsfld       class 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise''HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise'::_instance
      IL_0025: brtrue.s     IL_0031
      IL_0027: newobj       instance void 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise'::.ctor()
      IL_002c: stsfld       class 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise''HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise'::_instance

      // [43 21 - 43 22]
      IL_0031: nop
      IL_0032: leave.s      IL_003f
    } // end of .try
    finally
    {

      IL_0034: ldloc.2      // V_2
      IL_0035: brfalse.s    IL_003e
      IL_0037: ldloc.1      // V_1
      IL_0038: call         void [System.Threading]System.Threading.Monitor::Exit(object)
      IL_003d: nop

      IL_003e: endfinally
    } // end of finally

    // [44 17 - 44 18]
    IL_003f: nop

    // [45 17 - 45 34]
    IL_0040: ldsfld       class 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise''HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise'::_instance
    IL_0045: stloc.3      // V_3
    IL_0046: br.s         IL_0048

    // [46 13 - 46 14]
    IL_0048: ldloc.3      // V_3
    IL_0049: ret

  } // end of method ConstAndReadonlyExercise::get_Instance

  .method public hidebysig static void
    UpdateApplicationNameValue() cil managed
  {
    .maxstack 3
    .locals init (
      [0] class 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise''instance',
      [1] class [System.Runtime]System.Reflection.FieldInfo 'field'
    )

    // [50 9 - 50 10]
    IL_0000: nop

    // [51 13 - 51 59]
    IL_0001: newobj       instance void 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise'::.ctor()
    IL_0006: stloc.0      // 'instance'

    // [52 13 - 52 68]
    IL_0007: ldstr        "初始值: "
    IL_000c: ldloc.0      // 'instance'
    IL_000d: ldfld        string 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise'::_applicationName
    IL_0012: call         string [System.Runtime]System.String::Concat(string, string)
    IL_0017: call         void [System.Console]System.Console::WriteLine(string)
    IL_001c: nop

    // [55 13 - 55 73]
    IL_001d: ldloc.0      // 'instance'
    IL_001e: callvirt     instance class [System.Runtime]System.Type [System.Runtime]System.Object::GetType()
    IL_0023: ldstr        "_applicationName"
    IL_0028: callvirt     instance class [System.Runtime]System.Reflection.FieldInfo [System.Runtime]System.Type::GetField(string)
    IL_002d: stloc.1      // 'field'

    // [56 13 - 56 61]
    IL_002e: ldloc.1      // 'field'
    IL_002f: ldloc.0      // 'instance'
    IL_0030: ldstr        "HelloDotNetGuide_V3"
    IL_0035: callvirt     instance void [System.Runtime]System.Reflection.FieldInfo::SetValue(object, object)
    IL_003a: nop

    // [58 13 - 58 68]
    IL_003b: ldstr        "修改后: "
    IL_0040: ldloc.0      // 'instance'
    IL_0041: ldfld        string 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise'::_applicationName
    IL_0046: call         string [System.Runtime]System.String::Concat(string, string)
    IL_004b: call         void [System.Console]System.Console::WriteLine(string)
    IL_0050: nop

    // [60 9 - 60 10]
    IL_0051: ret

  } // end of method ConstAndReadonlyExercise::UpdateApplicationNameValue