C# 中 const 和 readonly 关键字的区别和用法
https://img2024.cnblogs.com/blog/1336199/202507/1336199-20250707215115958-2134306461.png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
对比维度constreadonly基础定义编译时常量，值在编译期确定运行时常量，值在运行时确定初始化时机必须在声明时初始化可在声明时或构造函数中初始化支持的数据类型仅支持基元类型（int, float, char, bool等）、string和null引用支持所有类型（包括自定义类和结构体）默认值要求必须显式初始化不需要显示初始化，值类型默认零值，引用类型默认null性能表现零开销访问（直接编译到IL）微小访问开销（作为实例/静态字段分配内存）线程安全天然线程安全实例字段需注意可见性，静态字段线程安全反射修改无法通过反射修改可通过反射强制修改IL元数据标记literal 标记initonly 标记使用场景声明常量字段或本地常量，常量可以是数字、布尔值、字符串或 null 引用等声明依赖注入对象、配置值、运行时计算值等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) // 0x00000000readonly 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.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
//
IL_0000: ldarg.0 // this
IL_0001: ldstr "HelloDotNetGuide"
IL_0006: stfld string 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise'::_applicationName
//
IL_000b: ldarg.0 // this
IL_000c: call instance void System.Object::.ctor()
IL_0011: nop
//
IL_0012: nop
//
IL_0013: ldarg.0 // this
IL_0014: ldstr "HelloDotNetGuide_V2"
IL_0019: stfld string 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise'::_applicationName
//
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 (
bool V_0,
object V_1,
bool V_2,
class 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise' V_3
)
//
IL_0000: nop
//
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
//
IL_000d: nop
//
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.Monitor::Enter(object, bool&)
IL_001e: nop
//
IL_001f: nop
//
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
//
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.Monitor::Exit(object)
IL_003d: nop
IL_003e: endfinally
} // end of finally
//
IL_003f: nop
//
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
//
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 (
class 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise''instance',
class System.Reflection.FieldInfo 'field'
)
//
IL_0000: nop
//
IL_0001: newobj instance void 'HelloDotNetGuide.CSharp语法.ConstAndReadonlyExercise'::.ctor()
IL_0006: stloc.0 // 'instance'
//
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.String::Concat(string, string)
IL_0017: call void System.Console::WriteLine(string)
IL_001c: nop
//
IL_001d: ldloc.0 // 'instance'
IL_001e: callvirt instance class System.Type System.Object::GetType()
IL_0023: ldstr "_applicationName"
IL_0028: callvirt instance class System.Reflection.FieldInfo System.Type::GetField(string)
IL_002d: stloc.1 // 'field'
//
IL_002e: ldloc.1 // 'field'
IL_002f: ldloc.0 // 'instance'
IL_0030: ldstr "HelloDotNetGuide_V3"
IL_0035: callvirt instance void System.Reflection.FieldInfo::SetValue(object, object)
IL_003a: nop
//
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.String::Concat(string, string)
IL_004b: call void System.Console::WriteLine(string)
IL_0050: nop
//
IL_0051: ret
} // end of method ConstAndReadonlyExercise::UpdateApplicationNameValueC#/.NET/.NET Core面试宝典
本文已收录至C#/.NET/.NET Core面试宝典中，更多C#关键字详解请前往C#/.NET/.NET Core面试宝典开放地址查阅。
[*]面试宝典地址：https://www.yuque.com/ysgstudyhard/da6e0c
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