浅谈C# 中 const 与 readonly的核心区别

public class ConstantDemo
{
    public const int MaxRetryCount = 3;
    public const string DefaultTitle = "C#常量解析";

    // ❌ 编译错误：声明时未赋值
    // public const double Pi;

    // ❌ 编译错误：不能在构造函数中修改
    // public ConstantDemo()
    // {
    //     MaxRetryCount = 5;
    // }
}

public class ReadonlyDemo
{
    // 声明时赋值
    public readonly int MinAge = 18;

    // 构造函数中赋值
    public readonly int UserId;
    public ReadonlyDemo(int userId)
    {
        UserId = userId;
    }

    // 静态 readonly：在静态构造函数中赋值
    public static readonly string Version;
    static ReadonlyDemo()
    {
        Version = "1.0.1";
    }
}

public class ConstScopeDemo
{
    public const int GlobalConst = 100;

    public void LocalConstDemo()
    {
        const string LocalMsg = "局部常量";
        Console.WriteLine(LocalMsg);
    }
}

public class ReadonlyScopeDemo
{
    public readonly int FieldReadonly = 50;

    public void LocalReadonlyError()
    {
        // ❌ 编译错误：readonly 不能修饰局部变量
        // readonly int x = 10;
    }
}

public const int ConstValue = 10;

public void UseConst()
{
    int a = ConstValue;
}

ldc.i4.s 10   // 直接压栈常量 10

public readonly int ReadonlyValue;

public ReadonlyDemo()
{
    ReadonlyValue = 20;
}

public void UseReadonly()
{
    int b = ReadonlyValue;
}

ldfld int32 ReadonlyValue

public class ConstStaticDemo
{
    public const int ConstStatic = 10;

    // ❌ 编译错误
    // public static const int Invalid = 20;
}

int x = ConstStaticDemo.ConstStatic;

public class ReadonlyStaticDemo
{
    public readonly int InstanceReadonly = 100;
    public static readonly int StaticReadonly = 200;
}

public class ConstReferenceDemo
{
    public const string ConstString = "Hello";
    public const object ConstNull = null;

    // ❌ 编译错误
    // public const List<int> ConstList = new List<int>();
}

public class ReadonlyReferenceDemo
{
    public readonly List<int> Numbers = new() { 1, 2, 3 };

    public void Modify()
    {
        Numbers.Add(4);   // ✅ 合法

        // ❌ 编译错误：不能重新赋值
        // Numbers = new List<int>();
    }
}

public const int MaxDays = 7;

public static readonly string ConnectionString;
static DbConfig()
{
    ConnectionString = LoadFromConfig();
}