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

    // ? 編譯錯(cuò)誤：聲明時(shí)未賦值
    // public const double Pi;

    // ? 編譯錯(cuò)誤：不能在構(gòu)造函數(shù)中修改
    // public ConstantDemo()
    // {
    //     MaxRetryCount = 5;
    // }
}

public class ReadonlyDemo
{
    // 聲明時(shí)賦值
    public readonly int MinAge = 18;

    // 構(gòu)造函數(shù)中賦值
    public readonly int UserId;
    public ReadonlyDemo(int userId)
    {
        UserId = userId;
    }

    // 靜態(tài) readonly：在靜態(tài)構(gòu)造函數(shù)中賦值
    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()
    {
        // ? 編譯錯(cuò)誤：readonly 不能修飾局部變量
        // readonly int x = 10;
    }
}

public const int ConstValue = 10;

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

ldc.i4.s 10   // 直接壓棧常量 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;

    // ? 編譯錯(cuò)誤
    // 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;

    // ? 編譯錯(cuò)誤
    // 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

        // ? 編譯錯(cuò)誤：不能重新賦值
        // Numbers = new List<int>();
    }
}

public const int MaxDays = 7;

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