c# 如何使用结构体实现共用体

#include <stdio.h>

union data{
    int n;
    char ch;
    short m;
};

int main(){
    union data a;
    printf("%d, %d\n", sizeof(a), sizeof(union data) );
    a.n = 0x40;
    printf("%X, %c, %hX\n", a.n, a.ch, a.m);
    a.ch = '9';
    printf("%X, %c, %hX\n", a.n, a.ch, a.m);
    a.m = 0x2059;
    printf("%X, %c, %hX\n", a.n, a.ch, a.m);
    a.n = 0x3E25AD54;
    printf("%X, %c, %hX\n", a.n, a.ch, a.m);

    return 0;
}

using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Explicit)]
public struct IpAddress
{
    // FieldOffset 表示偏移的位置（以字节为单位）
    // sizeof(int) = 4, sizeof(byte) = 1
    [FieldOffset(0)] public int Address;
    [FieldOffset(0)] public byte Byte1;
    [FieldOffset(1)] public byte Byte2;
    [FieldOffset(2)] public byte Byte3;
    [FieldOffset(3)] public byte Byte4;

    public IpAddress(int address) : this()
    {
        // 给 Address 赋值时，所有成员的值都会自动被修改
        Address = address;
    }

    public override string ToString() => $"{Byte1}.{Byte2}.{Byte3}.{Byte4}";
}

var ip = new IpAddress(new Random().Next());
Console.WriteLine($"{ip} = {ip.Address}");
ip.Byte1 = 192;
ip.Byte2 = 168;
Console.WriteLine($"{ip} = {ip.Address}");

using System;
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Explicit)]
public struct IpAddress
{
    [FieldOffset(0)] public int Address;
    [FieldOffset(0)] public byte Byte1;
    [FieldOffset(1)] public byte Byte2;
    [FieldOffset(2)] public byte Byte3;
    [FieldOffset(3)] public byte Byte4;

    public IpAddress(int address) : this()
    {
        Address = address;
    }

    public override string ToString() => $"{Byte1}.{Byte2}.{Byte3}.{Byte4}";
}

public enum Protocol : byte { Http, https, ftp, sftp, tcp };

[StructLayout(LayoutKind.Explicit)]
public struct Server
{
    [FieldOffset(0)] public IpAddress Address;
    [FieldOffset(4)] public ushort Port;
    [FieldOffset(6)] public Protocol Protocol;
    [FieldOffset(0)] public long Payload;

    public Server(IpAddress addr, ushort port, Protocol prot) : this()
    {
        Address = addr;
        Port = port;
        Protocol = prot;
    }

    public Server(long payload)
    {
        // 参数长度可能不足填满每个成员，所以这里先对成员设初始值
        Address = new IpAddress(0);
        Port = 80;
        Protocol = Protocol.http;

        // 填值
        Payload = payload;
    }

    public Server Copy() =>  new Server(Payload);

    public override string ToString() => $"{Protocol}://{Address}:{Port}";
}

var ip = new IpAddress(new Random().Next());
Console.WriteLine($"Size: {Marshal.SizeOf(ip)} bytes. Value: {ip.Address} = {ip}");

var s1 = new Server(ip, 8080, Protocol.https);
var s2 = new Server(s1.Payload);
s2.Address.Byte1 = 100;
s2.Protocol = Protocol.ftp;
Console.WriteLine($"Size: {Marshal.SizeOf(s1)} bytes. Value: {s1.Address} = {s1}");
Console.WriteLine($"Size: {Marshal.SizeOf(s2)} bytes. Value: {s2.Address} = {s2}");