Union Types in C can also contain Struct fields

suggest change

Apart from primitives, the Explicit Layout structs (Unions) in C#, can also contain other Structs. As long as a field is a Value type and not a Reference, it can be contained in a Union:

using System;
using System.Runtime.InteropServices;

// The struct needs to be annotated as "Explicit Layout"
[StructLayout(LayoutKind.Explicit)]
struct IpAddress
{
    // Same definition of IpAddress, from the example above
}

// Now let's see if we can fit a whole URL into a long

// Let's define a short enum to hold protocols
enum Protocol : short { Http, Https, Ftp, Sftp, Tcp }

// The Service struct will hold the Address, the Port and the Protocol
[StructLayout(LayoutKind.Explicit)]
struct Service
{
    [FieldOffset(0)] public IpAddress Address;
    [FieldOffset(4)] public ushort Port;
    [FieldOffset(6)] public Protocol AppProtocol;
    [FieldOffset(0)] public long Payload;

    public Service(IpAddress address, ushort port, Protocol protocol)
    {
        Payload = 0;
        Address = address;
        Port  = port;
        AppProtocol = protocol;
    }

    public Service(long payload)
    {
        Address = new IpAddress(0);
        Port = 80;
        AppProtocol = Protocol.Http;
        Payload = payload;
    }

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

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

We can now verify that the whole Service Union fits into the size of a long (8 bytes).

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

var s1 = new Service(ip, 8080, Protocol.Https);
var s2 = new Service(s1.Payload);
s2.Address.Byte1 = 100;
s2.AppProtocol = Protocol.Ftp;

Console.WriteLine($"Size: {Marshal.SizeOf(s1)} bytes. Value: {s1.Address} = {s1}.");
Console.WriteLine($"Size: {Marshal.SizeOf(s2)} bytes. Value: {s2.Address} = {s2}.");

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