C# Language How to use C# Structs to create a Union type (Similar to C Unions) Union Types in C# can also contain Struct fields
Example
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}.");
