How can I implement C/C++ "union" in C#?

C# doesn't natively support the C/C++ notion of unions. However, you can use the StructLayout(LayoutKind.Explicit) and FieldOffset attributes to create equivalent functionality. Try the given code below:

using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Explicit)]
struct Data {
  [FieldOffset(0)]
  public byte Byte1;
  [FieldOffset(1)]
  public byte Byte2;
  [FieldOffset(2)]
  public byte Byte3;
  [FieldOffset(3)]
  public byte Byte4;
  [FieldOffset(4)]
  public byte Byte5;
  [FieldOffset(5)]
  public byte Byte6;
  [FieldOffset(6)]
  public byte Byte7;
  [FieldOffset(7)]
  public byte Byte8;
  [FieldOffset(0)]
  public int Int1;
  [FieldOffset(4)]
  public int Int2;
}

One thing to be careful of is the endian-ness of the machine if you plan to run it on non-x86 platforms that may have differing endianness.

See http://en.wikipedia.org/wiki/Endianness for an explanation.