using System.Runtime.InteropServices;
全部書くとこんな感じです

namespace DeviceApplication1
{
public partial class Form1 : Form
{
[StructLayout(LayoutKind.Explicit)]
public unsafe struct UNION {
[FieldOffset(0)]　public fixed byte aaa[2];
[FieldOffset(0)]　public ushort bbb;
　　　　};
unsafe UNION ccc;

private void Form1_Load(object sender, EventArgs e)
{
unsafe
{
ccc.aaa[0] = 0x12;
}
　　　　}