gistfile1.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;

namespace ConsoleApplication6
{
    public unsafe struct TheStruct
    {
        public int Field1;
        public ushort Field2;
        public ushort Field3;
        private fixed byte someString [2];

        public string SomeString
        {
            set
            {
                var bytes =   Encoding.ASCII.GetBytes(value);
                fixed(byte* ptr = someString)
                {
                    ptr[0] = bytes[0];
                    ptr[1] = bytes[1];
                }
            }
        }
    }
    unsafe class Program
    {
        static void Main(string[] args)
        {
            var truct = new TheStruct {Field1 = 42, Field2 = 4, Field3 = 2, SomeString = "42"};

            using (var stream = File.OpenWrite("C:\\temp\\file.bin"))
            {
                int size = Marshal.SizeOf(truct);

                byte* ptr = (byte*) &truct;
                for (int i = 0; i < size; i++, ptr++)
                {
                    stream.WriteByte(*ptr);
                }
            }
        }
    }
}