.ToBytes(UInt64 value, Byte[] array, Int32 offset);
.ToBytes(UInt16 value, Byte[] array, Int32 offset);

public unsafe static byte[] GetBytes(long value)
{
    byte[] array = new byte[8];
    fixed (byte* ptr = array)
    {
            *(long*)ptr = value;
    }
    return array;
}

static unsafe void ToBytes(ulong value, byte[] array, int offset)
{
    fixed (byte* ptr = &array[offset])
        *(ulong*)ptr = value;
}

byte[] array = new byte[9];
ToBytes(0x1122334455667788, array, 1);

static void ToBytes(ulong value, byte[] array, int offset)
{
    byte[] valueBytes = BitConverter.GetBytes(value);
    Array.Copy(valueBytes, 0, array, offset, valueBytes.Length);
}

static void ToBytes(ulong value, byte[] array, int offset)
{
    for (int i = 0; i < 8; i++)
    {
        array[offset + i] = (byte)value;
        value >>= 8;
    }
}

static void ToBytes(ulong value, byte[] array, int offset) {
 unchecked {
  array[offset + 0] = (byte)(value >> (8*7));
  array[offset + 1] = (byte)(value >> (8*6));
  array[offset + 2] = (byte)(value >> (8*5));
  array[offset + 3] = (byte)(value >> (8*4));
  //...
 }
}

static class Extensions
{
    public static void ToBytes(this int n, byte[] destination, int offset)
    {
        var data = BitConverter.GetBytes(n);
        Array.Copy(data, 0, destination, offset, data.Length);
    }
}

byte[] destination = new byte[10];

123.ToBytes(destination,0);

public static void ToBytes(ulong value, byte[] array, int offset)
{
    GCHandle handle = GCHandle.Alloc(value, GCHandleType.Pinned);
    try
    {
        Marshal.Copy(handle.AddrOfPinnedObject(), array, offset, 8);
    }
    finally
    {
        handle.Free();
    }
}