byte[] tableauByte = new byte[] { 0x99 };
IntPtr unmanagedPointer = Marshal.AllocHGlobal(tableauByte.Length);
Marshal.Copy(tableauByte, 0, unmanagedPointer, tableauByte.Length);
try
{
  // Utilisation de unmanagedPointer
}
finally
{
    Marshal.FreeHGlobal(unmanagedPointer);
}