private static string GetUniqueKey()
{
  int maxSize = 8;
  int minSize = 5;
  char[] chars = new char[62];
  string a;
  a = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
  chars = a.ToCharArray();
  int size = maxSize;
  byte[] data = new byte[1];
  RNGCryptoServiceProvider crypto = new RNGCryptoServiceProvider();
  crypto.GetNonZeroBytes(data);
  size = maxSize;
  data = new byte[size];
  crypto.GetNonZeroBytes(data);
  StringBuilder result = new StringBuilder(size);
  foreach (byte b in data)
  {
    result.Append(chars[b % (chars.Length - 1)]);
  }
  return result.ToString();
}

for (int i = 0; i < 10; i++)
{
  string str = string.Format("{0}{1}", DateTime.Now.ToString("yyyyMMddHHmmss"), GetUniqueKey());
  Console.WriteLine(str);
}
Console.ReadKey();