How to generate a unique 8 digit number using c#

Random rand = new Random(100);
int ccc=  rand.Next(000000000, 999999999);

protected void form1_Load(object sender, EventArgs e)
{
   //Get the maximum ID from database
   int ID = //From Database.

   txtPostCode.Text=(ID + 1).ToString();

}

protected void btnSave_CLick(object sender,EventArgs e)
{
  // save button click

 // pass the text box value to database.
}

using System.Security.Cryptography; // Import this Dll
public string Get8Digits()
{
    var bytes = new byte[4];
    var rng = RandomNumberGenerator.Create();
    rng.GetBytes(bytes);
    uint random = BitConverter.ToUInt32(bytes, 0) % 100000000;
    return String.Format("{0:D8}", random);
}

DateTime.Now.ToString("MMddmmssff")