Random number

Hello guys!

I need your help.
This is code behind of my net page (C#).

My problem is check that the random number does not exist in my table tbl_1 where is stored in the field Nr_random.
If the random number generate exist in my table I need generate new number, else register the first random number generate.

how can i do it?
any help?
thank you.

Code:
                            Random RandomClass = new Random();
                            int RandomNumber = RandomClass.Next(100000000, 999999999);
                            
                            string vowels = "aeiou";
                            string name = strEMail_user;
                            name = new string(name.Where(c => !vowels.Contains(c)).ToArray());
                            string[] strresult = name.Split('.');
                            string stroutput = strresult[1].Substring(0, 3) + " " + strresult[0].Substring(0, 3);
                            string Nr_random = RandomNumber + "-" + stroutput.Replace(" ", "").ToUpper();