Generate Unique id Alphanumeric

try this

using System;
					
public class Program
{
	public static void Main()
	{
		string alphabets = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    	string small_alphabets = "abcdefghijklmnopqrstuvwxyz";
        string numbers = "1234567890";
 
		string characters = string.Empty;
		characters += alphabets + small_alphabets + numbers;
		string otp = string.Empty;
		for (int i = 0; i < 15; i++)
		{
			string character = string.Empty;
			do
			{
				int index = new Random().Next(0, characters.Length);
				character = characters.ToCharArray()[index].ToString();
			} while (otp.IndexOf(character) != -1);
			otp += character;
		}
		Console.WriteLine(otp);
	}
}

public static string GenerateUniqueId()
{
    DateTime now = DateTime.UtcNow;

    Random random = new Random();
    int randomNumber = random.Next(100000, 999999);

    string uniqueIdString = string.Format("{0:yyyyMMddHHmmss}{1}", now, randomNumber);

    SHA256 sha256 = SHA256.Create();
    byte[] hashBytes = sha256.ComputeHash(Encoding.UTF8.GetBytes(uniqueIdString));
    string hashString = BitConverter.ToString(hashBytes).Replace("-", "").ToLower();

    return hashString.Substring(0, 20);
}

Call method as below

var uniqueId =   GenerateUniqueId();

Sir I Want Genrerate Alphanumeric Like as (Re786hKhsoj@hl*kjguL) In this Formation.

Hello,

you can use Guid to generate uniq id,

string uniqueId = Guid.NewGuid().ToString();

OR

string uniqueID = string.Format("{0}{1}",
    Guid.NewGuid().ToString("N").Substring(0, 5),
    DateTime.Now.ToString("yyyyMMddHHmmssfff"));

// uniqueID will be a 19 character alphanumeric string, like:
// "3cd3a202108091313432"