using System;    

using System.Text;  

using System.Linq;

public class Program

{

    public static void Main()

    {

        Program p = new Program();

        //choose 3 characters prefix

        string Prefix = "NIX";

        Console.WriteLine("RequestID: " + p.GenerateRequestID(Prefix));

    }

    private string GenerateRequestID(string Prefix)

    {

        StringBuilder builder = new StringBuilder();

         Enumerable

       .Range(65, 26)

        .Select(e => ((char)e).ToString())

        .Concat(Enumerable.Range(97, 26).Select(e => ((char)e).ToString()))

        .Concat(Enumerable.Range(0, 10).Select(e => e.ToString()))

        .OrderBy(e => Guid.NewGuid())

        .Take(13)

        .ToList().ForEach(e => builder.Append(e));

        return Prefix + builder.ToString(); 

    }

}

​