using System;

using System.Linq;

public class Program

{

    public const string FullNamePattern = "{0} {1}";

    public static void Main()

    {

        string firstName = "Pawan";

        string lastName = "Rahate";

        Console.WriteLine(FormatUsername(firstName, lastName));

    }

    public static string FormatUsername(string firstName, string lastName)

        {

            string fullName = string.Format(FullNamePattern,

                !string.IsNullOrWhiteSpace(firstName) ? firstName.Trim() : string.Empty,

                !string.IsNullOrWhiteSpace(lastName) ? lastName.Substring(0,1).Trim() : string.Empty);

            return fullName.Trim();

        }

}

​

​