using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string text = "nameOf";

        string newText = text.FirstCharToUpper();

        Console.WriteLine(text + " => " + newText);

    }

}

​

public static class StringExtension

{

    public static string FirstCharToUpper(this string input)

    {

        switch (input)

        {

            case null: 

                throw new ArgumentNullException(nameof(input));

            case "": 

                throw new ArgumentException($"{nameof(input)} cannot be empty", nameof(input));

            default: 

                return input.First().ToString().ToUpper() + input.Substring(1);

        }

    }

}