C#

 // This program demonstrates string functions.
 
 using System;
 
 class Strings 
 {
     public static void Main (string[] args) 
     {
         String str = "Hello";
 
         Console.WriteLine("string: " + str);
         Console.WriteLine("string.ToLower(): " + str.ToLower());
         Console.WriteLine("string.ToUpper(): " + str.ToUpper());
         Console.WriteLine("string.IndexOf('e'): " + str.IndexOf('e'));
         Console.WriteLine("string.Length: " + str.Length);
         Console.WriteLine("string.Replace('H', 'j'): " + str.Replace('H', 'j'));
         Console.WriteLine("string(Substring(2, 2)): " + str.Substring(2, 2));
         Console.WriteLine("string.Trim(): " + str.Trim());
 
         String name = "Bob";
         double value = 123.456;
         Console.WriteLine(String.Format("{0} earned {1:$0.00}", name, value));
     }
 }

 // This program creates a file, adds data to the file, displays the file,
 // appends more data to the file, displays the file, and then deletes the file.
 // It will not run if the file already exists.
 //
 // References:
 //     https://www.mathsisfun.com/temperature-conversion.html
 //     https://en.wikibooks.org/wiki/C_Sharp_Programming
 
 using System;
 
 public class Files
 {
     public static void Main(String[] args)
     {
         string FILENAME = "~file.txt";
     
         if(System.IO.File.Exists(FILENAME))
         {
             System.Console.WriteLine("File already exists.\n");
         }
         else
         {
             CreateFile(FILENAME);
             ReadFile(FILENAME);
             AppendFile(FILENAME);
             ReadFile(FILENAME);
             DeleteFile(FILENAME);
         }
     }
     
     private static double CalculateFahrenheit(double celsius)
     {
         double fahrenheit;
 
         fahrenheit = celsius * 9 / 5 + 32;
         return fahrenheit;
     }
 
     private static void CreateFile(string filename)
     {
         System.IO.StreamWriter file;
         double celsius;
         double fahrenheit;
         
         try
         {
             file = System.IO.File.CreateText(filename);
             file.WriteLine("Celsius,Fahrenheit");
             for(celsius = 0; celsius <= 50; celsius++)
             {
                 fahrenheit = CalculateFahrenheit(celsius);
                 file.WriteLine(String.Format("{0:F1},{1:F1}", celsius, fahrenheit));
             }
             file.Close();
         }
         catch(Exception exception)
         {
             Console.WriteLine("Error creating " + filename);
             Console.WriteLine(exception.Message);
         }
     }
     
     private static void ReadFile(string filename)
     {
         System.IO.StreamReader file;
         string line;
 
         try
         {
             file = System.IO.File.OpenText(filename);
             while (true) 
             {
                 line = file.ReadLine();
                 if (line == null)
                 {
                     break;
                 }
                 Console.WriteLine(line);
             }
             file.Close();
             Console.WriteLine("");
         }
         catch(Exception exception)
         {
             Console.WriteLine("Error reading " + filename);
             Console.WriteLine(exception.Message);
         }
     }
     
     private static void AppendFile(string filename)
     {
         System.IO.StreamWriter file;
         double celsius;
         double fahrenheit;
         
         try
         {
             file = System.IO.File.AppendText(filename);
             for(celsius = 51; celsius <= 100; celsius++)
             {
                 fahrenheit = CalculateFahrenheit(celsius);
                 file.WriteLine(String.Format("{0:F1},{1:F1}", celsius, fahrenheit));
             }
             file.Close();
         }
         catch(Exception exception)
         {
             Console.WriteLine("Error appending to " + filename);
             Console.WriteLine(exception.Message);
         }
     }
     
     private static void DeleteFile(string filename)
     {
         try
         {
             System.IO.File.Delete(filename);
         }
         catch(Exception exception)
         {
             Console.WriteLine("Error deleting " + filename);
             Console.WriteLine(exception.Message);
         }
     }
 }