Create an application class named LetterDemo that instantiates objects of two classes named Letter and CertifiedLetter and that demonstrates all their methods.

The classes are used by a company to keep track of letters they mail to clients. The Letter class includes auto-implemented properties for the Name of the recipient and the Date mailed (stored as strings).

Next, include a ToString() method that overrides the Object classs ToString() method and returns a string that contains the name of the class (using GetType()) and the Letters data field values.

Create a child class named CertifiedLetter that includes an auto-implemented property TrackingNumber (of type string) that holds a tracking number for the letter.

What I have:

using System; //application namespace namespace LetterDemo { class Letter // definition of class Letter { // auto-implemented properties public string Name { get; set; } public string Date { get; set; } // override ToString public override string ToString() { string output =String.Format("Class Name: {0} | Recipient" +" Name: {1} | Date: {2} ",this.GetType(), Name, Date); return output;//return output } } // derived class class CertifiedLetter : Letter { // Auto-implemented property for tracking number public int TrackingNumber { get; set; } // override the ToString method public override string ToString() { string output =String.Format("Class Name: {0} | Recipient" +" Name: {1} | Date: {2} | " +"Tracking Number: {3} ",this.GetType(), Name, Date,TrackingNumber); return output; } } class Program //Test class { //entry point of application , main method static void Main(string[] args) { // create object of Letter Letter L = new Letter(); L.Name = "John"; //set Name L.Date = "12 November 2017"; //set Date // create object of CertifiedLetter CertifiedLetter CL = new CertifiedLetter(); CL.Name = "Mark";//set Name CL.Date = "11 November 2017"; //set Date CL.TrackingNumber = 123121; Console.WriteLine(L.ToString());//print details Console.WriteLine(CL.ToString());//print details Console.ReadKey();//used to hold the screen

Error I get:

Compilation failed: 4 error(s), 0 warnings NtTestf891301d.cs(10,8): error CS0246: The type or namespace name `Letter' could not be found. Are you missing `LetterDemo' using directive? NtTestf891301d.cs(11,8): error CS0841: A local variable `letter' cannot be used before it is declared NtTestf891301d.cs(12,8): error CS0841: A local variable `letter' cannot be used before it is declared NtTestf891301d.cs(13,24): error CS0841: A local variable `letter' cannot be used before it is declared