In-Class Activity for Pair Programming (& Exception Handling)
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     (courtesy of Ms. Coffman-Wolph, minor changes by Dr. Lilien)

Skills/Goals of this activity:
• Practice Windows Form Application programming
• Practice using MessageBox
• Practice exception handling (try/catch blocks)
• Learn about user-defined exceptions 
• >>> Practice pair programming <<<
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Create a small Windows Form application that does the following: 
After a user enters a number in the textbox and clicks the button,
the program will check the value entered for two things:
• Is it a number?
• Is it a number in the 0-9 range?

Requirements for Exceptions:
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• Must use try/catch blocks for FormatException.
• Must use a user-defined exception for making sure the number is in the
  0-9 range.

Steps:
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1) Create a form that contains a textbox and a button.

2) Create a class called TextboxFormatException that inherites from class Exception.

   2a) Note:  Class Exception already exists in C# so you do not need to create one.

   2b) This class has no variables, but contains constructor(s).  You should at least
       create a default constructor that calls the base constructor.

       • The base constructor will need 1 string parameter:  this string should
         contain information regarding the error (for example:  “Textbox format
         is incorrect.  Must be a number 0-9”).

3. Add code to the form to do the following:

   3a) Inside a try/catch block

       • Attempt to convert the information from the textbox into an integer.
         - You will need to catch a FormatException.
         - In the catch, you should clear the textbox and display a MessageBox
           that alerts the user that a formatting error has occured.

       • Check if the number is in the range of 0-9.
         - If the number is not inside this range, then you should clear the
           textbox and throw a TextboxRangeException.
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