Throwing an Exception

Introduction

As mentioned previously, the Exception class is equipped with a Message property that carries a message for the error that occurred. We also mentioned that the message of this property may not be particularly useful to a user. Fortunately, you can create your own message and pass it to an Exception object. To be able to receive custom messages, the Exception class provides the following constructor:

public Exception(string message);

As mentioned already, all exceptions classes derive directly or indirectly from Exception. As a result, all thoses classes are equipped with a default constructor and all of them inherit the constructor that takes a string as argument.

To use the constructor that uses a string parameter, in the section in which you are anticipating the error, type the throw keyword followed by a new instance of the Exception class using this constructor. Here is an example:

@{
    ViewBag.Title = "Calculations";
}

<h2>Calculations</h2>

@{
    string message = "";
    double result = 0.00;
    string oper = "";
    double number1 = 0.00, number2 = 0.00;


    if (IsPost)
    {
        try
        {
            number1 = double.Parse(Request["txtNumber1"]);
            number2 = double.Parse(Request["txtNumber2"]);
            oper = Request["txtNumber2"];

            if( (oper != "+") && (oper != "-") && (oper != "*") && (oper != "/") )
            {
                throw new Exception(oper);
            }

            switch (oper)
            {
                case "+":
                    result = number1 + number2;
                    break;
                case "-":
                    result = number1 - number2;
                    break;
                case "*":
                    result = number1 * number2;
                    break;
                case "/":
                    result = number1 / number2;
                    break;
                default:
                    message = "Bad Operation";
                    break;
            }

        }
        catch (Exception exc)
        {
            message = "The value you entered is not a valid number.";
        }
    }
}

@using (Html.BeginForm())
{
    <table>
        <tr>
            <td>Number 1:</td>
            <td>@Html.TextBox("txtNumber1", @number1)</td>
        </tr>
        <tr>
            <td>Operator:</td>
            <td>@Html.TextBox("txtOperator", @oper)</td>
        </tr>
        <tr>
            <td>Number 2:</td>
            <td>@Html.TextBox("txtNumber2", @number1)</td>
        </tr>
        <tr>
            <td>&nbsp;</td>
            <td><input type="submit" name="btnCalculate" value="Calculate" /></td>
        </tr>
        <tr>
            <td>Result:</td>
            <td>@Html.TextBox("txtResult", @result)</td>
        </tr>
    </table>

    <p>@message</p>
}

Practical Learning: Throwing an Exception

1. Start Microsoft Visual Studio. On the Visual Studio 2019 dialog box, click Create a New Project (if Microsoft Visual Studio was already opened, on the main menu of, click File -> New -> Project...)
2. In the list of projects templates, click Windows Forms App (.NET Framework)
3. Click Next
4. Change the project Name to Algebra1
5. Click Create
6. Design the form as follows:

   

   Control	(Name)	Text	Other Properties
   Label		Number 1
   Label		Operation
   Label		Number 2
   Label		Result
   Label		Operation
   TextBox	txtNumber1
   ComboBox	cbxOperators		Items: + - * /
   TextBox	txtNumber2
   Button	btnCalculate	Calculate
   TextBox	txtResult

7. Double-click the = button
8. Change the document as follows:

   using System;
   using System.Windows.Forms;
   
   namespace Algebra1
   {
       public partial class Form1 : Form
       {
           public Form1()
           {
               InitializeComponent();
           }
   
           private void btnCalculate_Click(object sender, EventArgs e)
           {
               try
               {
                   double result = 0.00;
                   double number1 = double.Parse(txtNumber1.Text);
                   double number2 = double.Parse(txtNumber2.Text);
                   string oper = cbxOperators.Text;
   
                   if ((oper != "+") && (oper != "-") && (oper != "*") && (oper != "/"))
                   {
                       throw new Exception(oper);
                   }
   
                   switch (oper)
                   {
                       case "+":
                           result = number1 + number2;
                           break;
                       case "-":
                           result = number1 - number2;
                           break;
                       case "*":
                           result = number1 * number2;
                           break;
                       case "/":
                           result = number1 / number2;
                           break;
                       default:
                           MessageBox.Show("Bad Operation", "Elementary Algebra");
                           break;
                   }
   
                   txtResult.Text = result.ToString();
               }
               catch (Exception)
               {
                   MessageBox.Show("The value you entered is not a valid number.",
                                   "Elementary Algebra");
               }
           }
       }
   }

9. To execute the project, on the main menu, click Debug -> Start Without Debugging

   

10. Click the = button:

    

11. In the Number 1 text box, type a number such as 29647.97
12. In the Operator combo box, select +
13. In the Number 2 text box, type a number such as 158.36
14. Click the = button:

    

15. In the Operator combo box, select one of the options and click the = button:

    

    

    

16. Close the form and return to Microsoft Visual Studio
17. On the main menu of Microsoft Visual Studio, click File -> New -> Project...
18. In the list of project templates of the Create a New Project dialog box, click Windows Forms App (.NET Framework)
19. On the main menu of Microsoft Visual Studio, click File -> New -> Project...
20. Click Next
21. Change the Name of the project to PledgeDistribution2
22. Click Create
23. Design the form as follows:

    

    Control	(Name)	Text	TextAlign
    Label		Organization for Fundraising Events
    Label		__________________________________
    Label		Two-Way Campaign Distribution
    Label		Amount to Allocate:
    TextBox	txtAllocation
    Label		In the Ratio:
    TextBox	txtPortion1		Center
    Label		:
    TextBox	txtPortion2		Center
    Button	btnCalculate	Calculate
    Label		__________________________________
    Label		Part 1 Receives:
    TextBox	txtPart1
    Label		Part 2 Receives:
    TextBox	txtPart2

24. Double-click the Calculate button
25. Change the document as follows:

    using System;
    using System.Windows.Forms;
    
    namespace PledgeDistribution2
    {
        public partial class Form1 : Form
        {
            public Form1()
            {
                InitializeComponent();
            }
    
            (int, int) DistributeParts(int amount, int portion1, int portion2)
            {
                int totalRatios = portion1 + portion2;
                int eachPart    = amount / totalRatios;
                int result      = eachPart * portion1;
    
                return (result, amount - result);
            }
    
            private void btnCalculate_Click(object sender, EventArgs e)
            {
                try
                {
                    int amtPledged = int.Parse(txtAllocation.Text);
                    int ratio1     = int.Parse(txtPortion1.Text);
                    int ratio2     = int.Parse(txtPortion2.Text);
    
                    (int first, int second) portions = DistributeParts(amtPledged, ratio1, ratio2);
    
                    txtPart1.Text = portions.first.ToString();
                    txtPart2.Text = portions.second.ToString();
                }
                catch (FormatException)
                {
                    MessageBox.Show("The value you entered for the amount to allocate " +
                                    "or pledge is not valid. Only natural numbers " +
                                    "(such as 200, 1250, 5000, or 100000) are allowed.",
                                    "Organization for Fundraising Events");
                }
            }
        }
    }

26. To execute the project, on the main menu, click Debug -> Start Without Debugging:

    

27. Set the values as follows:

    Amount to Allocate: 8500
    In the Ratio:       4:3

    

28. Click the Calculate button

    

29. Change the ratio to 0:0
30. Click the Calculate utton

    

31. Click the Quit button to close the form and return to your programming environment

Exceptions and Methods

There are various ways you can involve exceptions in a function or method. When it comes to functions and methods, one way you can use the throw option is to get out of the function or method at any time, for any reason. Consider the following code:

using System;

class Exercise
{
    public void ShowMessage()
    {
        throw new Exception();

        MessageBox.Show("This is the wonderful world of C# programming.");
    }
}

In this example, the line that holds the message box will never be reached. One way you can proceed in a function or method is to throw an exception if something happens. That is, you can create a conditional statement that would consider, or throw an exception.

Practical Learning: Throwing an Exception from a Method

1. Change the DistributeParts() method as follows:
   

   using System;
   using System.Windows.Forms;
   
   namespace PledgeDistribution2
   {
       public partial class Form1 : Form
       {
           public Form1() => InitializeComponent();
   
           (int, int) DistributeParts(int amount, int portion1, int portion2)
           {
               int totalRatios = portion1 + portion2;
               
               if (totalRatios == 0)
                   throw new FormatException("The value provided for one or both ratios is not valid.");
   
               int eachPart = amount / totalRatios;
               int result   = eachPart * portion1;
   
               return (result, amount - result);
           }
   
           private void btnCalculate_Click(object sender, EventArgs e)
           {
               try
               {
                   int amtPledged = int.Parse(txtAllocation.Text);
                   int ratio1     = int.Parse(txtPortion1.Text);
                   int ratio2     = int.Parse(txtPortion2.Text);
   
                   (int first, int second) portions = DistributeParts(amtPledged, ratio1, ratio2);
   
                   txtPart1.Text = portions.first.ToString();
                   txtPart2.Text = portions.second.ToString();
               }
               catch (FormatException)
               {
                   MessageBox.Show("The value you entered for the amount to allocate " +
                                   "or pledge is not valid. Only natural numbers " +
                                   "(such as 200, 1250, 5000, or 100000) are allowed.",
                                   "Organization for Fundraising Events");
               }
           }
       }
   }

2. To execute the project, on the main menu, click Debug -> Start Without Debugging
3. Click the Calculate button
4. Change the values as follows:

   Amount to Allocate: 8500
   In the Ratio:       0:0

5. Click the Calculate utton

   

6. Click OK on the message box
7. Close the form and return to your programming environment
8. One of the characteristics of a function is that it returns a value, which is done using the return keyword. Therefore, if a calculation made in a function may produce an error, you can create a conditional statement, such as an if...else or if...else if...else scenario where the last else is a throw condition.
   For an example, change the document as follows:

   using System;
   using System.Windows.Forms;
   
   namespace PledgeDistribution2
   {
       public partial class Form1 : Form
       {
           public Form1() => InitializeComponent();
   
           int DistributeFirstPart(int amount, int portion1, int portion2)
           {
               if (portion1 != 0)
               {
                   int totalRatios = portion1 + portion2;
   
                   int eachPart = amount / totalRatios;
                   int result = eachPart * portion1;
   
                   return result;
               }
               else
                   throw new FormatException("The value provided for the first ratio portion is not valid.");
           }
   
           int Subtract(int first, int second)
           {
               if (first == 0)
                   throw new FormatException("The value provided for the amount to pledge must be a positive number.");
   
               return first - second;
           }
   
           private void btnCalculate_Click(object sender, EventArgs e)
           {
               try
               {
                   int amtPledged = int.Parse(txtAllocation.Text);
                   int ratio1     = int.Parse(txtPortion1.Text);
                   int ratio2     = int.Parse(txtPortion2.Text);
   
                   int part1 = DistributeFirstPart(amtPledged, ratio1, ratio2);
                   int part2 = Subtract(amtPledged, part1);
   
                   txtPart1.Text = part1.ToString();
                   txtPart2.Text = part2.ToString();
               }
               catch (FormatException)
               {
                   MessageBox.Show("The value you entered for the amount to allocate " +
                                   "or pledge is not valid. Only natural numbers " +
                                   "(such as 200, 1250, 5000, or 100000) are allowed.",
                                   "Organization for Fundraising Events");
               }
           }
       }
   }

9. To execute the project, on the main menu, click Debug -> Start Without Debugging
10. In the Amount to Allocate text box, type a currency number such as 5000
11. In the Ratio text box, type 0
12. Click the Allocate button

    

13. Click OK on the message box
14. Close the form and return to your programming environment
15. On the main menu of Microsoft Visual Studio, click File -> Recent Projects and Solutions -> ... PledgeDistribution2.sln

Exceptions and Properties

If you create a non-automatic property that processes some data and returns a valid result, if there is a possibility that something could go wrong in the calculation, you can make the property throw an exception. You can perform some procesing in the set clause of the property. In that clause, you can throw an exception. Here is an example:

using System;

public class WorkDay
{
    private double tw;

    public WorkDay(double time)
    {
        tw = time;
    }

    public double TimeWorked
    {
        get
        {
            return tw;
        }
        set
        {
            if (tw > 24)
                throw new FormatException("Nobody can work more than 24 hours in one day.");

            tw = value;
        }
    }
}

Or you can handle the exception in the get clause.

Topics on Handling Exceptions

Catching Various Exceptions

In the above examples, when we anticipated some type of problem, we instructed the compiler to use our default catch section. We left it up to the compiler to find out when there was a problem and we provided a catch section to deal with it. A secttion of code with numerous or complex operations and requests can also produce different types of errors. You should be able to face different problems and deal with them individually, each by its own kind. To do this, you can create different catch sections, each made for a particular error. The formula to follow is:

try {
	// Code to Try
}
catch(Arg1)
{
	// One Exception
}
catch(Arg2)
{
	// Another Exception
}

The compiler would proceed from top-down:

1. Following the normal flow of the program, the compiler enters the try block
2. If no exception occurs in the try block, the rest of the try block executes.
   If an exception occurs in the try block, the compiler registers the type of error that occurred. If there is a throw line, the compiler registers it also:
   1. The compiler gets out of the try section
   2. The compiler examines the first catch section. If the first catch clause matches the thrown error, that catch section executes and the exception handling routine may end. If the first catch doesn't match the thrown error, the compiler proceeds with the next catch clause
   3. The compiler checks the next match, if any, and proceeds as in the first catch. This continues until the compiler finds a catch clause that matches the thrown error
   4. If one of the catch clauses matches the thrown error, its body executes. If no catch clause matches the thrown error, the compiler calls the Exception class and uses the default message

Multiple catch clauses are written if or when a try block is expected to throw different types of errors. For example, in our calculator, we want to consider only the addition, the subtraction, the multiplication, and the division. It is also likely that the user may type one or two invalid numbers. This leads us to know that our project can produce at least two types of errors. Based on this, we can address them using two catch clauses.

Obviously various bad things could happen when a form is used. Imagine that the user wants to perform a division. You need to tell the webpage what to do if the user enters the denominator as 0 (or 0.00). If this happens, one of the options you should consider is to display a message and get out. Fortunately, the .NET Framework provides the DivideByZeroException class to which we were introduced in the previous lesson to deal with an exception caused by division by zero. As done with the message passed to the Exception class, you can compose your own message and pass it to the DivideByZeroException(string message) constructor.

Practical Learning: Throwing Various Exceptions

1. Change the document as follows:

   using System;
   using System.Windows.Forms;
   
   namespace Algebra1
   {
       public partial class Form1 : Form
       {
           public Form1() => InitializeComponent();
   
           private void btnCalculate_Click(object sender, EventArgs e)
           {
               try
               {
                   double result = 0.00;
                   double number1 = double.Parse(txtNumber1.Text);
                   double number2 = double.Parse(txtNumber2.Text);
                   string oper = cbxOperators.Text;
   
                   if ((oper != "+") && (oper != "-") && (oper != "*") && (oper != "/"))
                   {
                       throw new Exception(oper);
                   }
   
                   switch (oper)
                   {
                       case "+":
                           result = number1 + number2;
                           break;
                       case "-":
                           result = number1 - number2;
                           break;
                       case "*":
                           result = number1 * number2;
                           break;
                       case "/":
                           result = number1 / number2;
                           break;
                       default:
                           MessageBox.Show("Bad Operation", "Elementary Algebra");
                           break;
                   }
   
                   txtResult.Text = result.ToString();
               }
               catch (FormatException)
               {
                   MessageBox.Show("You type an invalid number. Please correct it",
                                   "Elementary Algebra");
               }
               catch (Exception ex)
               {
                   MessageBox.Show("Operation Error: " + ex.Message + " is not a valid operator",
                                   "Elementary Algebra");
               }
           }
       }
   }

2. To exectute, on the main menu of Microsoft Visual Studio, click Debug -> Start Without Debugging
3. Close the form and return to Microsoft Visual Studio

Exception Filters

Sometimes you will have different controls that may throw the same type of exception. The problem is that these controls may present the same error message. For example, if you have more than two text boxes that expect numeric values, if a valid value is not provided, each of the controls would throw a FormatException exception. The problem is that when one of those controls throws the exception, you may need a way to either know what control threw the exception or what exception was thrown.

Filtering an exception consists of specifying a criterion by which a particular exception, among others, would be selected. The C# language provides a mechanism that allows you to get more details about an exception. To start, you must use a technique that would be applied to each particular error. One way to do this is to use the throw keyword to create a custom message when an exception is thrown. To help you select an exception, the C# language provides the when keyword. The formula to use it is:

try
{
	. . .
}
catch(exception-parameter) when (condition)
{
	. . .
}

In the catch clause, pass an exception-parameter as we have done so far. This is followed by the when keyword that is used as in an if() conditional statement. In its parentheses, provide a logical expression that can evaluate to true or false. If that expression is true, the body of the catch clause would execute.

Nesting an Exception

You can create an exception inside of another. This is referred to as nesting an exception. This is done by applying the same techniques we used to nest conditional statements. This means that you can write an exception that depends on, and is subject to, another exception. To nest an exception, write a try block in the body of the parent exception. The nested try block must be followed by its own catch clause(s). To effectively handle the exception(s), make sure you include an appropriate throw in the try block.

Practical Learning: Nesting an Exception

1. Change the document as follows:

   using System;
   using System.Windows.Forms;
   
   namespace Algebra1
   {
       public partial class Form1 : Form
       {
           public Form1() => InitializeComponent();
   
           private void btnCalculate_Click(object sender, EventArgs e)
           {
               try
               {
                   double result = 0.00;
                   double number1 = double.Parse(txtNumber1.Text);
                   double number2 = double.Parse(txtNumber2.Text);
   
                   if ((cbxOperators.Text != "+") && (cbxOperators.Text != "-") && (cbxOperators.Text != "*") && (cbxOperators.Text != "/"))
                       throw new Exception(cbxOperators.Text);
   
                   switch (cbxOperators.Text)
                   {
                       case "+":
                           result = number1 + number2;
                           break;
                       case "-":
                           result = number1 - number2;
                           break;
                       case "*":
                           result = number1 * number2;
                           break;
                       case "/":
                           try
                           {
                               if (number2 == 0)
                                   throw new DivideByZeroException("Division by zero is not allowed");
   
                               result = number1 / number2;
                           }
                           catch (DivideByZeroException ex)
                           {
                               MessageBox.Show(ex.Message, "Elementary Algebra");
                           }
                           break;
                       default:
                           MessageBox.Show("Bad Operation", "Elementary Algebra");
                           break;
                   }
   
                   txtResult.Text = result.ToString();
               }
               catch (FormatException) {
                   MessageBox.Show("You type an invalid number. Please correct it",
                                   "Elementary Algebra");
               }
               catch (Exception ex) {
                   MessageBox.Show("Operation Error: " + ex.Message + " is not a valid operator",
                                   "Elementary Algebra");
               }
           }
       }
   }

2. To exectute, on the main menu of Microsoft Visual Studio, click Debug -> Start Without Debugging
3. Close the form and return to Microsoft Visual Studio
4. Close your programming environment

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