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When a bad behavior occurs in your application, the program
is said to throw an exception. To customize the throwing of an exception, in the section
of code where you are anticipating the
error, type the throw keyword followed by a new instance of the Exception
class (or one of its derived classes) using the constructor that takes a string. Here is an example:
<script runat="server">
private void btnCalculateClick(object sender, EventArgs e)
{
double side;
double perimeter, area;
try {
side = double.Parse(txtSide.Text);
throw new Exception(side.ToString());
perimeter = side * 4;
area = side * side;
txtPerimeter.Text = perimeter.ToString();
txtArea.Text = area.ToString();
}
catch(Exception ex)
{
lblMessage.Text = "The value you typed, " + ex.Message + " is not valid.";
}
}
</script>
In reality, to effectively throw an exception, you should
write a conditional that specifies under what circumstance(s) the exception
should be thrown.
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Catching Various Exceptions
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A function or a method with numerous or complex
operations and requests can also produce different types of
errors. To catch various exceptions, you can create different catch sections, each made for a
particular error. The formula used would be:
try {
// Code to Try
}
catch(Arg1)
{
// One Exception
}
catch(Arg2)
{
// Another Exception
}
This makes it to create a catch block for each type
of exception. In the same way, you can write
different throw expressions if the code can produce different types of errors.
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 Practical Learning: Catching Various Exceptions
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- Click the square.aspx.cs tab
- To catch various exceptions, change the file as follows:
protected void btnCalculate_Click(object sender, EventArgs e)
{
double side = 0.00;
double perimeter, area;
try
{
side = double.Parse(txtSide.Text);
perimeter = side * 4;
area = side * side;
txtPerimeter.Text = perimeter.ToString();
txtArea.Text = area.ToString();
}
catch(FormatException)
{
lblMessage.Text = "You typed an invalid value for the side.";
}
catch (Exception)
{
lblMessage.Text = "Something bad happened to the web page. " +
"Contact the webmaster";
}
}
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- Click the index.cs tab
- Press Ctrl + F5 to test the web site
- Click the Square link
- In the Side text box, type something
- Click Calculate
- Return to your programming environment
You can create an exception inside of another. This
is referred to as nesting an 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(es) clause. To effectively handle the exception, make sure you include an
appropriate throw in the try block.
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Practical Learning:
Nesting Exceptions |
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- Click the square.aspx.cs tab
- To catch various exceptions, change the file as follows:
protected void btnCalculate_Click(object sender, EventArgs e)
{
double length = 0.00;
double height = 0.00;
double perimeter;
double area;
try
{
try
{
length = double.Parse(txtLength.Text);
}
catch (FormatException)
{
lblMessage.Text = "You typed an invalid value for the length.";
}
try
{
height = double.Parse(txtHeight.Text);
}
catch (FormatException)
{
lblMessage.Text = "You typed an invalid value for the height.";
}
perimeter = (length + height) * 2;
area = length * height;
txtPerimeter.Text = perimeter.ToString();
txtArea.Text = area.ToString();
}
catch
{
lblMessage.Text = "Something bad happened to the web page. " +
"Contact the webmaster";
}
}
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- Save all
- Return to the browser and refresh
- Click the Rectangle link
- In the Length text box, type 42.60
- In the Height text box, type 35.85
- Click Calculate
- Change the value in the Length text box to 42Period60
- Click Calculate
- Return to your programming environment
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Exceptions and Functions or Methods
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One of the most effective techniques used to deal
with code is to isolate assignments. Isolating assignments and handing them to method is an important matter in the area of application
programming. One of the ways you can use functions or methods in exception
routines is to have a central method that receives variables, and sends
them to other external methods. The external method tests the value of a
variable. If an exception occurs, the external method displays or
sends a throw. This throw can be picked up by the method that sent the
error.
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Creating an Exceptional Class
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The Exception class of the .NET Framework is a great
tool for handling exceptions. To deal with
particular errors, various classes are derived from Exception. If
for some reason neither the Exception class nor any of the Exception-based classes fulfills your requirement, you can derive a new
class from Exception or from one of the available Exception-based
classes.
To derive a class from Exception or from one of its
classes, simply follow the rules of class inheritance.
Here is an example of a class based on Exception:
public class CustomException : Exception
{
}
After deriving the class, you can add the necessary members
as you see fit. Remember that the primary characteristic of an exception is to
present a message to the user. In Exception-based classes, this message is
represented by the Message property. Therefore, if you want to prepare a custom
message for your class, you can override or new this property.
Once you have created and customized your exceptional class,
you can use it the same way you would another exception class, such as throwing
it.
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