Книга: C# 2008 Programmer

Handling Exceptions Using the try-catch Statement

Handling Exceptions Using the try-catch Statement

In C#, you can use the try-catch statement to enclose a block of code statements that may potentially cause exceptions to be raised. You enclose these statements within the catch block and that block to catch the different types of exceptions that may occur.

Using the previous example, you can enclose the statements that ask the user to input num1 and num2 and then performs the division within a try block. You then use the catch block to catch possible exceptions, like this:

static void Main(string[] args) {
 int num1, num2, result;
 try {
  Console.Write("Please enter the first number:");
  num1 = int.Parse(Console.ReadLine());
  Console.Write("Please enter the second number:");
  num2 = int.Parse(Console.ReadLine());
  result = num1 / num2;
  Console.WriteLine("The result of {0}/{1} is {2}", num1, num2, result);
 } catch (Exception ex) {
  Console.WriteLine(ex.Message);
 }
 Console.ReadLine();
}

The Exception class is the base class for all exceptions; that is, it catches all the various types of exceptions. The class contains the details of the exception that occurred, and includes a number of properties that help identify the code location, the type, the help file, and the reason for the exception. The following table describes these properties.

Property Description
Data Gets a collection of key/value pairs that provide additional user-defined information about the exception.
HelpLink Gets or sets a link to the help file associated with this exception.
HResult Gets or sets HRESULT, a coded numerical value that is assigned to a specific exception.
InnerException Gets the Exception instance that caused the current exception.
Message Gets a message that describes the current exception.
Source Gets or sets the name of the application or the object that causes the error.
StackTrace Gets a string representation of the frames on the call stack at the time the current exception was thrown.
TargetSite Gets the method that throws the current exception.

In the preceding program, if you type in a numeric value for num1 and then an alphabetical character for num2, the exception is caught and displayed like this:

Please enter the first number:6
Please enter the second number:a
Input string was not in a correct format.

If, though, you enter 0 for the second number, you get a different description for the error:

Please enter the first number:7
Please enter the second number:0
Attempted to divide by zero.

Notice that two different types of exceptions are caught using the same Exception class. The description of the exception is contained within the Message property of the Exception class.

You can use the ToString() method of the Exception class to retrieve more details about the exception, such as the description of the exception as well as the stack trace.

However, there are cases where you would like to print your own custom error messages for the different types of exceptions. Using the preceding code, you would not be able to do that — you would need a much finer way to catch the different types of possible exceptions.

To know the different types of exceptions that your program can cause (such as entering "a" for num1 or division by zero), you can set a breakpoint at a line within the catch block and try entering different values. When an exception is raised during runtime, IntelliSense tells you the error and the type of the exception raised. Figure 12-2 shows that the FormatException exception is raised when you enter a for num1.


Figure 12-2

If you are not sure what type of exception your program is going to raise during runtime, it is always safe to use the base Exception class. If not — if the exception that is raised does not match the exception you are trying to catch — a runtime error will occur. Here's an example:

static void Main(string[] args) {
 int num1, num2, result;
 try {
  Console.Write("Please enter the first number:");
  num1 = int.Parse(Console.ReadLine());
  Console.Write("Please enter the second number:");
  num2 = int.Parse(Console.ReadLine());
  result = num1 / num2;
  Console.WriteLine("The result of {0}/{1} is {2}", num1, num2, result);
 } catch (DivideByZeroException ex) {
  Console.WriteLine(ex.Message);
 }
 Console.ReadLine();
}

If a division-by-zero exception occurs (entering 0 for num2), the exception is caught. However, if you enter an alphabetic character for num1 or num2, a FormatException exception is raised. And because you are only catching the DivideByZeroException exception, this exception goes unhandled and a runtime error results.

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