Sams Teach Yourself C# 5.0 in 24 Hours (2013)

---

Try It Yourself: Working with Exceptions

To see how you can work with exceptions by throwing and catching several different exceptions, follow these steps:

1. Create a new console application.

2. In the Program.cs file, implement the following functions:

private static void ThrowsException()
{
Console.WriteLine("About to throw an InvalidOperationException");
throw new InvalidOperationException();
}

private static void PrintString(string message)
{
if (message == null)
{
throw new ArgumentNullException("message");
}

Console.WriteLine(message);
}

3. In the Main method of the Program.cs file, call the ThrowsException method.

4. Run the application using Ctrl+F5. You see the console window initially, as shown in Figure 11.1.

Figure 11.1. Console application just before throwing an exception.

If you run Windows Vista or later, you might see the dialog box shown in Figure 11.2.

Figure 11.2. Windows application failure dialog box checking for solutions.

Express for Desktops does not support JIT Debugging, so after this dialog box completes, you see the exception information displayed, as shown in Figure 11.3.

Figure 11.3. Runtime exception generated from the lack of type safety.

The information displayed is the exception information and indicates that this was an unhandled exception of type System.InvalidOperationException. It also displays the stack trace information, which includes the method call stack, filename, and line numbers. This stack trace information helps you to identify the location in your code where the exception occurred.

If you have one of the Visual Studio with MSDN editions installed, you see a dialog box, as shown in Figure 11.4, enabling you to close or debug the application.

Figure 11.4. Windows application failure dialog box closing the application.

5. If you press the Close program button, the console window should look like Figure 11.3. If you press the Debug button, you see the Visual Studio Just-In-Time Debugger dialog box.

6. Run the application again using Ctrl+F5, but this time press the Debug button when the dialog box shown in Figure 11.3 appears. This displays the Visual Studio Just-In-Time Debugger dialog box, as shown in Figure 11.5.

Figure 11.5. Visual Studio Just-In-Time Debugger dialog box.

If you press the Yes button, control returns to Visual Studio with the line containing the exception set to the current statement and the Exception Assistant dialog box displayed, as shown in Figure 11.6.

Figure 11.6. Exception Assistant.

By clicking the View Detail link under Actions, you can see the details of the exception, as shown in Figure 11.7.

Figure 11.7. Exception details dialog box.

7. Enclose the call to ThrowsException in a try/catch block that specifically catches an InvalidOperationException assigning it to a catch handler variable named ex. In the body of the catch handler, print out the value of the Message property.

8. Run the application using Ctrl+F5. You see the console window shown in Figure 11.8.

Figure 11.8. Handling the runtime exception.

Notice that this time, the application did not terminate, but instead displayed the message associated with the exception.

9. Change the body of Main to the following:

try
{
PrintString(null);
}
catch (InvalidOperationException ex)
{
Console.WriteLine(ex.GetType().Name);
Console.WriteLine(ex.Message);
}
catch (ArgumentNullException ex)
{
Console.WriteLine(ex.GetType().Name);
Console.WriteLine(ex.Message);
}
catch (Exception ex)
{
Console.WriteLine(ex.GetType().Name);
Console.WriteLine(ex.Message);
}

10. Run the application using Ctrl+F5. You see the console window shown in Figure 11.9.

Figure 11.9. Multiple catch handlers.

11. Now change the order of the catch handlers so that the catch(Exception) handler is first. You should notice that you immediately see two compiler errors indicating that the additional two catch handlers will not be executed because a previous handler catches the base exception type, as shown in Figure 11.10.

Figure 11.10. Multiple catch handlers in the wrong order.