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Notice the following features of the Example class:
The class implements the IDisposable interface.
The destructor calls Dispose.
The Dispose method is public and can be called at any time.
The Dispose method can safely be called multiple times. The variable disposed indicates whether the method has already been run. The scarce resource is released only the first time the method runs.
The Dispose method calls the static GC.SuppressFinalize method. This method stops the garbage collector from calling the destructor on this object, because the object has now been finalized.
All the regular methods of the class (such as SomeBehavior) check to see whether the object has already been disposed. If it has, they throw an exception.
Making Code Exception-Safe
In the following exercise, you will rewrite a small piece of code to make the code exceptionsafe. The code opens a text file, reads its contents one line at a time, writes these lines to a text box on a form on the screen, and then closes the text file. However, if an exception arises
as the file is read or as the lines are written to the text box, the call to close the text file will be bypassed. You will rewrite the code to use a using statement instead, ensuring that the
code is exception-safe.
Write a using statement
1.Start Microsoft Visual Studio 2008 if it is not already running.
2.Open the UsingStatement project, located in the \Microsoft Press\Visual CSharp Step by Step\Chapter 14\UsingStatement folder in your Documents folder.
3.On the Debug menu, click Start Without Debugging.
A Windows Presentation Foundation (WPF) form appears.
4.On the form, click Open File.
5.In the Open dialog box, move to the \Microsoft Press\Visual CSharp Step by Step\ Chapter 14\UsingStatement\UsingStatement folder in your Documents folder, and select the Window1.xaml.cs source file.
This is the source file for the application itself.
268Part II Understanding the C# Language
6.Click Open.
The contents of the file are displayed in the form, as shown here:
7.Close the form to return to Visual Studio 2008.
8.Open the Window1.xaml.cs file in the Code and Text Editor window, and then locate the openFileDialogFileOk method.
The method looks like this:
private void openFileDialogFileOk(object sender, System.ComponentModel.CancelEventArgs e)
{
string fullPathname = openFileDialog.FileName; FileInfo src = new FileInfo(fullPathname); fileName.Text = src.Name;
source.Clear();
TextReader reader = new StreamReader(fullPathname); string line;
while ((line = reader.ReadLine()) != null)
{
source.Text += line + “\n”;
}
reader.Close();
}
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The variables fileName, openFileDialog, and source are three private fields of the Window1 class. The problem with this code is that the call to reader.Close is not guaran-
teed to execute. If an exception occurs after opening the file, the method will terminate with an exception, but the file will remain open until the application finishes.
9.Modify the openFileDialogFileOk method, and wrap the code that processes the file in a using statement (including opening and closing braces), as shown in bold here. Remove the statement that closes the TextReader object.
private void openFileDialogFileOk(object sender, System.ComponentModel.CancelEventArgs e)
{
string fullPathname = openFileDialog.FileName; FileInfo src = new FileInfo(fullPathname); fileName.Text = src.Name;
source.Clear();
using (TextReader reader = new StreamReader(fullPathname))
{
string line;
while ((line = reader.ReadLine()) != null)
{
source.Text += line + “\n”;
}
}
}
You no longer need to call reader.Close because it will be invoked automatically by the Dispose method of the StreamReader class when the using statement completes.
This applies whether the using statement finishes naturally or terminates because of an exception.
10.On the Debug menu, click Start Without Debugging.
11.Verify that the application works as before, and then close the form.
If you want to continue to the next chapter:
Keep Visual Studio 2008 running, and turn to Chapter 15. If you want to exit Visual Studio 2008 now:
On the File menu, click Exit. If you see a Save dialog box, click Yes (if you are using Visual Studio 2008) or Save (if you are using Visual C# 2008 Express Edition) and save the project.
