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handles, resources that require an explicit activity that the garbage collector doesn t know about Note that finalizers will execute on their own thread, making their execution even less determinant This indeterminacy makes an unhandled exception within a finalizer (outside of the debugger) difficult to diagnose because the circumstances that led to the exception are not clear From the user s perspective, the unhandled exception will be thrown relatively randomly and with little regard for any action the user was performing For this reason, take care to avoid exceptions within finalizers Use defensive programming techniques such as checking for nulls (refer to Listing 920)
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Deterministic Finalization with the using Statement The problem with finalizers on their own is that they don t support deterministic finalization (the ability to know when a finalizer will run) Rather, finalizers serve the important role of a backup mechanism for cleaning up resources if a developer using a class neglects to call the requisite cleanup code explicitly For example, consider the TemporaryFileStream that not only includes a finalizer but also a Close() method The class uses a file resource that could potentially consume a significant amount of disk space The developer using TemporaryFileStream can explicitly call Close() in order to restore the disk space Providing a method for deterministic finalization is important because it eliminates a dependency on the indeterminate timing behavior of the finalizer Even if the developer fails to call Close() explicitly, the finalizer will take care of the call The finalizer will run later than if it was called explicitly, but it will be called Because of the importance of deterministic finalization, the Base Class Library includes a specific interface for the pattern and C# integrates the pattern into the language The IDisposable interface defines the details of the pattern with a single method called Dispose(), which developers call on a resource class to dispose of the consumed resources Listing 921 demonstrates the IDisposable interface and some code for calling it
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Listing 921: Resource Cleanup with IDisposable
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using System; using SystemIO class TemporaryFileStream : IDisposable { public TemporaryFileStream() { _File = new FileInfo(PathGetTempFileName()); _Stream = new FileStream( FileFullName, FileModeOpenOrCreate, FileAccessReadWrite); } ~TemporaryFileStream() { Close(); } public FileStream Stream { get { return _Stream; } } readonly private FileStream _Stream; public FileInfo File { get { return _File; } } readonly private FileInfo _File; public void Close() { if(Stream != null) { StreamClose(); } if(File != null) { FileDelete(); } // Turn off calling the finalizer SystemGCSuppressFinalize(this); } #region IDisposable Members public void Dispose() { Close(); } #endregion }
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class Program { // static void Search() { TemporaryFileStream fileStream = new TemporaryFileStream(); // Use temporary file stream; // fileStreamDispose(); // } }
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The steps for both implementing and calling the IDisposable interface are relatively simple However, there are a couple of points you should not forget First, there is a chance that an exception will occur between the time TemporaryFileStream is instantiated and Dispose() is called If this happens, Dispose() will not be invoked and the resource cleanup will have to rely on the finalizer To avoid this, callers need to implement a try/finally block Instead of coding such a block explicitly, C# provides a using statement expressly for the purpose The resultant code appears in Listing 922
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Listing 922: Invoking the using Statement
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class Program { // static void Search() { using (TemporaryFileStream fileStream1 = new TemporaryFileStream(), fileStream2 = new TemporaryFileStream()) { // Use temporary file stream; } } }
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The resultant CIL code is identical to the code that would be created if there was an explicit try/finally block, where fileStreamDispose() is called in
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the finally block The using statement, however, provides a syntax shortcut for the try/finally block Within a using statement, you can instantiate more than one variable by separating each variable with a comma The key is that all variables are of the same type and that they implement IDisposable To enforce the use of the same type, the data type is specified only once rather than before each variable declaration
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Garbage Collection and Finalization The IDisposable pattern contains one additional important call Back in Listing 921, the Close() method included a call to SystemGCSuppressFinalize() (captured again in Listing 923) Its purpose was to remove the TemporaryFileStream class instance from the finalization (f-reachable) queue
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