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POMSDN Dispose() example erroneous? (when to set managed references to null)
 

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  1. POMSDN Dispose() example erroneous? (when to set managed references to null)
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    <p><a href="http://msdn.microsoft.com/en-us/library/fs2xkftw.aspx#Y630" rel="nofollow noreferrer">MSDN's example pattern</a> for implementing a Dispose() method depicts setting the reference to a disposed managed resource to null (<code>_resource = null</code>), but does so outside the <code>if (disposing)</code> block:</p> <pre><code>protected virtual void Dispose(bool disposing) { // If you need thread safety, use a lock around these // operations, as well as in your methods that use the resource. if (!_disposed) { if (disposing) { if (_resource != null) _resource.Dispose(); Console.WriteLine("Object disposed."); } // Indicate that the instance has been disposed. _resource = null; _disposed = true; } } </code></pre> <p>Shouldn't <code>_resource = null</code> be placed inside this code block? If a call to <code>Dispose(false)</code> is made then <code>_resource</code> will be null and unable to be subsequently disposed! ??</p> <p>Of course, <code>Dispose(false)</code> is only called (in practice) by the runtime during finalization. But if <code>_resource</code> wasn't previously disposed, what is the need to set it to null at this point when the object (including the <code>_resource</code> member field) is about to be garbage collected?</p> <hr> <p>[end of original question]</p> <h1>Follow up:</h1> <p>After much reading, it appears setting the reference to null is not necessary, but may be a good idea for "heavy" member objects if you have reason to believe the containing class (the one being disposed) might not be garbage collected soon.</p> <p>Know that object disposal is no assurance that the object has been "released" by consuming code. The disposed object might be kept around (in a collection or otherwise) for various purposes, or just in error. I can imagine an application that uses objects from a collection then disposes of them, but keeps them in the collection for a later process to perform removal and log final state (or something like that... who knows...)</p> <p><strong>Conclusions:</strong></p> <ol> <li>Setting references to "heavy" member objects to null releases them for garbage collection even if the disposed object is not released.</li> <li>It is overkill to clear references for all objects.</li> <li>Hence, placement of the <code>_resource = null</code> statement (the original question) is not important for two reasons: (A) Using it at all is only something to think about after reading the above; (B) In the MSDN example, it executes for both <code>Dispose(true)</code> and <code>Dispose(false)</code>, but the latter only occurs when the object is finalized and just about to be garbage collected anyway!</li> </ol> <p>Thus, my preference will be to place <code>_resource = null</code> inside the most inner <code>if</code> block:</p> <pre><code>if (disposing) { if (_resource != null) { _resource.Dispose(); _resource = null; } } </code></pre> <p>This keeps all the <code>_resource</code> code together. Further thoughts, anyone?</p> <p>More reading:</p> <ul> <li><a href="https://stackoverflow.com/questions/456551/in-the-disposebool-method-implementation-shouldnt-one-set-members-to-null">In the Dispose(bool) method implementation, Shouldn&#39;t one set members to null?</a></li> <li><a href="https://stackoverflow.com/questions/2926869/c-do-you-need-to-dispose-of-objects-and-set-them-to-null">Do you need to dispose of objects and set them to null?</a></li> <li><a href="http://www.codeproject.com/KB/dotnet/idisposable.aspx" rel="nofollow noreferrer">http://www.codeproject.com/KB/dotnet/idisposable.aspx</a> (Very good, and long!)</li> </ul>
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    1. USKevin P. Rice
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    1. COIt serves little purpose, but does little harm... Actually, I'd be more annoyed by a: the writing to `Console`, and b: writing to `Console` even if it is already disposed and we didn't dispose it (see missing braces on the `if`).
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      1. POMSDN Dispose() example erroneous? (when to set managed references to null)
      1. USMarc Gravell
    2. COHaha! The `Console.Writeline()` is sample code. Would Trace.Writeline() please you better? Also, there is no missing brace--it's on the end of the `if` line (not StyleCop compliant).
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      1. POMSDN Dispose() example erroneous? (when to set managed references to null)
      1. USKevin P. Rice
    3. COlook again; it writes "Object disposed." even if `_resource` was null and it therefore didn't call `.Dispose()`. I mean the *inner-most* `if`.
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      1. USMarc Gravell
 

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