This team created several foundational pieces, such as Visual Studio MEF<\/a> and the Visual Studio Text data model platform<\/a>. Together, these technologies and the innovations that followed them re-invigorated the Visual Studio ecosystem and enabled some of its most distinctive features, like CodeLens<\/a>.<\/p>\n After a decade of iteration, the VS IDE team has accumulated a list of best practices for using these components from extensions without adversely impacting Visual Studio performance. In-process extensions can do just about anything, so in the words of \u2018Uncle Ben\u2019, \u201cWith great power comes great responsibility\u201d<\/em>.<\/p>\n The Future of Visual Studio Extensions<\/a> details how we\u2019ll improve Visual Studio\u2019s resiliency in future releases by running extensions in a separate process so they can\u2019t impact the IDE\u2019s performance. For now, though, here\u2019s some DOs and DON\u2019Ts for crafting leak free VS Editor extensions with the existing in-process extensibility model.<\/p>\n At its base level, the Visual Studio text editor is a collection of MEF parts<\/a>. For a class to be a MEF part, all that is required is that it be decorated with an [Export] attribute in C#, like the following, be declared in a DLL registered in the VSIX manifest as a MefComponent<\/a>, and be imported somewhere by the IDE or another MEF part.<\/p>\n In the example above, we are exporting a MEF part, implementing ITextViewCreationListener, and importing a service that implements IContentTypeRegistryService from the IDE.<\/p>\n MEF has some important properties that you should make note of to avoid MEF related leaks.<\/p>\n Consider a second MEF part example: an extender is exporting\u00a0an ITextViewCreationListener, that will be signaled whenever a new text editor is opened, storing a reference to the text view for later use.<\/p>\n The state in the class has a fixed size (one text view, not a continuously growing collection), and it\u2019s stored in a local variable, so at first glance, this code seems harmless.<\/p>\n The implication is that a single leaked object can transitively leak much more than expected<\/strong><\/span>. With this example, closing your solution and opening another solution would consume the memory of having both solutions open simultaneously!<\/p>\n Since MEF parts live until Visual Studio is shutdown, it is important to be careful about how you use them to avoid memory leaks.<\/p>\n In addition to MEF, Visual Studio relies heavily on C# events<\/a> to compose extensions into the editor. Under all the syntactic sugar, an event in C# is effectively just a collection of subscriber delegates that get invoked by the class implementing the event one by one when an event is \u2018raised\u2019. One implication of this is that every class implementing an event has a strong reference to any current subscribers, preventing those objects from being garbage collected.<\/p>\n In the above example, we are once again doing something fairly innocuous: we\u2019re creating a \u2018classification tagger<\/a>\u2019, commonly used to provide code colorization, and subscribing to EditorOptions.OptionChanged, so we can update the colorization if some global option changes.<\/p>\n Subscribing each produced ITagger to OptionChanged, constructs a strong reference to this tagger. Since IEditorOptionsFactoryService.GlobalOptions is referenced by a MEF part (IEditorOptionsFactoryService), it is kept alive for the entirety of the VS session, and anything it references is too.<\/p>\n In this example, every time a document is opened and closed, we would leak another instance of MyTagger and anything it references. The tagger references the view, and is targeting C#, so we\u2019d also leak the entire Roslyn workspace, and a unique copy of every C# file that user opened during their VS session<\/strong>!<\/p>\n Now that we\u2019ve established the risks of memory leaks from C# event listeners, here are some best practices you can adopt in your extension.<\/p>\n Other common sources of memory leaks are:<\/p>\n To diagnose leaks in your Visual Studio extension you can use either of the following memory debugging tools.<\/p>\nMEF Part Memory Leaks<\/h2>\n
What are MEF Parts<\/h3>\n
[Export(typeof(ITextViewCreationListener))]\r\ninternal class MyPart : ITextViewCreationListener\r\n{\r\n private readonly Lazy<IContentTypeRegistryService> contentTypeRegistryService;\r\n\r\n [ImportingConstructor]\r\n public MyPart(Lazy<IContentTypeRegistryService> contentTypeRegistryService)\r\n {\r\n this.contentTypeRegistryService = contentTypeRegistryService;\r\n }\r\n\r\n public void TextViewCreated(ITextView textView)\r\n {\r\n }\r\n}<\/pre>\nProperties of MEF<\/h3>\n
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Common MEF Part Leaks<\/h3>\n
[Export(typeof(ITextViewCreationListener))]\r\ninternal sealed class MyPart : ITextViewCreationListener\r\n{\r\n private readonly Lazy<IContentTypeRegistryService> contentTypeRegistryService;\r\n\r\n private ITextView mostRecentTextView;\r\n\r\n [ImportingConstructor]\r\n public MyPart(Lazy<IContentTypeRegistryService> contentTypeRegistryService)\r\n {\r\n this.contentTypeRegistryService = contentTypeRegistryService;\r\n }\r\n\r\n public void TextViewCreated(ITextView textView)\r\n {\r\n this.mostRecentTextView = textView;\r\n }\r\n}<\/pre>\nThe Problem<\/h3>\n
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Best Practices for MEF parts<\/h3>\n
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C# Event Listener leaks<\/h2>\n
[Export(typeof(ITextViewCreationListener))]\r\n[Name(\"Colorization tagger\")]\r\n[ContentType(\"CSharp\")]\r\n[TagType(typeof(IClassificationTag))]\r\ninternal sealed class MyPart : IViewTaggerProvider\r\n{\r\n private readonly Lazy<IEditorOptionsFactoryService> optionsFactoryService;\r\n\r\n [ImportingConstructor]\r\n public MyPart(Lazy<IEditorOptionsFactoryService> optionsFactoryService)\r\n {\r\n this.optionsFactoryService = optionsFactoryService;\r\n }\r\n\r\n public ITagger<T> CreateTagger<T>(ITextView textView, ITextBuffer buffer) where T : ITag\r\n {\r\n return new MyTagger(textView, this.optionsFactoryService.Value) as ITagger<T>;\r\n }\r\n}\r\n\r\ninternal sealed class MyTagger : ITagger<IClassificationTag>\r\n{\r\n private readonly ITextView textView;\r\n\r\n public MyTagger(\r\n ITextView textView,\r\n IEditorOptionsFactoryService optionsFactoryService)\r\n {\r\n this.textView = textView;\r\n optionsFactoryService.GlobalOptions.OptionsChanged += this.OnOptionChanged;\r\n }\r\n\r\n public event EventHandler<SnapshotSpanEventArgs> TagsChanged;\r\n\r\n public IEnumerable<ITagSpan<IClassificationTag>> GetTags(NormalizedSnapshotSpanCollection spans)\r\n {\r\n return Array.Empty<ITagSpan<IClassificationTag>>();\r\n }\r\n\r\n private void OnOptionChanged(object sender, EditorOptionChangedEventArgs e)\r\n {\r\n }\r\n}<\/pre>\n![\"Image](\"https:\/\/devblogs.microsoft.com\/visualstudio\/wp-content\/uploads\/sites\/4\/2021\/09\/Picture3.png\")
<\/a><\/p>\nBest Practices for C# Event Listeners<\/h3>\n
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Other Common Leaks<\/h2>\n
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private void OnLoaded(object sender, RoutedEventArgs e)\r\n{\r\n var textDp = DepedencyPropertyDescriptor.FromProperty(TextBlock.TextProperty, typeof(TextBlock));\r\n textDp.AddValueChanged(this, this.OnTextChanged);\r\n\r\n this.RenderHighlighedSpans();\r\n}<\/pre>\nDiagnosing Leaks<\/h2>\n
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![\"Image](\"https:\/\/devblogs.microsoft.com\/visualstudio\/wp-content\/uploads\/sites\/4\/2021\/09\/Picture4.png\")
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