Visual Studio 2010 を使用していますが、先週または Visual Studio から次のようなメッセージが表示されます。「Visual Studio で例外が発生しました。これは拡張機能が原因である可能性があります。」
私のActivityLog.xmlは、これを問題として記録していることを明らかにしています:
<entry>
<record>321</record>
<time>2012/09/20 16:52:36.126</time>
<type>Error</type>
<source>Editor or Editor Extension</source>
<description>System.ComponentModel.Composition.CompositionException: The composition produced a single composition error. The root cause is provided below.
Review the CompositionException.Errors property for more detailed information.

1) Object reference not set to an instance of an object.

Resulting in: An exception occurred while calling the 'OnImportsSatisfied' method on type 'Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSKeyword'.

Resulting in: Cannot activate part 'Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSKeyword'.
Element: Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSKeyword --> Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSKeyword

Resulting in: Cannot get export 'Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSKeyword (ContractName="Microsoft.VisualStudio.Text.Classification.EditorFormatDefinition")' from part 'Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSKeyword'.
Element: Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSKeyword (ContractName="Microsoft.VisualStudio.Text.Classification.EditorFormatDefinition") --> Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSKeyword

 at System.ComponentModel.Composition.Hosting.CompositionServices.GetExportedValueFromComposedPart(ImportEngine engine, ComposablePart part, ExportDefinition definition)
 at System.ComponentModel.Composition.Hosting.CatalogExportProvider.GetExportedValue(ComposablePart part, ExportDefinition export, Boolean isSharedPart)
 at System.ComponentModel.Composition.Hosting.CatalogExportProvider.CatalogExport.GetExportedValueCore()
 at System.ComponentModel.Composition.Primitives.Export.get_Value()
 at System.ComponentModel.Composition.ExportServices.GetExportedValueFromLazy[T](Export export)
 at System.ComponentModel.Composition.ExportServices.<>c__DisplayClass4`2.<CreateStronglyTypedExportOfTM>b__1()
 at System.Lazy`1.CreateValue()
 at System.Lazy`1.LazyInitValue()
 at System.Lazy`1.get_Value()
 at Microsoft.VisualStudio.Text.Utilities.GuardedOperations.InstantiateExtension[TExtension](Object errorSource, Lazy`1 provider)</description>
</entry>
<entry>
<record>322</record>
<time>2012/09/20 16:52:36.202</time>
<type>Error</type>
<source>Editor or Editor Extension</source>
<description>System.ComponentModel.Composition.CompositionException: The composition produced a single composition error. The root cause is provided below. Review the CompositionException.Errors property for more detailed information.

1) Object reference not set to an instance of an object.

Resulting in: An exception occurred while calling the 'OnImportsSatisfied' method on type 'Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSComment'.

Resulting in: Cannot activate part 'Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSComment'.
Element: Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSComment --> Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSComment

Resulting in: Cannot get export 'Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSComment (ContractName="Microsoft.VisualStudio.Text.Classification.EditorFormatDefinition")' from part 'Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSComment'.
Element: Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSComment (ContractName="Microsoft.VisualStudio.Text.Classification.EditorFormatDefinition") --> Microsoft.VisualStudio.Web.Exports.ExportedClassificationFormatCSSComment

 at System.ComponentModel.Composition.Hosting.CompositionServices.GetExportedValueFromComposedPart(ImportEngine engine, ComposablePart part, ExportDefinition definition)
 at System.ComponentModel.Composition.Hosting.CatalogExportProvider.GetExportedValue(ComposablePart part, ExportDefinition export, Boolean isSharedPart)
 at System.ComponentModel.Composition.Hosting.CatalogExportProvider.CatalogExport.GetExportedValueCore()
 at System.ComponentModel.Composition.Primitives.Export.get_Value()
 at System.ComponentModel.Composition.ExportServices.GetExportedValueFromLazy[T](Export export)
 at System.ComponentModel.Composition.ExportServices.<>c__DisplayClass4`2.<CreateStronglyTypedExportOfTM>b__1()
 at System.Lazy`1.CreateValue()
 at System.Lazy`1.LazyInitValue()
 at System.Lazy`1.get_Value()
 at Microsoft.VisualStudio.Text.Utilities.GuardedOperations.InstantiateExtension[TExtension](Object errorSource, Lazy`1 provider)</description>
</entry>
これを引き起こす特定のアクションはないようです。VSを最初に起動したときに発生することもあれば、しばらく机を離れて戻った後に発生することもあります。また、ログから、Visual Studio CSS Editing Package をロードした直後に発生するようです。MVC4 をインストールしましたが、新しい CSS (CSS3 だと思いますか?) の問題かもしれません。
とにかく、誰かが以前にこれに遭遇し、何らかの解決策を持っていたかどうか疑問に思っています.