If you’re working on a MEF (Managed Extensibility Framework) application, you’re probably familiar with its ups and down. On the one hand, MEF is a powerful plugin system, that’s a pleasure to work with once you know it well.

On the other hand, we all encountered strange failures that originated with MEF. Such failures are a nightmare to debug and identify.

There are several reasons why a MEF failure might happen. A common reason is when an exception is thrown in a MEF ImportingConstructor. For example:

public class LicenseProvider: ILicenseProvider
    public LicenseProvider(ILicenseChecker checker)
        //The following checks license on the server, but the WiFi just disconnected 
        //and an Exception is thrown
        bool licenseValid = checker.CheckLicenseValid();

This sort of exceptions can be missed and occur in production.

The result is not very pleasant:

An exception has been encountered. This may be caused by an extension.

Preventing the Exception

While working on the OzCode extension, I created a simple Roslyn Analyzer to add try/catch statements to all MEF Importing-Constructors. Let’s see it in action:

The **MefImportExceptionAnalyzer** does the following:
  1. Adds a Diagnostic that finds a MEF ImportingConstructor with content that’s not entirely wrapped in a try/catch statement.
  2. Provides a Code Fix to handle the problem.
  3. The Code Fix will:
    1. Add a try/catch statement around the entire content
    2. Add ErrorNotificationLogger.LogErrorWithoutShowingErrorNotificationUI(“Error in MEF ctor”, e); inside the catch.
    3. Add a using statement that for the static class ErrorNotificationLogger

Now, when a similar exception happens in production, it will be reported and as a bonus won’t crash the extension with a nasty error dialog.

This Analyzer is available as NuGet as OzCode.VisualStudioExtensionAnalyzers

Writing the Roslyn Analyzer

The code for this analyzer is available on GitHub .

If you’re interested in how such an analyzer is made, or want to change the behavior of the existing analyzer, then read my tutorial Creating a Roslyn Analyzer that wraps code in a try/catch statement .