SSRS ReportViewer NullReferenceException on Dispose

I recently assisted on troubleshooting an error in a utility application where an exception was being thrown on the dispose of a Microsoft.Reporting.WebForms.ReportViewer.  The environmental conditions were pretty specific, so it’s possible you’d never see something like this in your environment.  But if you do, here’s how you can work around it.

The specific condition is that we have a shared library of code for both desktop and web applications.  One of the functions in that library takes some parameters for an SSRS report and returns a byte array for a rendered PDF of the report.  Because the library initially was used exclusively by the website, the WebForms version of the ReportViewer was used.  As time went on, the library was used by desktop apps and windows services.  That’s when the trouble began.

So, if you are using a WebForms.ReportViewer in a desktop application, you may get this exception when disposing the instance.  Digging into the decompiled code for the ReportViewer control suggested it was because there was no HttpContext available.  For us, the long-term fix was clear: use the WinForms version of the ReportViewer.  In the short term though, adding this line of code resolved the error:

If HttpContext.Current Is Nothing Then HttpContext.Current = New HttpContext(New HttpRequest(IO.Path.GetRandomFileName, "", ""), New HttpResponse(IO.TextWriter.Null))

This created an HttpContext where there was none before, and the ReportViewer instance was able to be disposed without an error.