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IIS 5.x Wildcard Mapping and ELMAH

The Problem

As described in this thread, ELMAH does not work correctly with IIS5x in combination with Wildcard Mapping. The symptom of this is that you will see HTTP 404 errors when accessing elmah.axd/stylesheet, elmah.axd/rss and others.

The reason for this is because the ErrorLogPageFactory uses Request.PathInfo (i.e. the PATH_INFO server variable) to determine which handler to use. With Wildcard Mapping, this variable gets calculated slightly differently, so ELMAH cannot identify which handler to use.

NB The ideas and basis for the solution to this problem have come from this forum posting.

The Solution

To overcome this you can add the FixIIS5xWildcardMappingModule to your application as follows:

<httpModules>
  <add name="FixIIS5xWildcardMappingModule" 
       type="Elmah.FixIIS5xWildcardMappingModule, Elmah"/>
</httpModules>

NB the thread mentioned at the start of this article offers alternative solutions, but they have since been superseded by this one.

Potential Issues

Firstly, this solution does not work with ASP.NET 1.0. This is because HttpContext.RewritePath does not exist in version 1.0 of the .NET Framework.

Secondly, there is a small possibility that the module will not work with ASP.NET 1.1 either. Version 1.1 of the .NET Framework does not include methods that can be used directly to ascertain the path attribute in the httpHandler declaration for Elmah.ErrorLogPageFactory. The module instead loads the web.config file into an XmlDocument and performs an XPath query to select the appropriate node. In the unlikely event that the web.config file cannot be successfully loaded, a default handler path of elmah.axd is assumed. Should this assumption be false, then the module will cease to function correctly.