Tuesday, 23 July 2013

Specifying an Assembly's Location with MEF

Today I came across a problem with dynamically loading assemblies using MEF, when the assembly being loaded has a dependency on yet another assembly. In my case this was SignalR.

Assembly Host (.exe) -> Assembly (.dll) -> SignalR (.dll)

I was typically seeing the error:
Could not load file or assembly 'AssemblyNameHere, PublicKeyToken=xxxxxxxxxxxxxxxxx' or one of its dependencies. The system cannot find the file specified.

The reason for this is because the CLR is looking for the dependency in the directory in which the assembly host is running, and not the location of the assembly MEF loaded in.

The way around this is in this MSDN article. By using the probing tag in the app.config, you can specify the directory to look for dependencies. The caveat with using probing is you can only specify sub-directories of the assembly host, so if you want to use an absolute path to another place on the disk then probing isn't the solution, but I'd argue whether you needed it in another place anyway.

The config change required should look something like this:

<configuration>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <probing privatePath="Modules\"/>
    </assemblyBinding>
  </runtime>
</configuration>

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