Some times when upgrading your library you may get this error:

CS0433: The type 'Mastercode' exists in both ....

Even if you erase the cache, it still creates the cache in the temporary ASP net files....

in this case, you may have to add the line <remove assembly="*"/> inside the assemblies section in the compilation section of your web.config file.

like this:

<assemblies>
<remove assembly="*"/>
<add assembly="Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />
</assemblies>

Please let us know if this post helps you....