Judging from the error message you are getting it seems that you are using some source control under Visual Studio such as Team Foundation Server. Your project is source controlled and you have placed the Elmah.dll
assembly in a folder outside of the location of the project which is under source control.
So for example your source controlled project is mapped to c:\myproject
and you have extracted the elmah assembly into c:\elmah
and have referenced it from this location. The problem with this approach is that the source control will no longer be aware of the location of this dependent assembly.
So one possibility to solve this problem is to add the dependent assembly under source control into the same folder: for example c:\myproject\Reference Assemblies\Elmah
and then reference it from there.
Or simply use the official NuGet package
.