In windows application i would do it like ClassLibrary1.Properties.Resources.file.xml
By default, the class generated to access the resources is internal, so you can't access it from another assembly. If you need to make it public, select the .resx file, go to its properties, and change the Custom tool property from "ResXFileCodeGenerator" to "PublicResXFileCodeGenerator". This custom tool generates public classes, which should solve your problem.
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