I have an application (not document-based) which allows you to open a certain file type in it, but when I drag the file into the app’s icon, it logs *** +[Controller readableTypes]: selector not recognized, even though everything works fine. Should I igonore this, or is there something I’m missing?

See DebuggingTechniques, particularly the section on debugging uncaught exceptions.

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OK, thanks.

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There is already a page [DebugUnrecognizedSelector] that topically addresses this problem by title.

OpenFileInNonDocumentApplication deals with the specific subject of OpeningAFile in a non-document application

HowToOpen deals with opening a file using the C standard library