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Monotouch Exception: This class is not key value coding-compliant …

February 13th, 2013

While writing an iPad application in Monotouch I started getting the following exception while running the program in the iPad simulator:

Objective-C exception thrown.  Name: NSUnknownKeyException Reason: [<MainView 0x99d1460> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key buttonSetup.

Stunned by how unhelpful the exception message was I tried to track down the cause of the error. The only clue the error message gave me was that buttonSetup was the name of a UIButton I had previously had on my main view in the application. However, I had renamed the button so theoretically there were no references to the button left in the project. After reopening the xib for the view in XCode, I tried to track down any remaining reference to the button with no luck until after a few minutes clicking round, I found the following view in Xcode – the Connections Inspector view – which showed a remaining reference to the old button name in the Referencing Outlets:

monotouch-remove-referencing-outlet

To get to the Referencing Outlets section, I clicked on the renamed button (in the pane shown on the left in the pic. above) then opened the Connections Inspector using the toolbar button shown on the top right in the pic. The red rectangle on the bottom right shows the extra referencing outlet to the old control name that I had to remove to get the project to run without an exception. Removing the buttonSetup referencing outlet was easy – just click the x on the File’s Owner widget.