Why does my iOS 9 app fail with: System.Exception: Failed to marshal the Objective-C object?

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You may see an error of this form:

System.Exception: Failed to marshal the Objective-C object ... Could not find an existing managed instance for this object ...

API changes in iOS 9 require that a callback constructor be used when calling unmanaged code, as the underlying API now expects it. Use the following line to add the callback constructor to the class:

public foo (IntPtr handle) : base (handle)

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