Question

I have a simple button to be added to a UINavigationBar, Min OS is 3.2 and we compile with the default compiler. I have MT 5.0:

            saleImg = UIImage.FromFileUncached (shoppingImg);
            saleButton = new UIBarButtonItem ("", UIBarButtonItemStyle.Bordered, (s, o) => {ItunesHelper.Open (); });
            saleButton.SetBackgroundImage(saleImg,UIControlState.Normal,UIBarMetrics.Default);
            saleButton.SetBackgroundImage(saleImg,UIControlState.Selected, UIBarMetrics.Default);
            saleButton.SetBackgroundImage(saleImg,UIControlState.Highlighted, UIBarMetrics.Default);

But when run it throws the error:

Objective-C exception thrown. Name: NSInvalidArgumentException Reason: -[UIBarButtonItem setBackgroundImage:forState:barMetrics:]: unrecognized selector sent to instance 0x76d4d00

Answer 1

The UIBarButtonItem.SetBackgroundImage method was introduced with iOS 5. Since you are getting an “unrecognized selector…” message, I can only assume that you are running the app on a device with an iOS version older than 5.0.

If you want your app to be backwards compatible, create a button and initialize the UIBarButtonItem with the UIBarButtonItem(UIView) constructor:

UIButton button = UIButton.FromType(UIButtonType.Custom);
button.SetBackgroundImage(saleImg, UIControlState.Normal);
button.TouchUpInside += (s, o) => {ItunesHelper.Open (); };
UIBarButtonItem saleButton = new UIBarButtonItem(button);

Setting the minimum OS version does not mean that MonoDevelop or the compiler will inform you of API compatibilities. It is merely a setting in Info.plist which will determine if your app will appear on the App Store for devices of that version or not.