imagepicker.cs

var imagePicker = new UIImagePickerController();
var actionSheet = new UIActionSheet (null, null, "Cancel", null, new string[] {"Take photo", "Choose Existing"});

actionSheet.Clicked += delegate(object a, UIButtonEventArgs b) {
    if (b.ButtonIndex == 0) {
        imagePicker.SourceType = UIImagePickerControllerSourceType.Camera;
    } else if (b.ButtonIndex == 1) {
        imagePicker.SourceType = UIImagePickerControllerSourceType.PhotoLibrary;
    }
    if (b.ButtonIndex != actionSheet.CancelButtonIndex) { 
        NavigationController.PresentViewController(imagePicker, true, null);
    }
};
actionSheet.ShowFromTabBar(TabBarController.TabBar);