C# / MonoTouch EventHandler becomes null after component displayed; delegate going missing?

I've got a UIPickerViewModel that's supposed to fire an event when its selected value changes, but by the time Selected is called, the EventHandler is null. Code (somewhat simplified):

public class MyPickerViewModel: UIPickerViewModel {
    public event EventHandler SelectionChanged;

 开发者_运维技巧   public void PrintWhetherSelectionChangedIsNull() {
         System.Console.WriteLine(SelectionChanged == null ? "yes" : "no");
    }

    public override void Selected(UIPickerView picker, int row, int comp) {
        if (SelectionChanged != null) { // it always is, though
            SelectionChanged(this, new EventArgs());
        }
    }
}

public class SomeView: UIView {
    SomeView(MyPickerViewModel model) {
        model.SelectionChanged += delegate(object s, EventArgs e) { 
            DoSomething();
        }

        model.PrintWhetherSelectionChangedIsNull(); // prints "no"
    }

    private void DoSomething() { /* never called */ }
}

I'm pretty sure there's only one instance of the model, and that the EventHandler is non-null before becoming null again.

I'm guessing the delegates I've added to the EventHandler are getting garbage collected or something, but that's just a guess. Can anyone point me to a good primer on C# / iOS object lifecycles?

Update: I tried using a named EventHandler, an instance variable on the view, and the problem's still there. So it's not just a garbage collection issue.

Try updating the SomeView so it stores the model value, just to be sure, it does not release the reference for itself for some reason.

public class SomeView: UIView { 

    MyPickerViewModel _model;

    SomeView(MyPickerViewModel model) {  
        _model = model;

        _model.SelectionChanged += delegate(object s, EventArgs e) {   
            DoSomething();  
        }  

        _model.PrintWhetherSelectionChangedIsNull(); // prints "no"  
    }  

    private void DoSomething() { /* should be called now */ }  
}