What could be null here?

Question

My app is crashing after I clear the Bing Map of its pushpins and then open a flyout. This is what I see in the debugger:

The textblock is definitely not null - it is defined declaratively in XAML.

The List of String (CurrentlyMappedPhotosets) is not null - as you can see, it contains a string / has a count of 1.

The code just before what's seen on the scream shot is:

int count = App.CurrentlyMappedPhotosets.Count;

What could be null here?

Just before this happens, I call ClearMap:

private void ClearMap()
{
    var mapLayerChildren = from c in DataLayer.Children select c;
    var kinderGarten = mapLayerChildren.ToArray();
    for (int i = 0; i < kinderGarten.Count(); i++)
    {
        if (kinderGarten[i] is Pushpin)
        {
            DataLayer.Children.Remove(kinderGarten[i]);
        }
    }

    CloseInfobox();

    App.CurrentlyMappedPhotosets.Clear(); 
    if (null != App.photosetLocationCollection)
    {
        App.photosetLocationCollection.Clear();
    }
    appbarbtnClearMap.IsEnabled = false;
    UpdateGUI(false);
}

...which calls UpdateGUI():

private void UpdateGUI(bool resizeMap)
{
    appbarbtnRenamePhotoset.IsEnabled = App.CurrentlyMappedPhotosets.Count > 0;
    if (resizeMap)
    {
        ResizeMap();
    }
}

Answer 1

Have you looked if the value present in the collection is not null? Any non primitive type class is by default null if not constructed with a value.

So you can have as many null values of the type you are trying to have.

I suggest that you do a test by changing the line to:

textblock0.Text = App.CurrentlyMappedPhotosets[0].HasValue ? App.CurrentlyMappedPhotosets[0].Value : "";