ComboBox crashing WPF application because SelectionChanged is firing too soon

问题

Here's my XAML:

<ComboBox x:Name="cloneSelector"
          Width="80"
          SelectedIndex="0"
          VerticalAlignment="Top"
          Margin="10"
          SelectionChanged="cloneSelector_SelectionChanged">
    <ComboBoxItem Content="All" />
    <ComboBoxItem Content="Top" />
    <ComboBoxItem Content="Middle" />
    <ComboBoxItem Content="Bottom" />
    <ComboBoxItem Content="None" />
</ComboBox>

And here is the SelectionChanged handler (as requested by a comment):

cloneSelection = (CloneFormat) cloneSelector.SelectedIndex;

var frameSize = videoDevice.VideoResolution.FrameSize;

    switch (cloneSelection)
    {
        case CloneFormat.Top:
            cloneRect = new Rectangle(0, 0, frameSize.Width, (frameSize.Width* 9) / 16);
            break;
        case CloneFormat.Middle:
            cloneRect = new Rectangle(0, 100, frameSize.Width, (frameSize.Width* 9) / 16);
            break;
        case CloneFormat.Bottom:
            cloneRect = new Rectangle(0, 200, frameSize.Width, (frameSize.Width* 9) / 16);
            break;
        default:
            cloneRect = new Rectangle(0, 0, frameSize.Width, frameSize.Height);
            break;
    }

My application runs if SelectedIndex, the SelectionChanged handler, or the ComboBoxItems are removed. The problem seems to be that the handler is accessing cloneSelector too soon and throwing a null object exception. Is there an easy way to prevent this behavior while still maintaining my functionality (i.e. not having SelectedIndex start at -1)?

Also, if I didn't test my project so frequently I would've had no idea what I did to break it. It builds successfully and fails silently. When I debug, the Exception Unhandled box in the No Symbols Loaded designer says

System.Reflection.TargetInvocationException: 'Exception has been thrown by the target of an invocation.'

Inner Exception

NullReferenceException: Object reference not set to an instance of an object.

And I can see that it was thrown in PresentationFramework.dll but it gives me no information about what was null or what's causing the error. How do I get better debugging info, i.e. get it to tell me that the ComboBox is the source of the crash?

回答1:

It turns out my ComboBox wasn't even the null reference, it was my video resolution object. All the more reason I need more specific debugging info.

来源：https://stackoverflow.com/questions/44051514/combobox-crashing-wpf-application-because-selectionchanged-is-firing-too-soon

标签

wpf

xaml

combobox

crash

nullreferenceexception