CODEWITHSUNDEEP
c#wpfcombobox
D. Ziegler
D. Ziegler

Reputation: 23

ComboBox and ComboBoxItem names are null

I have a list of messages that is sorted by the selected option (time, name or group ID) from a ComboBox. I added a ComboBox with Items to my xaml:

<ComboBox x:Name="sort_dropdown" HorizontalAlignment="Left" Margin="828,219,0,0" VerticalAlignment="Top" Width="120" SelectedIndex="0">
        <ComboBoxItem x:Name="time_sort" Content="Time" Selected="time_sort_Selected"/>
        <ComboBoxItem x:Name="nickname_sort" Content="Nickname" Selected="nickname_sort_Selected"/>
        <ComboBoxItem x:Name="groupId_sort" Content="Group ID" Selected="groupId_sort_Selected"/>
    </ComboBox>

to have the list reordered, I wrote a function in the cs file:

private void reorder()
    {
        if (time_sort.IsSelected)
            fill_chat(MessageSorting.DateSort(Globals.MSG_LOCAL_LIST, ascendingOrder));
        else if (nickname_sort.IsSelected)
            fill_chat(MessageSorting.UserNameSort(Globals.MSG_LOCAL_LIST, ascendingOrder));
        else fill_chat(MessageSorting.GroupIdSort(Globals.MSG_LOCAL_LIST, ascendingOrder));
    }

When Run, I get a NullReferenceException. And upon debugging I found that the objects name reference to null.

Examples: time_sort = null. sort_dropdown = null. and etc.

Changing attributes on the xaml doesn't help because there's no reference to read those attributes. Thanks.

Update: This isn't the main window, and I noticed when I run the code that this window class runs before I call the .show function on it. For some reason the code in it is executed without being called.

Update 2: I have the following code:

public MainLoggedWindow()
    {

        MessageBox.Show("Before initialization");
        InitializeComponent();
        MessageBox.Show("after initialization");
    }

when running, the first MessageBox shows up but the second doesn't. the reorder() function is called from the following function:

        private void order_dropdown_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        ascendingOrder = !ascendingOrder;
        reorder();
    }

ascendingOrder is a bool that saves the current sorting order. There's another comboBox with 2 items (Ascending or Descending), Every time the user changes it's choice this function runs.

Update 3: Following the Call Stack while debugging shows that initializeComponent(); calls order_dropdown_SelectionChanged(...) which calls reorder().

Solved.

Narrowing down the problem: initializeComponent(); Caused order_dropdown_SelectionChanged(...) to run.

Explanation: On the ComboBox declaration in the xaml, selectedIndex attribute is defined like this: SelectedIndex="0". So part of the initialization is creating the ComboBox and setting selectedindex=0 which instantly triggers the order_dropdown_SelectionChanged(...) function. Now since the ComboBox items weren't created yet the name reference was null and the program crashed.

Solution: Don't define both selectedIndex attribute and _SelectionChanged(...) in the xaml. Personally I removed the selectedIndex attribute and defined it programmatically after initializeComponent(); is called.

Upvotes: 0

Views: 79

Answers (1)

Richardissimo
Richardissimo

Reputation: 5773

As part of the InitializeComponent it is setting the selection of your order_dropdown; but this is occurring before the named controls have been set up. So your reorder method needs to defend against that by wrapping its current contents within a test for if(time_sort!=null) as a way of determining whether the initialize has completed.

Upvotes: -1

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