WPF Combobox binding: can't change selection.

Question

After wasting hours on this, following on the heels of my Last Problem, I'm starting to feel that Framework 4 is a master of subtle evil, or my PC is haunted.

I have three comboboxes and a textbox on a WPF form, and I have an out-of-the-box Subsonic 3 ActiveRecord DAL. When I load this "edit record" form, the comboboxes fill correctly, they select the correct items, and the textbox has the correct text. I can change the TextBox text and save the record just fine, but the comboboxes CANNOT BE CHANGED. The lists drop down and highlight, but when you click on an item, the item selected stays the same.

Here's my XAML:

        <StackPanel Orientation="Horizontal" Margin="10,10,0,0">
            <TextBlock Width="80">Asset</TextBlock>
            <ComboBox Name="cboAsset" Width="180"  
              DisplayMemberPath="AssetName"
              SelectedValuePath="AssetID" 
              SelectedValue="{Binding AssetID}" ></ComboBox>
        </StackPanel>
        <StackPanel Orientation="Horizontal" Margin="10,10,0,0">
            <TextBlock Width="80">Status</TextBlock>
            <ComboBox Name="cboStatus" Width="180" 
              DisplayMemberPath="JobStatusDesc"  SelectedValuePath="JobStatusID"  
              SelectedValue="{Binding JobStatusID}" ></ComboBox>
        </StackPanel>
        <StackPanel Orientation="Horizontal" Margin="10,10,0,0">
            <TextBlock Width="80">Category</TextBlock>
            <ComboBox Name="cboCategories" Width="180" 
              DisplayMemberPath="CategoryName"
              SelectedValuePath="JobCategoryID"
              SelectedValue="{Binding JobCategoryID}" ></ComboBox>
        </StackPanel>
        <StackPanel Orientation="Horizontal" Margin="10,10,0,0">
            <TextBlock Width="80">Reason</TextBlock>
            <TextBox Name="txtReason" Width="380" Text="{Binding Reason}"/>
        </StackPanel>

Here are the relevant snips of my code (intJobID is passed in):

    SvcMgrDAL.Job oJob;
    IQueryable<SvcMgrDAL.JobCategory> oCategories = SvcMgrDAL.JobCategory.All().OrderBy(x => x.CategoryName);
    IQueryable<SvcMgrDAL.Asset> oAssets = SvcMgrDAL.Asset.All().OrderBy(x => x.AssetName);
    IQueryable<SvcMgrDAL.JobStatus> oStatus = SvcMgrDAL.JobStatus.All();

        cboCategories.ItemsSource = oCategories;
        cboStatus.ItemsSource = oStatus;
        cboAsset.ItemsSource = oAssets;
        this.JobID = intJobID;
        oJob = SvcMgrDAL.Job.SingleOrDefault(x => x.JobID == intJobID);
        this.DataContext = oJob;

Things I've tried:

-Explicitly setting IsReadOnly="false" and IsSynchronizedWithCurrentItem="True"
-Changing the combobox ItemSources from IQueryables to Lists.
-Building my own Job object (plain vanilla entity class using INotifyPropertyChanged).
-Every binding mode for the comboboxes.
-ItemsSource="{Binding}"

The Subsonic DAL doesn't implement INotifyPropertyChanged, but I don't see as it'd need to for simple binding like this. I just want to be able to pick something from the dropdown and save it.

Comparing it with my last problem (link at the top of this message), I seem to have something really wierd with data sources going on. Maybe it's a Subsonic thing?

EDIT: For some reason the set accessor is hit only on the AssetID property and only the first time. WPF is now heading for WTF :)

EDIT 2: You gotta be kidding me- I've removed the binding (ie it only has a displaymemberpath, a valuememberpath and an itemssouce) and it's STILL doing it! It accepts your first selection, and then won't change.

Answer 1

Sounds like the field is somehow readonly, or that your change isn't being persisted. After the binding sets the new value, it will re-read the property to ensure that it was actually changed. If your property returns the old value, then it'll be re-selected in the combo box, giving the appearance that the value never changed.

I don't know that DAL, but can you step through the property setter code? You might also have an issue with type conversion.

EDIT reading your comment about the red rectangle -- it sounds as though your property (or something to do with the binding) is raising an exception. Unless, of course, you're using data validation in your UI. You might turn 'Break on all exceptions' in the debugger's settings, assuming you're using Visual Studio.

EDIT 2 You should check the VS Output pane for any error messages related to binding. You can also read this blog post which gives more info on debugging bindings.

Answer 2

It's hard to tell from a small sample of your code but try commenting out the line:

//this.DataContext = oJob; 

and see if this helps.

Setting the DataContext and ItemsSource might be causing a conflict.

Answer 3

I've narrowed it down to the Subsonic objects used as ComboBoxItems. If you create a new class that uses exactly the same code as the relevant parts of the Subsonic one, it works. If you use POCOs/datatables for the combos and Subsonic for the record being edited, it works. But if you use Subsonic for both, it doesn't.

I had hoped to extend the subsonic objects and not have to code a full-blown BLL tier. Looks like I'm faced with doing that or throwing out Subsonic for the DAL. I might post a more specific question for the Subsonic folks.

Many thanks to all who contributed.

Answer 4

Did you write any global style for your combo box which may have a bug or something missing? Or are you using pure default styles for your combobox? Try removing any default styles applied.

Are you wiring up any events? If your code hooks up for event like PreviewMouseLeftButtonUp and marks event as handled then probably combobox may ignore and wont select anything.