Why MVVM/WPF ComboBox SelectedItem is null in multibinding to other control's Visibility?

0
votes

EDIT: I bound to same property (SearchType) that combobox binds to -> works fine. I still would like to know why my first solution described here does not work.

I have

public enum SearchType
{
    NetworkObjects,
    Customers
}

In ViewModel contructor:

public  SearchViewModel()
{
    SearchType = Panels.SearchType.NetworkObjects;

In xaml:

<UserControl.Resources>
    <xpui:ConvertSearchTypeToVisibility x:Key="searchtypetovisibilityconverter" />
</UserControl.Resources>

<ComboBox
            Name="SearchTypeComboBox"
            ItemsSource="{Binding Path=SearchTypes}"
            SelectedItem="{Binding Path=SearchType, Mode=TwoWay}">
...
<DataGrid.Visibility>
   <MultiBinding Converter="{StaticResource searchtypetovisibilityconverter}">
       <Binding RelativeSource="{RelativeSource Self}" Path="Name"/>
           <Binding ElementName="SearchTypeComboBox" Path="SelectedItem" />
   </MultiBinding>
</DataGrid.Visibility>

Converter:

public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
    string gridName = (string)values[0];
    SearchType searchType = (SearchType)values[1];

In Convert-method values have 2 items and values[1]==null. Also if I take away the binding SelectedItem is SearchType.NetworkObjects as set in ViewModel constructor. What do I do wrong?

2
wpfxamlmvvmmultibinding
What is SearchTypes, and does SearchType.NetworkObjects exist in it? Also, are you getting an exception; could you use a MultiDataTrigger instead? – Kcvin
I would consider that a valid approach. Now your controls depend on the view model, where if you removed the ComboBox and your DataGrid depended on it, it wouldn't compile. Also, it may be showing up as null during control initialization process, and then be set shortly after rendering. – Kcvin
@matti Could you post all of the code for your converter? I think I may have an answer but don't know what the expected behavior is – Kcvin
How many times it hit converter. When do you apply data context? – HungDL
sometimes it happens to me. I was not sure 100% it is, just my experience. But anyway, happy in helping you ^^ – HungDL

2 Answers

1
votes

I expect that there is something going wrong in code that you haven't posted. I wrote up a solution with very similar behavior using the provided code and there was no case where values[1] == null unless I removed the ComboBox.SelectedItem binding.

Here is the working sample.

1
votes

The problem is that in platform InitializeComponent in code behind is called before the DataContext is set by the platform I use. Therefore the Converter is called with the unbound (default) values, which in this case is null for the SelectedItem. The solution is to check the values-array and especially values[1] and return the Bindin.DoNothing if the value is null (or anything but SearchType).

Guess this is good practice in general. Thanks to @Neil and @NETScape to point this out.

   public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (values != null && values.Length == 2 && values[0] is string && values[1] is SearchType)
        {
            string gridName = (string)values[0];
            SearchType searchType = (SearchType)values[1];

            if ((gridName == "ObjectSearchResults" && searchType == SearchType.NetworkObjects) ||
                (gridName == "CustomerSearchResults" && searchType == SearchType.Customers))
            {
                return Visibility.Visible;
            }
            return Visibility.Collapsed;
        }
        return Binding.DoNothing;
    }