CODEWITHSUNDEEP
c#wpfxamlcombobox
ProfK
ProfK

Reputation: 51094

Why is this ComboBox data binding not working?

I have a small test window, like this:

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <ComboBox x:Name="Combo" DisplayMemberPath="Word" ItemsSource="{Binding Numbers}" HorizontalAlignment="Left" Margin="115,27,0,0" VerticalAlignment="Top" Width="120" />
    </Grid>
</Window>

With code-behind:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        Numbers = new ObservableCollection<NumberWord>
                      {
                          new NumberWord {Number = 1, Word = "One"},
                          new NumberWord {Number = 2, Word = "Two"},
                          new NumberWord {Number = 3, Word = "Three"}
                      };
        Combo.ItemsSource = Numbers;
    }
    public ObservableCollection<NumberWord> Numbers { get; set; }
}

I keep seeing answers to my other binding questions that state the explicit setting of Combo.ItemsSource = Numbers; is unnecessary because I have the binding ItemsSource="{Binding Numbers}". I have also been told multiple times that I need not set a DataContext on Combo because the whole window is the data context, and Combo inherits this data context.

My question is, why do I always - not just this combo - set ItemsSource or other binding properties explicitly in code behind. Why is the XAML data binding not working?

Upvotes: 1

Views: 2208

Answers (3)

UFO
UFO

Reputation: 410

you need to implement INotifyPropertyChanged. then use: 

this.DataContext = this;

Upvotes: 0

Nitin Purohit
Nitin Purohit

Reputation: 18578

DataContext is one of the DependancyProperties which the child control inherits from the Parent if the local value is not set.

Hence if you set the DataContext of Window to something, the child controls including your ComboBox will have it.

Now, in your case, you have not set the DataContext of your Window hence your Binding of ItemsSource is not working

If in the constructor of your window, after initializeComponent you do

DataContext = this;

then you need not to set the ItemsSource of Combobox in code behind and your binding of ItemsSource will work on your Combo.

Upvotes: 1

Clemens
Clemens

Reputation: 128136

The binding ItemsSource="{Binding Numbers}" needs a source object, namely one that has a public Numbers property. In your case this is the MainWindow instance.

So either you set the source explicitly by something like this:

ItemsSource="{Binding Numbers,
    RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=Window}}"

or you set the source object by setting a DataContext, e.g. in code behind:

public MainWindow()
{
    InitializeComponent();
    DataContext = this;
    ...
}

In case you need to change the value of the Numbers property later, you also have to add a property change notification mechanism, like implementing the INotifyPropertyChanged interface.

This is all well explained in the Data Binding Overview article on MSDN.

Upvotes: 2

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