Why Doesn't My WPF ComboBox SelectedItem/SelectedValue Update When Bound to a Custom List?

WPF ComboBox and Custom Lists: Binding Challenges and Solutions

Binding a WPF ComboBox to a custom list involves using DisplayMemberPath and SelectedValuePath to control how data is displayed and selected. However, a common problem is the failure of SelectedItem/SelectedValue to update correctly. This often stems from an improperly set DataContext.

The DataContext Problem

The DataContext dictates which object's properties are used for data binding within a given element. Forgetting to set it correctly for your ComboBox is a frequent cause of binding issues.

Solution: Setting the DataContext

To fix this, ensure the ComboBox's DataContext points to the object containing the property you're binding to (SelectedValue or SelectedItem). Example:

<ComboBox 
    DataContext="{Binding RelativeSource={RelativeSource AncestorType={x:Type Window}}}" 
    DisplayMemberPath="Name" 
    ItemsSource="{Binding Path=PhonebookEntries}" 
    SelectedValue="{Binding Path=PhonebookEntry}" 
    SelectedValuePath="Name">
</ComboBox>

This binds the ComboBox to a Window's DataContext (typically your ViewModel), ensuring access to the ViewModel's properties.

Important Considerations:

• Matching Property Names: Your custom list's class must have a public property matching both DisplayMemberPath and SelectedValuePath (in this case, "Name").
• CollectionView Issues (Older .NET): In .NET 4.5 and earlier, using CollectionView directly can cause binding problems. A derived class is recommended. This limitation is absent in .NET 4.6 and later.

Successful Binding: A Summary

By correctly setting the DataContext, using appropriate DisplayMemberPath and SelectedValuePath values, and verifying your custom list's properties, you can reliably bind a WPF ComboBox to your custom data.

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