WPF ComboBox binding

Question

How can I make a ComboBox to display the selected item?

I have following ComboBox declared in XAML:

<ComboBox Margin="4 0 2 0";
ItemsSource="{Binding YAxes}"
SelectedItem="{Binding SelectedYAxis, Mode=TwoWay}"
DisplayMemberPath="AxisTitle"
SelectedValuePath="AxisTitle"/>

Code behind:

private AxisCollection _yAxes;
public AxisCollection YAxes
{
    get => _yAxes;
    private set
    {
        _yAxes = value;
        OnPropertyChanged("YAxes");
    }
}

private IAxis _selectedXAxis;
public IAxis SelectedXAxis
{
    get => _selectedXAxis;
    set
    {
        _selectedXAxis = value;
        OnPropertyChanged(nameof(SelectedXAxis));
    }
}

On items selection setter on SelectedYAxis fires, but ComboBox does never update its text.

Answer 1

HI,
without knowing your code it's impossible to solve your problem. Try following demo. Select line in upper ComboBox change the displaying Text in Label and update the lower ComboBox.

XAML

<Window x:Class="Window90"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="Window90" Height="450" Width="800">
  <Window.DataContext>
    <local:Window90VM/>
  </Window.DataContext>
  <StackPanel>
    <ComboBox  Margin="4 0 2 0"
                            ItemsSource="{Binding YAxes}"
                            SelectedItem="{Binding SelectedYAxis, Mode=TwoWay}"
                            DisplayMemberPath="AxisTitle"
                            SelectedValuePath="AxisTitle"/>
    <Label Content="{Binding SelectedYAxis.AxisTitle}" Height="100"/>
    <ComboBox  Margin="4 0 2 0"
                            ItemsSource="{Binding YAxes}"
                            SelectedItem="{Binding SelectedYAxis, Mode=TwoWay}"
                            DisplayMemberPath="AxisTitle"
                            SelectedValuePath="AxisTitle"/>
  </StackPanel>
</Window>

And ViewModel and data class:

Imports System.Collections.ObjectModel
Imports System.ComponentModel
Imports System.Runtime.CompilerServices

Public Class Window90VM
  Implements INotifyPropertyChanged

  Public Sub New()
    For i = 1 To 10
      col.Add(New Data With {.AxisTitle = $"Titel {[i]}"})
    Next
    cvs.Source = col
  End Sub

  Private cvs As New CollectionViewSource
  Private col As New ObservableCollection(Of Data)

  Public ReadOnly Property YAxes As ICollectionView
    Get
      Return cvs.View
    End Get
  End Property

  Private _selectedYAxis As Data
  Public Property SelectedYAxis As Data
    Get
      Return Me._selectedYAxis
    End Get
    Set(value As Data)
      Me._selectedYAxis = value
      OnPropertyChanged()
    End Set
  End Property

#Region " PropertyChanged"
  Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
  Friend Sub OnPropertyChanged(<CallerMemberName> Optional propName As String = "")
    RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propName))
  End Sub
#End Region

  Public Class Data
    Public Property AxisTitle As String
  End Class

End Class