WPF ComboBox SelectionChanged event to command not firing

Question

I have the following XAML of a ComboBox which has a code-behind SelectionChanged event handler and another Command property of the ViewModel. I have set the SelectedIndex property to 0. Now when I run the project, the code-behind handler is invoked, but the Command is not executed. What I want is that the Command should be executed for SelectedIndex=0 the first time the View is loaded.

<ComboBox Name="listComboBox" SelectionChanged="listComboBox_SelectionChanged" SelectedIndex="0" SelectedValuePath="Content" Margin="5,0" Height="35" Width="150" VerticalAlignment="Center" HorizontalAlignment="Left">
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="SelectionChanged">
            <i:InvokeCommandAction Command="{Binding ListTypeComboSelectionChangedCmd}" CommandParameter="{Binding ElementName=listComboBox, Path=SelectedValue}"/>
        </i:EventTrigger>
     </i:Interaction.Triggers>
     <ComboBoxItem Content="ItemOne" />
     <ComboBoxItem Content="ItemTwo" />
     <ComboBoxItem Content="ItemThree" />
</ComboBox>

Update

Code-behind event handler:

private void listComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e) { }

ICommand object:

public ICommand ListTypeComboSelectionChangedCmd 
{ 
    get { return new RelayCommand<string>(ListTypeComboSelectionChangedCmdExec); }
    private set;
}

ICommand Handler:

private void ListTypeComboSelectionChangedCmdExec(string listType) { }

Solution 1

Bind SelectedValue to a Property on the view model.

In the Property set{...} block do your logic there or call

ListTypeComboSelectionChangedCmdExec(value)

See Binding ComboBox SelectedItem using MVVM

Solution 2

In my case, I use handler in the code behind and connect it to ModelView as below.

var viewModel = (MyViewModel)DataContext;
if (viewModel.MyCommand.CanExecute(null))
    viewModel.MyCommand.Execute(null);

Please check this link: Call Command from Code Behind

Comments

[Lucifer]

That's a smart solution, but I would have preferred to keep the setter clean. Also I was more interested in knowing as to why such a functionality doesn't work, where the SelectionChanged event fires but it does not execute the underlying Command. Thanks anyway.

[Richard June]

Why not Bind SelectedValue to the Property, but in your view OnLoaded event handler, set it to the first value. Setter logic is sub optimal

[Michal Ciechan]

@Lucifer bit late of a reply, but my guess would be that the event gets fired before the behaviour gets 'Attached'.