How to Add a Colmmand to a ComboBox - wpf

Question

I am attempting to add command capability to a ComboBox. After some searching, I decided on the following approach as being the simplist:
1) Add System.Windows.Interactivity.dll to my References
2) Add the following to my XAML
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
3) Add the following to my ComboBox
<i:Interaction.Triggers>
<i:EventTrigger EventName="SelectionChanged">
<i:InvokeCommandAction Command="{Binding ChangePlanner}" />
</i:EventTrigger>
</i:Interaction.Triggers>
I have two questions:
A) Is this the most straightforward approach? If not, what is?
B) If this is the right approach, why does it not work? That is, my ChangePlanner Sub is not being invoked.

Answer 1

Here is a quick working sample using the triggers with a ComboBox:
ViewModel
public class ShellViewModel : BindableBase
{
private string _selectedItem;
public string Title => "Sample";
public ObservableCollection<string> Items
{
get;
} = new ObservableCollection<string>(new[] { "A", "B", "C" });
public string SelectedItem
{
get => _selectedItem;
set => SetProperty(ref _selectedItem, value);
}
public ICommand ChangeCommand => new DelegateCommand<string>(s => Debug.WriteLine($"Command Executed: {s}"));
}
View
<Window x:Class="Poc.MainWindow"
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:viewModels="clr-namespace:Poc.ViewModels"
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
mc:Ignorable="d"
Title="{Binding Title}" Height="350" Width="525">
<Window.DataContext>
<viewModels:ShellViewModel />
</Window.DataContext>
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"></RowDefinition>
<RowDefinition Height="*"></RowDefinition>
</Grid.RowDefinitions>
<ComboBox Grid.Row="0" ItemsSource="{Binding Items}" SelectedItem="{Binding SelectedItem}">
<i:Interaction.Triggers>
<i:EventTrigger EventName="SelectionChanged">
<i:InvokeCommandAction Command="{Binding ChangeCommand}" CommandParameter="{Binding SelectedItem}"></i:InvokeCommandAction>
</i:EventTrigger>
</i:Interaction.Triggers>
</ComboBox>
</Grid>
Haven't seen your code posted yet, but I am going to guess that you were trying to bind to a method (and not an ICommand).

Answer 2

Is this the most straightforward approach? If not, what is?
The most straightforward and MVVM friendly approach would be to bind the SelectedItem of the ComboBox to a source property of your view model and handle any logic, or invoke the command, in the setter of this one:
private object _selectedItem;
public object SelectedItem
{
get { return _selectedItem; }
set
{
_selectedItem = value;
ChangePlanner.Execute(null);
}
}
why does it not work?
Impossible to say based on the information you have provided. Make sure that ChangePlanner is a public property of the DataContext of the ComboBox that returns an ICommand to begin with.