ComboBox's SelectedValue doesn't update the bound property automatically

Question

ComboBox's SelectedValue doesn't update the bound property automatically

Emon Haque 3,176

I've a ComboBox in one of my Views:

<ComboBox Grid.Row="1"
          IsSynchronizedWithCurrentItem="True"
          ItemsSource="{Binding VacantSpaces}"
          DisplayMemberPath="Name"
          SelectedValuePath="Id"
          SelectedValue="{Binding NewObject.SpaceId, Mode=OneWayToSource, UpdateSourceTrigger=PropertyChanged}"/>

when I launch the app and change selection, SelectedValue gets assigned to NewObject.SpaceId. On a button click I new up the NewObject and filter out the space that was selected in the ComboBox this way:

protected override void renewNewObject()
{
    var spaceId = NewObject.SpaceId;
    NewObject = new Lease()
    {
        Id = ++MainVM.maxLeaseId,
        PlotId = NewObject.PlotId,
        //SpaceId = ,
        TenantId = NewObject.TenantId,
        DateStart = DateTime.Now
    };
    OnPropertyChanged(nameof(NewObject));
    NewReceivable.LeaseId = NewObject.Id;

    var space = MainVM.spaces.First(x => x.Id == spaceId);
    space.IsVacant = false;
    space.OnPropertyChanged(nameof(space.IsVacant));
    //var current = VacantSpaces.CurrentItem as Space;
    App.Current.Dispatcher.Invoke(ReceivableHeads.Refresh);;
}

I don't assign the SpaceId to the NewObject when I renew and at that point SpaceId is null. When it hits this line space.OnPropertyChanged(nameof(space.IsVacant)), ComboBox's selection changes but it neither updates the current, VacantSpaces.CurrentItem, nor assigns NewObject 's SpaceId automatically.

If I hookup an event handler on VacantSpaces.CurrentChanged, I see the the CurrentItem changes there but the SpaceId remains null and that event handler is called when it gets out of the renewNewObject function so I've to manually assign SpaceId there!

Why doesn't the ComboBox update NewObject.SpaceId automatically? How to make this assignment automatic?

Accepted answer

1 additional answer

Your answer

Answer 1

Probably, it's not possible with Mode=OneWayToSource. To make it automatic, what I've done is: first removed the Mode:

<cc:ComboField Grid.Row="1" Label="Space"   
                Source="{Binding VacantSpaces}"  
                Display="Name"  
                SelectedValuePath="Id"  
                SelectedValue="{Binding NewObject.SpaceId}"/>

now, after the execution of renewNewObject function, the combobox's SelectedItem becomes null so to remove the null from ComboBox, I've to move CurrentItem manually in Combo's OnItemsChanged:

protected override void OnItemsChanged(NotifyCollectionChangedEventArgs e)  
{  
    switch (e.Action)  
    {  
        ...  
        case NotifyCollectionChangedAction.Remove:  
        case NotifyCollectionChangedAction.Reset:  
            if (e.NewItems == null && !Items.IsEmpty)  
                Items.MoveCurrentToFirst();  
            break;  
    }  
}

I'd that in Reset before because CurrentItem remains null when Plots/Spaces.IsEmpty return false, now I've to have that in Remove as well!

Answer 2

Hi Emon,
I didn't unsterstand you problem. You bind SelectedValue to property "SpaceId" of an object in ItemsSource. After select line in ComboBox the selected "Id" ("SelectedValuePath") will be written in NewObject in "SpaceId" ("SelectedValue"). Try following demo:

XAML:

<Window x:Class="WpfApp1.Window007"  
        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:WpfApp007"  
        mc:Ignorable="d"  
        Title="Window007" Height="450" Width="800">  
  <Window.DataContext>  
    <local:ViewModel/>  
  </Window.DataContext>  
  <StackPanel>  
    <ComboBox Grid.Row="1"  
           IsSynchronizedWithCurrentItem="True"  
           ItemsSource="{Binding VacantSpaces}"  
           DisplayMemberPath="Name"  
           SelectedValuePath="Id"  
           SelectedValue="{Binding NewObject.SpaceId, Mode=OneWayToSource, UpdateSourceTrigger=PropertyChanged}"/>  
    <Button Content="New Lease" Command="{Binding}"/>  
    <Label Content="{Binding NewObject.IsVacant}"/>  
  </StackPanel>  
</Window>

Classes:

using System;  
using System.Collections.Generic;  
using System.ComponentModel;  
using System.Linq;  
using System.Runtime.CompilerServices;  
using System.Windows;  
using System.Windows.Input;  
  
namespace WpfApp007  
{  
  public class ViewModel : ICommand, INotifyPropertyChanged  
  {  
    public ViewModel()  
    {  
      VacantSpaces.Add(new Lease() { Id = 1, Name = "Name 1" });  
      VacantSpaces.Add(new Lease() { Id = 2, Name = "Name 2" });  
      VacantSpaces.Add(new Lease() { Id = 3, Name = "Name 3" });  
      VacantSpaces.Add(new Lease() { Id = 4, Name = "Name 4" });  
      NewObject = new Lease() { Id = 5, Name = "Name 5", IsVacant = true };  
      OnPropertyChanged(nameof(NewObject));  
    }  
    public Lease NewObject { get; set; }  
  
    public List<Lease> VacantSpaces { get; set; } = new List<Lease>();  
  
    protected void renewNewObject()  
    {  
      var spaceId = NewObject.SpaceId;  
      NewObject = new Lease()  
      {  
        Id = ++MainVM.maxLeaseId,  
        PlotId = NewObject.PlotId,  
        //SpaceId = ,  
        TenantId = NewObject.TenantId,  
        DateStart = DateTime.Now  
      };  
      OnPropertyChanged(nameof(NewObject));  
      NewReceivable.LeaseId = NewObject.Id;  
      var space = MainVM.spaces.First(x => x.Id == spaceId);  
      space.IsVacant = false;  
      space.OnPropertyChanged(nameof(space.IsVacant));  
      //var current = VacantSpaces.CurrentItem as Space;  
      Application.Current.Dispatcher.Invoke(ReceivableHeads.Refresh); ;  
    }  
  
    public void Execute(object parameter) => renewNewObject();  
  
    public event EventHandler CanExecuteChanged;  
    public bool CanExecute(object parameter) => true;  
  
    public event PropertyChangedEventHandler PropertyChanged;  
    private void OnPropertyChanged([CallerMemberName] string propName = "") =>  
      PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));  
  
    private class MainVM  
    {  
      public static int maxLeaseId { get; internal set; } = 99;  
      public static List<Lease> spaces { get; internal set; } = new List<Lease>() { new Lease() { Id = 3 } };  
    }  
  
    private class NewReceivable  
    {  
      public static int LeaseId { get; internal set; }  
    }  
  
    private class ReceivableHeads  
    {  
      internal static void Refresh()  
      {  
        //throw new NotImplementedException();  
      }  
    }  
  }  
  
  
  
  public class Lease : INotifyPropertyChanged  
  {  
    public int Id { get; set; }  
    public int SpaceId { get; set; }  
    public int PlotId { get; set; }  
    public int TenantId { get; set; }  
    public DateTime DateStart { get; set; }  
    public string Name { get; set; }  
  
    public bool IsVacant { get; set; }  
  
    public event PropertyChangedEventHandler PropertyChanged;  
    public void OnPropertyChanged([CallerMemberName] string propName = "") =>  
      PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));  
  }  
}

Result:

See Debug:

Emon Haque 3,176 Reputation points

2020-10-24T14:26:29.963+00:00

anonymous user-3316, ComboBox's selection changes automatically. See the Space combobox:

First, the selecteditem was new, as soon as I click the button, new gets removed by a filter in the ICollectionView, VacantSpaces, and the selecteditem in the combobox changes to old1 BUT look at the Debug output: Old SpaceId: 1 & New SpaceId: 1, it doesn't change!
Peter Fleischer (former MVP) 19,341 Reputation points

2020-10-24T14:32:45.35+00:00

Hi Emon,
I create demo with your posted code without changes. I only add code for other classes and methods which you haven't post. And in my demo I see the changes. The Id from selected data item in ComboBox will be written in SpaceId in newObject (see my demo below).
Emon Haque 3,176 Reputation points

2020-10-24T15:06:13.413+00:00

anonymous user-3316, I didn't post all code. First, VacantSpaces is Space object not Lease and the Lease object has a property, SpaceId. Lease object's SpaceId is bound to the Id of selected Space of VacantSpaces. As soon as I let out (click the button) the SelectedItem, vacant space, of the combobox, it becomes occupied and at that point I renew the NewObject (Lease) and set the IsVacant property of previously selected Space to false and call OnPropertyChanged to filter it out of the ComboBox.

If you want to inspect the code, here it is, download the second one RentManager with ICollectionView. I've removed the bin, obj and .vs folders to reduce the size so first add a plot in Plot section of the UI, then add 2 spaces in the Space section and finally add a tenant in Tenant section to work in Lease section. Lease View is in View/Add/AddLeaseView.xaml and the related VM is in ViewModel/Add/AddLeaseVM.cs
Emon Haque 3,176 Reputation points

2020-10-24T16:33:53.29+00:00

anonymous user-3316, also note, NewObject .SpaceId remains null in renewNewObject function. When it gets out of renewNewObject function, the VacantSpaces.CurrentItem changes as the SelectedItem of the ComboBox changes BUT that doesn't update the NewObject .SpaceId!

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ComboBox's SelectedValue doesn't update the bound property automatically

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