We Keep Coding

WPF: access ElementName in a list within a list on the XAML side

I'm having a hard time solving a potential newbie problem: I've got a ObservableCollection<TopItem> MyTopItems that I display in a ListView. The type TopItem contains a string TopName and an ObservableCollection<NestedItem> NestedItems. The type NestedItem contains only a string NestedName.
My problematic is quite simple: I want to retrieve information on the nested item that I select, on the XAML side.
Right now, I can retrieve the selected item of TopItems quite easily, but I can't retrieve the selected item of NestedItems.
I know that I can bind the selected item (for TopItems and NestedItems) in the view model, but in my case it's almost pointless because I've got no use for it in the view model. Plus, I'd really like to know how to do it on the XAML side!
Enough talk, now comes the code.
A class to implement to INotifyPropertyChanged interface that I'm gonna use in my models and view model; not the cleanest way of doing, but it's for the sake of the demo. This class is just there to see the big picture, just know that it works well:
using System.ComponentModel;
using System.Runtime.CompilerServices;
namespace WpfSelectItemInDoubleList.Utils
{
public abstract class INPCBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected void RaisedPropertyChanged([CallerMemberName]string propertyName = null)
{
if (this.PropertyChanged != null)
{
PropertyChanged.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
protected bool SetProperty<T>(ref T storage, T value, [CallerMemberName]string propertyName = null)
{
if (Equals(storage, value))
{
return false;
}
else
{
storage = value;
RaisedPropertyChanged(propertyName);
return true;
}
}
}
}
Comes the NestedItem type:
using WpfSelectItemInDoubleList.Utils;
namespace WpfSelectItemInDoubleList.Model
{
public class NestedItem : INPCBase
{
private string _NestedName;
public string NestedName
{
get { return this._NestedName; }
set
{
SetProperty(ref this._NestedName, value);
}
}
public NestedItem(string nestedName)
{
NestedName = nestedName;
}
}
}
The TopItem type:
using System.Collections.ObjectModel;
using WpfSelectItemInDoubleList.Utils;
namespace WpfSelectItemInDoubleList.Model
{
public class TopItem : INPCBase
{
private string _TopName;
public string TopName
{
get { return this._TopName; }
set
{
SetProperty(ref this._TopName, value);
}
}
private ObservableCollection<NestedItem> _NestedItems;
public ObservableCollection<NestedItem> NestedItems
{
get { return this._NestedItems; }
set
{
SetProperty(ref this._NestedItems, value);
}
}
public TopItem(string topName)
{
TopName = topName;
}
}
}
The view model:
using System.Collections.ObjectModel;
using WpfSelectItemInDoubleList.Model;
using WpfSelectItemInDoubleList.Utils;
namespace WpfSelectItemInDoubleList.ViewModel
{
public class MainWindowViewModel : INPCBase
{
private ObservableCollection<TopItem> _TopItems;
public ObservableCollection<TopItem> TopItems
{
get { return this._TopItems; }
set
{
SetProperty(ref this._TopItems, value);
}
}
public MainWindowViewModel()
{
TopItems = new ObservableCollection<TopItem>();
for (int i = 0; i < 5; i++)
{
var topItem = new TopItem($"top item {i}")
{
NestedItems = new ObservableCollection<NestedItem>()
};
for (int j = 0; j < 5; j++)
{
var nestedItem = new NestedItem($"NI {j}");
topItem.NestedItems.Add(nestedItem);
}
TopItems.Add(topItem);
}
}
}
}
Finally, the most important part: the XAML!:
<Window x:Class="WpfSelectItemInDoubleList.View.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:local="clr-namespace:WpfSelectItemInDoubleList"
xmlns:vm="clr-namespace:WpfSelectItemInDoubleList.ViewModel"
mc:Ignorable="d"
Title="List in list" Height="350" Width="525">
<Window.DataContext>
<vm:MainWindowViewModel />
</Window.DataContext>
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="*" />
<RowDefinition Height="50" />
<RowDefinition Height="50" />
</Grid.RowDefinitions>
<ListView x:Name="TopItemsLV" Grid.Row="0" Margin="10" HorizontalContentAlignment="Stretch" ItemsSource="{Binding TopItems, Mode=TwoWay}" SelectionMode="Single">
<ListView.ItemTemplate>
<DataTemplate>
<Grid HorizontalAlignment="Stretch">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="100" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<TextBlock x:Name="TopNameTB" Grid.Column="0" Text="{Binding TopName}" TextWrapping="Wrap" VerticalAlignment="Center" />
<StackPanel Grid.Column="1">
<ListView x:Name="NestedItemsLV" ItemsSource="{Binding NestedItems}" BorderThickness="0" HorizontalAlignment="Center" SelectionMode="Single">
<ListView.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel Background="Transparent" Orientation="Horizontal" />
</ItemsPanelTemplate>
</ListView.ItemsPanel>
<ListView.ItemTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<TextBlock Text="{Binding NestedName}" />
</StackPanel>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</StackPanel>
<ContentControl Grid.Row="1" />
</Grid>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
<ContentControl Margin="10" Grid.Row="1" Content="{Binding ElementName=TopItemsLV, Path=SelectedItem.TopName}" />
<ContentControl Margin="10" Grid.Row="2" Content="{Binding ElementName=NestedItemsLV, Path=SelectedItem.NestedName}" />
</Grid>
</Window>
The interesting part is the second ContentControl. The first one is working well, but the second doesn't: nothing is showing when I select a nested item. A hint is given to me by intellisense: it sees the TopItemsLV, but not the NestedItemsLV.
Prepare for the most beautiful UI ever. Please don't stole it from me, I'm planning to make millions out of it! Just kidding.
As you can see, the selected item from TopItems is showing, but not the selected item from NestedItems. Any idea why?
Thanks :)

EDIT: Skip the first solution. It's more appropriate for really simple views. Scroll down to my second solution instead.
If you are only binding a single ItemsControl (e.g., ListView) to this list of TopItem instances, then you could just the default collection view manage the selected items for you. That's probably the simplest way to do this.
First, set IsSynchronizedWithCurrentItem="True" on both TopItemsLV and NestedItemsLV.
Then, change your content control bindings as follows:
<ContentControl Content="{Binding Path=TopItems/TopName}" />
<ContentControl Content="{Binding Path=TopItems/NestedItems/NestedName}" />
The / separator in a binding path means "drill down into the currently selected item". The selected item is maintained by the default collection view for both your TopItems collection and each NestedItems collection. The default collection view is what you would get if you called CollectionViewSource.GetDefaultView.
Better Solution
The conventional MVVM approach would be to add a SelectedItem property alongside your TopItems and NestedItems collections. Make sure they fire property change events. The property type should match the corresponding collection's element type. If these properties start out with a null value, then nothing will be selected initially, which is what you want.
On both list views, set SelectedItem="{Binding SelectedItem, Mode=TwoWay}". Remove the IsSynchronizedWithCurrentItem settings from my original answer.
Adjust your content control bindings as follows:
<ContentControl Content="{Binding Path=SelectedItem.TopName}" />
<ContentControl Content="{Binding Path=SelectedItem.SelectedItem.NestedName}" />
Attach a new event handler to NestedItemsLV:
<ListView x:Name="NestedItemsLV"
GotFocus="OnNestedItemsLVGotFocus"
... />
In your view's code-behind, implement the handler as follows:
private void OnNestedItemsLVGotFocus(object sender, RoutedEventArgs e)
{
var viewModel = this.DataContext as MainWindowViewModel;
var parentItem = (sender as FrameworkElement)?.DataContext as TopItem;
if (viewModel != null && parentItem != null)
viewModel.SelectedItem = parentItem;
}
I think you'll agree that this solution works better.

How to switch views within one window from button in child view in WPF MVVM?

I have an issue with switching views in a WPF MVVM app.
When clicking on menu items defined in the main view, switching works fine.
When clicking on a button in a child view, switching does not work as expected.
If I set contentcontrol in child view (where the button is) as well as parent view,
the child view gets displayed mixed with previous displayed view, a button from one view and background from the one I want to switch to.
Without it, the debugger shows something happening, similar steps in the ViewModelBase class to what happened when choosing from the menu mentioned above but no visual changes in the window.
I have commands in a ViewmodelBase (that all viewmodels inherit from either directly or through a mainviewmodel) class that gets called from bindings such as in the XAML above.
CurrentViewModel is a property in ViewModelBase that is used to determine which view gets displayed. In the constructor of ViewModelBase i set commands for example:
CategoryVMCommand = new RelayCommand(() => ExecuteCategoryVMCommand());
(RelayCommand from the line above comes from the MVVM light framework,
although its not necessary for the solution to use that framework)
I found many tutorials and answers for similar problems, but couldnt get any of them to work. For example I tried, without success, using IOC for a similar problem in the below link:
MVVM Main window control bind from child user control
Here are some of the code involved and description of what Im doing:
Main Window:
<Grid>
<ContentControl Content="{Binding CurrentViewModel}" />
<DockPanel Margin="0,0,0,50">
<Menu DockPanel.Dock="Top">
<MenuItem Header="_File">
<MenuItem Header="_Open" Command="{Binding CategoryVMCommand}"/>
<MenuItem Header="_Close"/>
<MenuItem Header="_Save"/>
</MenuItem>
<MenuItem Header="_New">
<MenuItem Header="_Create" Command="{Binding MainControlVMCommand}"/>
</MenuItem>
</Menu>
<StackPanel></StackPanel>
</DockPanel>
</Grid>
</Window>
Then I select Menu item New, the following view is displayed:
<UserControl x:Class="WpfApplication1.MainControl"
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"
d:DesignHeight="300"
d:DesignWidth="300"
mc:Ignorable="d">
<Grid>
<!--<ContentControl Content="{Binding CurrentViewModel, Mode=OneWay}" />-->
<TextBlock HorizontalAlignment="Left" Margin="10,20,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Height="39" Width="144" FontSize="24"><Run Language="en-gb" Text="TITLE"/><LineBreak/><Run Language="en-gb"/></TextBlock>
<Button Content="Open category" HorizontalAlignment="Left" Margin="10,136,0,0" VerticalAlignment="Top" Width="153" Height="63" Command="{Binding CategoryVMCommand}" />
<Button Content="Create new category" HorizontalAlignment="Left" Margin="10,218,0,0" VerticalAlignment="Top" Width="153" Height="63"/>
<ListBox HorizontalAlignment="Left" Height="145" Margin="293,136,0,0" VerticalAlignment="Top" Width="201" Background="#FFDDDDDD"/>
<TextBlock HorizontalAlignment="Left" Margin="293,107,0,0" TextWrapping="Wrap" Text="Recently Used" VerticalAlignment="Top" FontSize="18"/>
</Grid>
</UserControl>
button open category clicked, and Currentviewmodel set code executes (depending on ContenControl in MainControl view being commented out or not either
return or assigned), then The ExecuteCategoryCommand get executed. Then the line with the expected command in ViewModelBase constructor executes, although
either no change or the mixed result i mentioned originally
ViewModelBase class:
namespace ViewModel
{
public class ViewModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private ViewModelBase _currentViewModel;
public ICommand CategoryVMCommand { get; private set; }
public ICommand MainControlVMCommand { get; private set; }
protected void NotifyPropertyChanged( String propertyName = "")
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
public ViewModelBase()
{
MainControlVMCommand = new RelayCommand(() => ExecuteMainControlVMCommand());
CategoryVMCommand = new RelayCommand(() => ExecuteCategoryVMCommand());
}
public ViewModelBase CurrentViewModel
{
get
{
return _currentViewModel;
}
set
{
if (_currentViewModel == value)
return;
_currentViewModel = value;
NotifyPropertyChanged("CurrentViewModel");
}
}
protected void ExecuteCategoryVMCommand()
{
CurrentViewModel = null;
CurrentViewModel = new CategoryVM();
}
protected void ExecuteMainControlVMCommand()
{
CurrentViewModel = null;
CurrentViewModel = new MainControlVM();
}
}
}
So my question is how can I click the button in the child view, send command from ViewModelBase, set CurrentViewModel, and successfully switch views within one window without any visual remains of the previously displayed view?
Thanks for any help.

ListBox not correctly changing ContentControl's ViewModel

I'm still pretty new to WPF and I decided to change the application I am developing to start following the MVVM pattern as best as I could. I am running into a problem when I try to have a list box dictate the view model of a content control. I've been stuck on this for a while and searching the internet is not producing answers for me.
For some reason a new instance of the view model the list box contains is being generated as the data context of the content control. When I was debugging I made sure that the list box contains the view models it should, and that the item I select on the list box is indeed the item that the list box is selecting, however the content control changing based on the selection. There is a view model populating the content control, however it is not in the collection the list box populates from. And I can somehow delete the view model in the content control via my remove button. But when I make a selection change on the list box, or add a new item to the collection it populates the content control with a new view model that once again is not in the collection. I have no clue why it is doing this, or what in my code would suggest this behavior.
I made a simple application to try and figure out what I'm doing wrong. It replicates my problem perfectly. I'm pretty sure the buttons don't adhere to MVVVM (supposed to run a command contained in the view model to adhere to MVVM from what I've been reading) but that is not my main concern right now as the problem exists without the buttons.
MainWindow.xml
<Window
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication1" x:Class="WpfApplication1.MainWindow"
Title="MainWindow" Height="440" Width="436">
<Window.DataContext>
<local:mwvm/>
</Window.DataContext>
<Window.Resources>
<DataTemplate DataType="{x:Type local:ucvm}">
<local:uc/>
</DataTemplate>
</Window.Resources>
<Grid>
<Button Content="a" HorizontalAlignment="Left" Margin="323,351,0,0" VerticalAlignment="Top" Width="95" Click="Button_Click"/>
<Button Content="r" HorizontalAlignment="Left" Margin="323,378,0,0" VerticalAlignment="Top" Width="95" Click="Button_Click_1"/>
<ContentControl Margin="10,10,110,10" Content="{Binding SelectedItem, ElementName=lb_UCs}"/>
<ListBox x:Name="lb_UCs" HorizontalAlignment="Left" Height="336" Margin="323,10,0,0" VerticalAlignment="Top" Width="95" ItemsSource="{Binding UCs}" DisplayMemberPath="CoolText"/>
</Grid>
</Window>
MainWindow.xaml.cs
public partial class PanelPartsView : UserControl
{
private PanelPartsViewModel _DC;
public PanelPartsView()
{
InitializeComponent();
_DC = DataContext as PanelPartsViewModel;
}
private void btn_Remove_Click(object sender, RoutedEventArgs e)
{
_DC.Panels.Remove(lb_Panels.SelectedItem as PartsViewModel);
}
private void btn_Add_Click(object sender, RoutedEventArgs e)
{
var pvm = new PartsViewModel();
_DC.Panels.Add(pvm);
lb_Panels.SelectedItem = pvm;
System.Console.WriteLine("lb_Panels.selecteditem = {0}", ((PartsViewModel)lb_Panels.SelectedItem).PanelName);
System.Console.WriteLine("cc_PanelParts.content = {0}", ((PartsViewModel)cc_PanelParts.Content).PanelName);
}
}
mwvm
class mwvm
{
private ObservableCollection<ucvm> _UCs = new ObservableCollection<ucvm>();
public ObservableCollection<ucvm> UCs
{
get { return _UCs; }
}
public mwvm()
{
//this is for for testing, the real application would be purely dynamic
_UCs.Add(new ucvm());
_UCs.Add(new ucvm());
_UCs.Add(new ucvm());
}
}
uc.xaml
<UserControl
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:WpfApplication1" x:Class="WpfApplication1.uc"
mc:Ignorable="d" d:DesignWidth="300" Height="90">
<Grid>
<Grid.DataContext>
<local:ucvm/>
</Grid.DataContext>
<Button Content="{Binding CoolText}" Margin="10,10,10,0" Height="44" VerticalAlignment="Top"/>
<TextBox Height="23" Margin="10,59,10,0" TextWrapping="Wrap" Text="{Binding CoolText}" VerticalAlignment="Top"/>
</Grid>
</UserControl>
uc.xaml.cs
public partial class uc : UserControl
{
public uc()
{
InitializeComponent();
}
}
ucvm.cs
class ucvm : INotifyPropertyChanged
{
private static int i = 1;
private string _CoolText = "<" + i++ + ">" + System.DateTime.Now.ToLongTimeString();
public string CoolText
{
get { return _CoolText; }
set
{
_CoolText = value;
NPC("CoolText");
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void NPC(string s)
{
if (PropertyChanged != null) PropertyChanged(this, new PropertyChangedEventArgs(s));
}
}
I have also tried binding the content control like so...
<ContentControl Content="{Binding SelectedUCVMl, Mode=OneWay}"/>
<ListBox x:Name="lb_UCs" ItemsSource="{Binding UCs}" SelectedItem="{Binding SelectedUCVM}" DisplayMemberPath="CoolText"/>
...and so...
<ContentControl Content="{Binding UCs/}"/>
<ListBox x:Name="lb_UCs" ItemsSource="{Binding UCs}" IsSynchronizedWithCurrentItem="True" DisplayMemberPath="CoolText"/>
but to no avail.
Any help would be greatly appreciated.

It looks like you just have to remove this part from uc.xaml:
<Grid.DataContext>
<local:ucvm/>
</Grid.DataContext>
This syntax creates a new instance of the view model, each time an instance of uc.xaml is created, which of course isn't what you want. You want the data context of uc.xaml instances to inherit the instance currently selected in the list box.

How to access objects in code from XAML

I am new to WPF and am trying to understand how to use data binding to bind the controls on my window to objects in my code behind. I see several questions about accessing XAML objects from the codebehind, but that's not what I'm looking for. I already know how to do that.
label1.Content = LabelText;
listbox1.ItemsSource = ListItems;
I have also seen answers about how to access a class in the codebehind from XAML.
<local:MyClass x:Key="myClass" />
But I don't see how to apply that to a specific instance of the class. Here is an example of what I'm trying to do. The 'Bindings' are obviously incorrect. That is what I need help with.
public partial class MainWindow : Window
{
private string _labelText;
private List<string> _listItems = new List<string>();
public MainWindow()
{
InitializeComponent();
_labelText = "Binding";
_listItems.Add("To");
_listItems.Add("An");
_listItems.Add("Object");
}
public string LabelText
{
get { return _labelText; }
set { _labelText = value; }
}
public List<string> ListItems
{
get { return _listItems; }
set { _listItems = value; }
}
}
<Window x:Class="SO_Demo.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="SO Demo" Height="160" Width="225">
<Grid DataContext="MainWindow">
<Label x:Name="label1" Width="80" Height="25" Margin="12,12,0,0"
Content="{Binding Path=LabelText}"
HorizontalAlignment="Left" VerticalAlignment="Top" />
<ListBox x:Name="listbox1" Width="100" Height="60" Margin="12,44,0,0"
ItemsSource="{Binding Path=ListItems}" DisplayMemberPath="ListItems"
HorizontalAlignment="Left" VerticalAlignment="Top" />
</Grid>
</Window>
The books and tutorials I have read make it sound like this should be very simple. What am I missing?

While you can DataBind directly to the class in the manner you're attempting, it is not how this is commonly done. The recommended approach is to create an object (ViewModel) that aggregates all the model data you want displayed in your UI, and then set that ViewModel as the DataContext of your View (Window in this case). I would recommend reading about MVVM, which is how most WPF application are built. But the example below can get you started.
Here is a simple example based on your sample above:
ViewModel
public class MyViewModel : INotifyPropertyChanged
{
private string _title;
private ObservableCollection<string> _items;
public string LabelText
{
get { return _title; }
set
{
_title = value;
this.RaisePropertyChanged("Title");
}
}
public ObservableCollection<string> ListItems {
get { return _items; }
set
{
_items = value; //Not the best way to populate your "items", but this is just for demonstration purposes.
this.RaisePropertyChanged("ListItems");
}
}
//Implementation of INotifyPropertyChanged
public event PropertyChangedEventHandler PropertyChanged;
protected void RaisePropertyChanged(string propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
CodeBehind
public partial class MainWindow : Window
{
private MyViewModel _viewModel;
public MainWindow()
{
InitializeComponent();
_viewModel = new MyViewModel();
//Initialize view model with data...
this.DataContext = _viewModel;
}
}
View (Window)
<Window x:Class="SO_Demo.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="SO Demo" Height="160" Width="225">
<Grid>
<Label x:Name="label1" Width="80" Height="25" Margin="12,12,0,0" Content="{Binding Path=LabelText}"
HorizontalAlignment="Left" VerticalAlignment="Top" />
<ListBox x:Name="listbox1" Width="100" Height="60" Margin="12,44,0,0"
ItemsSource="{Binding Path=ListItems}"
HorizontalAlignment="Left" VerticalAlignment="Top" />
</Grid>
</Window>

<Grid DataContext="MainWindow"> is invalid.
If you want to reference the window you must either:
<Window x:Name="MyWindow">
<Grid DataContext="{Binding ElementName=MyWindow}"/>
</Window>
or
<Grid DataContext="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}}"/>

Passing value from child window to parent window using WPF and MVVM pattern

I have parent window which has textBox called "SchoolName", and a button called "Lookup school Name".
That Button opens a child window with list of school names. Now when user selects school Name from child window, and clicks on "Use selected school" button. I need to populate selected school in parent view's textbox.
Note: I have adopted Sam’s and other people’s suggestion to make this code work. I have updated my code so other people can simply use it.
SelectSchoolView.xaml (Parent Window)
<Window x:Class="MyProject.UI.SelectSchoolView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Parent" Height="202" Width="547">
<Grid>
<TextBox Height="23" Width="192"
Name="txtSchoolNames"
Text="{Binding Path=SchoolNames, UpdateSourceTrigger=PropertyChanged,
Mode=TwoWay}"
/>
<Label Content="School Codes" Height="28" HorizontalAlignment="Left"
Margin="30,38,0,0" Name="label1" VerticalAlignment="Top" />
<Button Content="Lookup School Code" Height="30" HorizontalAlignment="Left"
Margin="321,36,0,0" Name="button1" VerticalAlignment="Top" Width="163"
Command="{Binding Path=DisplayLookupDialogCommand}"/>
</Grid>
</Window>
SchoolNameLookup.xaml (Child Window for Look up School Name)
<Window x:Class="MyProject.UI.SchoolNameLookup"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:toolkit="http://schemas.microsoft.com/wpf/2008/toolkit"
Title="SchoolCodeLookup" Height="335" Width="426">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="226*" />
<RowDefinition Height="70*" />
</Grid.RowDefinitions>
<toolkit:DataGrid Grid.Row="0" Grid.Column="1" x:Name="dgSchoolList"
ItemsSource="{Binding Path=SchoolList}"
SelectedItem="{Binding Path=SelectedSchoolItem, Mode=TwoWay}"
Width="294"
AutoGenerateColumns="False"
CanUserAddRows="False"
CanUserDeleteRows="False"
CanUserResizeRows="False"
CanUserSortColumns="True"
SelectionMode="Single">
<Button Grid.Row="1" Grid.Column="1" Content="Use Selected School Name"
Height="23" Name="btnSelect" Width="131" Command="{Binding
Path=UseSelectedSchoolNameCommand}" />
</Grid>
</Window>
SchoolNameLookupViewModel
private string _schoolNames;
public string SchoolNames
{
get { return _schoolNames; }
set
{
_schoolNames= value;
OnPropertyChanged(SchoolNames);
}
}
private ICommand _useSelectedSchoolNameCommand;
public ICommand UseSelectedSchoolNameCommand{
get
{
if (_useSelectedSchoolNameCommand== null)
_useSelectedSchoolNameCommand= new RelayCommand(a =>
DoUseSelectedSchollNameItem(), p => true);
return _useSelectedSchoolNameCommand;
}
set
{
_useSelectedSchoolNameCommand= value;
}
}
private void DoUseSelectedSchoolNameItem() {
StringBuilder sfiString = new StringBuilder();
ObservableCollection<SchoolModel> oCol =
new ObservableCollection<SchoolModel>();
foreach (SchoolModel itm in SchollNameList)
{
if (itm.isSelected) {
sfiString.Append(itm.SchoolName + "; ");
_schoolNames = sfiString.ToString();
}
}
OnPropertyChanged(SchoolNames);
}
private ICommand _displayLookupDialogCommand;
public ICommand DisplayLookupDialogCommand
{
get
{
if (_displayLookupDialogCommand== null)
_displayLookupDialogCommand= new
RelayCommand(a => DoDisplayLookupDialog(), p => true);
return _displayLookupDialogCommand;
}
set
{
_displayLookupDialogCommand= value;
}
}
private void DoDisplayLookupDialog()
{
SchoolNameLookup snl = new SchoolNameLookup();
snl.DataContext = this; //==> This what I was missing. Now my code works as I was expecting
snl.Show();
}

My solution is to bind both the windows to the same ViewModel, then define a property to hold the resulting value for codes, lets call it CurrentSchoolCodes, Bind the label to this property. Make sure that CurrentSchoolCodes raises the INotifyPropertyChanged event.
then in the DoUseSelectedSchoolNameItem set the value for CurrentSchoolCodes.
For properties in your models I suggest you to load them as they are required(Lazy Load patttern). I this method your property's get accessor checks if the related field is still null, loads and assigns the value to it.
The code would be like this code snippet:
private ObservableCollection<SchoolModel> _schoolList;
public ObservableCollection<SchoolModel> SchoolList{
get {
if ( _schoolList == null )
_schoolList = LoadSchoolList();
return _schoolList;
}
}
In this way the first time your WPF control which is binded to this SchoolList property tries to get the value for this property the value will be loaded and cached and then returned.
Note: I have to say that this kind of properties should be used carefully, since loading data could be a time consuming process. And it is better to load data in a background thread to keep UI responsive.

The Solution Sam suggested here is a correct one.
What you didn't get is that you should have only one instance of you viewmodel and your main and child page should refer to the same one.
Your viewmodel should be instanciated once: maybe you need a Locator and get the instance there... Doing like this the code in your ctor will fire once, have a look at the mvvmLight toolkit, I think it will be great for your usage, you can get rid of those Classes implementing ICommand too...
You can find a great example of using that pattern here:
http://blogs.msdn.com/b/kylemc/archive/2011/04/29/mvvm-pattern-for-ria-services.aspx
basically what happens is this:
you have a Locator
public class ViewModelLocator
{
private readonly ServiceProviderBase _sp;
public ViewModelLocator()
{
_sp = ServiceProviderBase.Instance;
// 1 VM for all places that use it. Just an option
Book = new BookViewModel(_sp.PageConductor, _sp.BookDataService);
}
public BookViewModel Book { get; set; }
//get { return new BookViewModel(_sp.PageConductor, _sp.BookDataService); }
// 1 new instance per View
public CheckoutViewModel Checkout
{
get { return new CheckoutViewModel(_sp.PageConductor, _sp.BookDataService); }
}
}
that Locator is a StaticResource, in App.xaml
<Application.Resources>
<ResourceDictionary>
<app:ViewModelLocator x:Key="Locator" d:IsDataSource="True" />
</ResourceDictionary>
</Application.Resources>
in your views you refer you viewmodels trough the Locator:
DataContext="{Binding Book, Source={StaticResource Locator}}"
here Book is an instance of BookViewModel, you can see it in the Locator class
BookViewModel has a SelectedBook:
private Book _selectedBook;
public Book SelectedBook
{
get { return _selectedBook; }
set
{
_selectedBook = value;
RaisePropertyChanged("SelectedBook");
}
}
and your child window should have the same DataContext as your MainView and work like this:
<Grid Name="grid1" DataContext="{Binding SelectedBook}">