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WPF .NET unable to add new instance to ObservableCollection

I am new to WPF and losing my mind with issues. I have a view, viewmodel and model. I want the user user to fill in some information in the view, press button to confirm and then have a new instance of the model (with the user specified parameters) added to the ObservableCollection and to my local database.
View: (unrelated stuff hidden)
<TextBox DataContext="{DynamicResource RiderequestViewModel}" Margin="15,0,15,0" Grid.Column="1" Grid.Row="1" FontSize="12" Height="25" Text="{Binding Riderequest.Time}"/>
<TextBox DataContext="{DynamicResource RiderequestViewModel}" Margin="15,0,15,0" Grid.Column="1" Grid.Row="2" FontSize="12" Height="25" Text="{Binding Riderequest.LocationFrom}"/>
<TextBox DataContext="{DynamicResource RiderequestViewModel}" Margin="15,0,15,0" Grid.Column="1" Grid.Row="3" FontSize="12" Height="25" Text="{Binding Riderequest.LocationTo}"/>
<Button DataContext="{DynamicResource RiderequestViewModel}" x:Name="nextBtn" Grid.Column="0" Grid.ColumnSpan="2" Grid.Row="5" Content="Verder" Width="150" Foreground="White" Command="{Binding AddRiderequestCommand}" Click="NextBtn_Click"/>
ViewModel RiderequestViewModel:
namespace Drink_n_Drive.ViewModel
{
class RiderequestViewModel: BaseViewModel
{
private Riderequest riderequest;
private ObservableCollection<Riderequest> riderequests;
public ObservableCollection<Riderequest> Riderequests
{
get
{
return riderequests;
}
set
{
riderequests= value;
NotifyPropertyChanged();
}
}
public Riderequest Riderequest
{
get
{
return riderequest;
}
set
{
riderequest= value;
NotifyPropertyChanged();
}
}
public ICommand AddRiderequestCommand { get; set; }
public ICommand ChangeRiderequestCommand { get; set; }
public ICommand DeleteRiderequestCommand { get; set; }
public RiderequestViewModel()
{
LoadRiderequests(); //load existing from DB
LinkCommands(); //Link ICommands with BaseCommands
}
private void LoadRiderequests()
{
RiderequestDataService riderequestDS = new RiderequestDataService();
Riderequests= new ObservableCollection<Riderequests>(riderequestDS .GetRiderequests());
}
private void LinkCommands()
{
AddRiderequestCommand = new BaseCommand(Add);
ChangeRiderequestCommand = new BaseCommand(Update);
DeleteRiderequestCommand = new BaseCommand(Delete);
}
private void Add()
{
RiderequestDataService riderequestDS = new RitaanvraagDataService();
riderequestDS.InsertRiderequest(riderequest); //add single (new) instance to the DB
LoadRiderequests(); //Reload ObservableCollection from DB
}
private void Update()
{
if (SelectedItem != null)
{
RiderequestDataService riderequestDS = new RiderequestDataService();
riderequestDS.UpdateRiderequest(SelectedItem);
LoadRiderequests(); //refresh
}
}
private void Delete()
{
if (SelectedItem != null)
{
RiderequestDataService riderequestDS = new RiderequestDataService();
riderequestDS.DeleteRiderequest(SelectedItem);
LoadRiderequests();
}
}
private Riderequest selectedItem;
public Riderequest SelectedItem
{
get { return selectedItem; }
set
{
selectedItem = value;
NotifyPropertyChanged();
}
}
}
}
Pressing the button simply does nothing and I don't know why. I also have a diffrent page where I want to show a datagrid of all instances in the ObservableCollection like this:
<DataGrid Grid.Row="1" Grid.ColumnSpan="2" Grid.RowSpan="3" DataContext="{DynamicResource RitaanvragenViewModel}" ItemsSource="{Binding Ritaanvragen}" SelectedItem="{Binding SelectedItem}" />
But the grid just shows completly empty. I have added some dummydata to my DB but still doesn't work.
My appologies for the mix of English and Dutch in the code.

I'm not 100% sure about it but i would do something like this:
As for first step I would change the TextBox to look like this:
<TextBox DataContext="{DynamicResource Ritaanvraag}" Margin="15,0,15,0" Grid.Column="1" Grid.Row="1" FontSize="12" Height="25" Text="{Binding Path=Time, Mode=OneWayToSource}"/>
There's no need to pass your ViewModel to it as a DataSource because your View's first few meta-data related lines should already define what ViewModel does it belong to.
When you not specify the type of your binding, it will use a default binding type which depends on the current object. You're using a TextBox now so it will have a TwoWay binding by default.
If you only want to accept data from the user and you don't want to show the data if your model has any then you should use OneWayToSource. (Note: OneWay is a direction between source -> view.)
I would also remove the DataSource from your DataGrid because you already set it's ItemSource:
<DataGrid Grid.Row="1" Grid.ColumnSpan="2" Grid.RowSpan="3" ItemsSource="{Binding Ritaanvragen}" SelectedItem="{Binding SelectedItem}" />

WPF MVVM moving a UserControl from one ObservableCollection to another by event

I have a checklist view that has 2 ScrollViewers. One checklist is for incomplete items, the other is for complete items. They are populated by 2 separate observable collections and bound to by ItemsControls.
The UserControl has a button, when clicked will move that 'check' to the other collection.
Currently the way I have this setup is in the ViewModel that's the DataContext for the UserControl there is a public event that is subscribed to by the main window's VM by using:
((CheckItemVM) ((CheckListItem) cli).DataContext).CompleteChanged += OnCompleteChanged;
where cli is the checklist item.
then the OnCompleteChanged finds the appropriate View object by using:
foreach (object aCheck in Checks)
{
if (aCheck.GetType() != typeof (CheckListItem)) continue;
if (((CheckListItem) aCheck).DataContext == (CheckItemVM) sender)
{
cliToMove = (CheckListItem) aCheck;
break;
}
}
It's pretty obvious this breaks MVVM and I'm looking for a way around it (CheckListItem is the View, and CheckItemVM is it's DataContext ViewModel). Reasoning for the boxed type is I've got another UserControl that will have instances inside both, which are basically section labels, and I need to be able to sort my observable collections where there is an association between the checklistitem to a specific section by name.

This can be done in MVVM using commands, and bindings....
The idea that I propouse here is to create a command in the Windows view model, that manage the check command, and this command to receive the item view model in the params, then manage the the things in the command. I'm going to show you a simple example, using MvvmLight library:
The model:
public class ItemViewModel : ViewModelBase
{
#region Name
public const string NamePropertyName = "Name";
private string _name = null;
public string Name
{
get
{
return _name;
}
set
{
if (_name == value)
{
return;
}
RaisePropertyChanging(NamePropertyName);
_name = value;
RaisePropertyChanged(NamePropertyName);
}
}
#endregion
#region IsChecked
public const string IsCheckedPropertyName = "IsChecked";
private bool _myIsChecked = false;
public bool IsChecked
{
get
{
return _myIsChecked;
}
set
{
if (_myIsChecked == value)
{
return;
}
RaisePropertyChanging(IsCheckedPropertyName);
_myIsChecked = value;
RaisePropertyChanged(IsCheckedPropertyName);
}
}
#endregion
}
A simple model with two property, one for the name (an identifier) and another for the check status.
Now in the Main View Model, (or Windows view model like you want)....
First the Collections, one for the checked items, and another for the unchecked items:
#region UncheckedItems
private ObservableCollection<ItemViewModel> _UncheckedItems;
public ObservableCollection<ItemViewModel> UncheckedItems
{
get { return _UncheckedItems ?? (_UncheckedItems = GetAllUncheckedItems()); }
}
private ObservableCollection<ItemViewModel> GetAllUncheckedItems()
{
var toRet = new ObservableCollection<ItemViewModel>();
foreach (var i in Enumerable.Range(1,10))
{
toRet.Add(new ItemViewModel {Name = string.Format("Name-{0}", i), IsChecked = false});
}
return toRet;
}
#endregion
#region CheckedItems
private ObservableCollection<ItemViewModel> _CheckedItems;
public ObservableCollection<ItemViewModel> CheckedItems
{
get { return _CheckedItems ?? (_CheckedItems = GetAllCheckedItems()); }
}
private ObservableCollection<ItemViewModel> GetAllCheckedItems()
{
var toRet = new ObservableCollection<ItemViewModel>();
foreach (var i in Enumerable.Range(11, 20))
{
toRet.Add(new ItemViewModel { Name = string.Format("Name-{0}", i), IsChecked = true });
}
return toRet;
}
#endregion
And the command:
#region CheckItem
private RelayCommand<ItemViewModel> _CheckItemCommand;
public RelayCommand<ItemViewModel> CheckItemCommand
{
get { return _CheckItemCommand ?? (_CheckItemCommand = new RelayCommand<ItemViewModel>(ExecuteCheckItemCommand, CanExecuteCheckItemCommand)); }
}
private void ExecuteCheckItemCommand(ItemViewModel item)
{
//ComandCode
item.IsChecked = true;
UncheckedItems.Remove(item);
CheckedItems.Add(item);
}
private bool CanExecuteCheckItemCommand(ItemViewModel item)
{
return true;
}
#endregion
The magic here could be in the Data binding, in this case I used command parameter and the FindAncestor binding, check the Data Template:
<DataTemplate x:Key="UncheckedItemDataTemplate">
<Grid>
<StackPanel Orientation="Horizontal">
<TextBlock HorizontalAlignment="Left" TextWrapping="Wrap" Text="{Binding Name}" VerticalAlignment="Top"/>
<CheckBox HorizontalAlignment="Left" VerticalAlignment="Top" IsChecked="{Binding IsChecked}" IsEnabled="False"/>
<Button Content="Check" Width="75" Command="{Binding DataContext.CheckItemCommand, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:MainWindow}}}" CommandParameter="{Binding Mode=OneWay}"/>
</StackPanel>
</Grid>
</DataTemplate>
<DataTemplate x:Key="CheckedItemDataTemplate">
<Grid>
<StackPanel Orientation="Horizontal">
<TextBlock HorizontalAlignment="Left" TextWrapping="Wrap" Text="{Binding Name}" VerticalAlignment="Top"/>
<CheckBox HorizontalAlignment="Left" VerticalAlignment="Top" IsChecked="{Binding IsChecked}" IsEnabled="False"/>
</StackPanel>
</Grid>
</DataTemplate>
One data template for checked items, and another for unchecked items. Now the usage, this is simpler:
<ListBox Grid.Row="2" Margin="5" ItemsSource="{Binding UncheckedItems}" ItemTemplate="{DynamicResource UncheckedItemDataTemplate}"/>
<ListBox Grid.Row="2" Margin="5" Grid.Column="1" ItemsSource="{Binding CheckedItems}" ItemTemplate="{DynamicResource CheckedItemDataTemplate}"/>
This is a cleaner solution, hope is helps.

Dynamic user control change - WPF

I'm developing an app in WPF and I need to change in runtime a content of a ContentControl depending than the user selected on ComboBox.
I have two UserControls and at my combo exists two itens, corresponding each one each.
First usercontrol:
<UserControl x:Class="Validator.RespView"
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"
mc:Ignorable="d"
d:DesignHeight="167" d:DesignWidth="366" Name="Resp">
<Grid>
<CheckBox Content="CheckBox" Height="16" HorizontalAlignment="Left" Margin="12,12,0,0" Name="checkBox1" VerticalAlignment="Top" />
<ListBox Height="112" HorizontalAlignment="Left" Margin="12,43,0,0" Name="listBox1" VerticalAlignment="Top" Width="168" />
<Calendar Height="170" HorizontalAlignment="Left" Margin="186,0,0,0" Name="calendar1" VerticalAlignment="Top" Width="180" />
</Grid>
Second usercontrol:
<UserControl x:Class="Validator.DownloadView"
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"
mc:Ignorable="d"
d:DesignHeight="76" d:DesignWidth="354" Name="Download">
<Grid>
<Label Content="States" Height="28" HorizontalAlignment="Left" Margin="12,12,0,0" Name="label1" VerticalAlignment="Top" />
<ComboBox Height="23" HorizontalAlignment="Left" Margin="12,35,0,0" Name="comboBox1" VerticalAlignment="Top" Width="120" />
<RadioButton Content="Last 48 hs" Height="16" HorizontalAlignment="Left" Margin="230,42,0,0" Name="rdbLast48" VerticalAlignment="Top" />
<Label Content="Kind:" Height="28" HorizontalAlignment="Left" Margin="164,12,0,0" Name="label2" VerticalAlignment="Top" />
<RadioButton Content="General" Height="16" HorizontalAlignment="Left" Margin="165,42,0,0" Name="rdbGeral" VerticalAlignment="Top" />
</Grid>
At MainWindowView.xaml
<Window x:Class="Validator.MainWindowView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:sys="clr-namespace:System;assembly=mscorlib"
xmlns:du="clr-namespace:Validator.Download"
xmlns:resp="clr-namespace:Validator.Resp"
Title="Validator" Height="452" Width="668"
WindowStartupLocation="CenterScreen" ResizeMode="NoResize">
<Window.Resources>
<DataTemplate DataType="{x:Type du:DownloadViewModel}">
<du:DownloadView/>
</DataTemplate>
<DataTemplate DataType="{x:Type resp:RespViewModel}">
<resp:RespView/>
</DataTemplate>
</Window.Resources>
<Grid>
<ComboBox ItemsSource="{Binding Path=PagesName}"
SelectedValue="{Binding Path=CurrentPageName}"
HorizontalAlignment="Left" Margin="251,93,0,0"
Name="cmbType"
Width="187" VerticalAlignment="Top" Height="22"
SelectionChanged="cmbType_SelectionChanged_1" />
<ContentControl Content="{Binding CurrentPageViewModel}" Height="171" HorizontalAlignment="Left" Margin="251,121,0,0" Name="contentControl1" VerticalAlignment="Top" Width="383" />
</Grid>
</Window>
I assigned to the DataContext of the MainView, the viewmodel below:
public class MainWindowViewModel : ObservableObject
{
#region Fields
private ICommand _changePageCommand;
private ViewModelBase _currentPageViewModel;
private ObservableCollection<ViewModelBase> _pagesViewModel = new ObservableCollection<ViewModelBase>();
private readonly ObservableCollection<string> _pagesName = new ObservableCollection<string>();
private string _currentPageName = "";
#endregion
public MainWindowViewModel()
{
this.LoadUserControls();
_pagesName.Add("Download");
_pagesName.Add("Resp");
}
private void LoadUserControls()
{
Type type = this.GetType();
Assembly assembly = type.Assembly;
UserControl reso = (UserControl)assembly.CreateInstance("Validator.RespView");
UserControl download = (UserControl)assembly.CreateInstance("Validator.DownloadView");
_pagesViewModel.Add(new DownloadViewModel());
_pagesViewModel.Add(new RespViewModel());
}
#region Properties / Commands
public ICommand ChangePageCommand
{
get
{
if (_changePageCommand == null)
{
_changePageCommand = new RelayCommand(
p => ChangeViewModel((IPageViewModel)p),
p => p is IPageViewModel);
}
return _changePageCommand;
}
}
public ObservableCollection<string> PagesName
{
get { return _pagesName; }
}
public string CurrentPageName
{
get
{
return _currentPageName;
}
set
{
if (_currentPageName != value)
{
_currentPageName = value;
OnPropertyChanged("CurrentPageName");
}
}
}
public ViewModelBase CurrentPageViewModel
{
get
{
return _currentPageViewModel;
}
set
{
if (_currentPageViewModel != value)
{
_currentPageViewModel = value;
OnPropertyChanged("CurrentPageViewModel");
}
}
}
#endregion
#region Methods
private void ChangeViewModel(IPageViewModel viewModel)
{
int indexCurrentView = _pagesViewModel.IndexOf(CurrentPageViewModel);
indexCurrentView = (indexCurrentView == (_pagesViewModel.Count - 1)) ? 0 : indexCurrentView + 1;
CurrentPageViewModel = _pagesViewModel[indexCurrentView];
}
#endregion
}
On MainWindowView.xaml.cs, I wrote this event to do the effective change:
private void cmbType_SelectionChanged_1(object sender, SelectionChangedEventArgs e)
{
MainWindowViewModel element = this.DataContext as MainWindowViewModel;
if (element != null)
{
ICommand command = element.ChangePageCommand;
command.Execute(null);
}
}
The app run ok and I inspected the application with WPFInspector and saw that the view changes when the combobox is changed internally, but the ContentControl still empty visually..
Sorry about the amount of code that I posted and my miss of knowledge but I'm working with this a long time and can't solve this problem.
Thanks

Issues:
Firstly don't ever create View related stuff in the ViewModel (UserControl). This is no longer MVVM when you do that.
Derive ViewModels from ViewModelBase and not ObservableObject unless you have a compelling reason to not use ViewModelBase when using MVVMLight. Keep ObservableObject inheritence for Models. Serves as a nice separation between VM's and M's
Next you do not need to make everything an ObservableCollection<T> like your _pagesViewModel. You do not have that bound to anything in your View's so it's just a waste. Just keep that as a private List or array. Check what a type actually does in difference to a similar other one.
Not sure about this one, maybe you pulled this code snippet as a demo, but do not use margins to separate items in a Grid. Your Layout is essentially just 1 Grid cell and the margins have the items not overlap. If you're not aware of that issue, Check into WPF Layout Articles.
Please don't forget principles of OOP, Encapsulation and sorts when writing a UI app. When having Properties like CurrentPageViewModel which you don't intend the View to switch make the property setter private to enforce that.
Don't resort to code-behind in the View too soon. Firstly check if it's only a View related concern before doing so. Am talking about your ComboBox SelectionChanged event handler. Your purpose of that in this demo is to switch the Bound ViewModel which is held in the VM. Hence it's not something that the View is solely responsible for. Thus look for a VM involved approach.
Solution:
You can get a working example of your code with the fixes for above from Here and try it out yourself.
Points 1 -> 5 are just basic straightforward changes.
For 6, I've created a SelectedVMIndex property in the MainViewModel which is bound to the SelectedIndex of the ComboBox. Thus when the selected index flips, the property setter after updating itself updates the CurrentPageViewModel as well such as
public int SelectedVMIndex {
get {
return _selectedVMIndex;
}
set {
if (_selectedVMIndex == value) {
return;
}
_selectedVMIndex = value;
RaisePropertyChanged(() => SelectedVMIndex);
CurrentPageViewModel = _pagesViewModel[_selectedVMIndex];
}
}

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}">