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Set up a binding between two properties in code behind

In my view, I have a ListBox with some templated items that contain buttons.
<ListBox x:Name="MyListBox" ItemTemplate="{DynamicResource DataTemplate1}"
ItemsSource="{Binding MyItems}">
</ListBox>
And the template for generated items:
<DataTemplate x:Key="DataTemplate1">
<StackPanel Orientation="Horizontal">
<Button Width="50" Click="Button_Click" />
</StackPanel>
</DataTemplate>
When user clicks a button on one of those ListBox items, I want to send the index of that ListBox item to my ViewModel.
So figured to use Binding as it seems to be the way in MVVM. But I'm struggling to set up a binding in code between two properties.
My View code is as follows:
public partial class ItemView : UserControl
{
ViewModel.ItemViewModel VM;
public ItemView()
{
InitializeComponent();
VM = new ViewModel.ItemViewModel();
this.DataContext = VM;
}
private int clickedItemIndex;
public int ClickedItemIndex { get => clickedItemIndex; set => clickedItemIndex = value; }
private void Button_Click(object sender, RoutedEventArgs e)
{
var ClickedItem = (sender as FrameworkElement).DataContext;
ClickedItemIndex = MyListBox.Items.IndexOf(ClickedItem);
}
}
I get the index and set it to ClickedItemIndex property,
I also have property in my ViewModel:
public int SomeInt { get; set; }
Now how do I set up a binding between these two properties?
I'm quite new to MVVM and still learning it. So, maybe this not the correct approach. But I need to have a way for each individual listbox item to be able to call upon an effect in more global viewmodel. For example, if I wanted to have a "Remove" button on each of the listbox items, I would somehow need to send the index to the viewmodel and call the removeItem method with index as the parameter. Or is there a better way to do similar things?

I have a sample app created just for this scenario. I know it seems a lot of code at first glance. Copy this code in your project, that will help debug and get a hang of it(MVVM, databinding, commands and so on).
usercontrol.xaml
<UserControl x:Class="WpfApplication1.UserControl1"
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"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300">
<Grid>
<Grid.Resources>
<DataTemplate DataType="{x:Type local:Model}">
<StackPanel Orientation="Horizontal">
<Label Content="{Binding Path=Name}"/>
<Button Command="{Binding RelativeSource={RelativeSource AncestorType=UserControl},Path=DataContext.UpdateCommand}"
CommandParameter="{Binding}"
Content="Update"/>
<Button Command="{Binding RelativeSource={RelativeSource AncestorType=UserControl},Path=DataContext.RemoveCommand}"
CommandParameter="{Binding}"
Content="Remove"/>
</StackPanel>
</DataTemplate>
</Grid.Resources>
<ListBox ItemsSource="{Binding Models}">
</ListBox>
</Grid>
usercontrol.cs
public partial class UserControl1 : UserControl
{
public UserControl1()
{
InitializeComponent();
DataContext = new ViewModel();
}
}
View model
public class ViewModel : INotifyPropertyChanged
{
private Models _Models;
public Models Models
{
get { return _Models; }
set { _Models = value;
PropertyChanged(this, new PropertyChangedEventArgs(nameof(Models)));
}
}
public ViewModel()
{
Models = new Models();
UpdateCommand = new Command(o => true, UpdateItem);
RemoveCommand = new Command(o => true, RemoveItem);
}
void RemoveItem(object item)
{
Model m = (item as Model);
Models.Remove(m);
}
void UpdateItem(object item)
{
Model m = (item as Model);
m.Name = m.Name + " updated";
}
public event PropertyChangedEventHandler PropertyChanged = delegate { };
public ICommand UpdateCommand { get; private set; }
public ICommand RemoveCommand { get; private set; }
}
Icommand implementation
public class Command : ICommand
{
private readonly Func<object, bool> _canExe;
private readonly Action<object> _exe;
public Command(Func<object,bool> canExecute,Action<object> execute)
{
_canExe = canExecute;
_exe = execute;
}
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
public bool CanExecute(object parameter)
{
return _canExe(parameter);
}
public void Execute(object parameter)
{
_exe(parameter);
}
}
Model and a collection of models
public class Models : ObservableCollection<Model>
{
public Models()
{
Add(new Model ());
Add(new Model ());
Add(new Model ());
Add(new Model ());
}
}
public class Model : INotifyPropertyChanged
{
static int count = 0;
public Model()
{
Name = "Model "+ ++count;
}
private string _Name;
public string Name
{
get { return _Name; }
set { _Name = value;
PropertyChanged(this, new PropertyChangedEventArgs(nameof(Name)));
}
}
public event PropertyChangedEventHandler PropertyChanged = delegate { };
}

You don't need to use a Button in order to select the item. When you click/tap on the item it will get automatically selected.
Then simply bind ListBox.SelectedIndex to your view model property SomeInt and it will update on every selection.
Data binding overview in WPF
You can also get the item itself by binding ListBox.SelectedItem to your view model.
You can handle new values by invoking a handler from the property's set method:
ViewModel.cs
class ViewModel : INotifyPropertyChanged
{
private int currentItemIndex;
public int CurrentItemIndex
{
get => this.currentItemIndex;
set
{
this.currentItemIndex = value;
OnPropertyChanged();
// Handle property changes
OnCurrentItemIndexChanged();
}
}
private MyItem currentItem;
public MyItem CurrentItem
{
get => this.currentItem;
set
{
this.currentItem = value;
OnPropertyChanged();
}
}
protected virtual void OnCurrentItemIndexChanged()
{
// Handle the new this.CurrentItemIndex value
}
// Implementation of INotifyPropertyChanged
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
ItemView .xaml
<UserControl>
<UserControl.DataContext>
<ViewModel />
</UserControl.DataContext>
<ListBox ItemsSource="{Binding MyItems}"
SelectedIndex="{Binding CurrentItemIndex}"
SelectedItem="{Binding CurrentItem}" />
</UserControl>

Create a ViewModel with sub ViewModel

Is there a proper way to create a C#/WPF ViewModel containing subViewModel ?
Objective is:
I have a MainWindow. That window is use to read/create images. There is a button on that windows who switch between 2 UserControl one with IHM used to read image, the other one used to create.
The MainWindow has a MainWindowViewModel with :
command switch
image length
application parameters
I want that both UserControls can acces to MainWindowViewModel field/properties and have they own commands.
Construction will be something like this:
public partial class ReadUserControl : UserControl
{
public ReadUserControl()
{
InitializeComponent();
DataContext = MainViewModel.ReadViewModel;
}
}
public partial class CreateUserControl : UserControl
{
public CreateUserControl()
{
InitializeComponent();
DataContext = MainViewModel.CreateViewModel;
}
}
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
DataContext = MainViewModel;
}
}
For example, if a MainViewModel contain a field ImageWidth setting ImageWidth in CreateUserControl change the value for ReadUserControl.
I hope to have been clear, I don't know how design my MainViewModel to achieve this result
EDIT1:
I've created the MainWindowViewModel as a Singleton but i'm still unable to get MainViewModel.CreateViewModel and MainViewModel.ReadViewModel
public class MainWindowViewModel : ViewModelBase
{
private static MainWindowViewModel _instance = null;
public static MainWindowViewModel Instance
{
get
{
if (_instance == null)
_instance = new MainWindowViewModel();
return _instance;
}
}
private MainWindowViewModel()
: base()
{
}
#region CreateViewModel
/* How to create ? */
#endregion
#region ReadViewModel
/* How to create ? */
#endregion
}

Your example will work. At least if you have made your MainViewModel a Singleton.
A more professional approach might be an Constructor-Injection like this.
public partial class ReadUserControl : UserControl
{
public ReadUserControl(MainViewModel vm)
{
InitializeComponent();
DataContext = vm.ReadViewModel;
}
}
With such DependencyInjections you can achieve a higher level of abstraction, since your UserControls can be generalized. (They will all have the same Constructor)
On the other hand, you give every such UserControl the ability, to manipulate the MainViewModel, not aware of side-effects.
In your special case, it would be more safe, to pass only the needed parameters to the UserControl, instead of giving them a bunch of informations, they will never need.
public partial class ReadUserControl : UserControl
{
public ReadUserControl(Icommand command, int imageLength, AppParams appParams)
{
InitializeComponent();
...
// Do with your Constructorparameters what ever you have to
}
}
Edit:
Here a small, dumb implementation of how it could be done:
Code
public class MainViewModel : INotifyPropertyChanged {
private INotifyPropertyChanged _selectedViewModel;
public MainViewModel() {
var cmd = new RelayCommand(x => {
MessageBox.Show("HelloWorld");
}, x => true);
this.RVM = new ReadViewModel(cmd);
this.WVM = new WriteViewModel(cmd);
this.SelectedViewModel = WVM;
}
private ICommand _switchViewModelCommand;
public ICommand SwitchViewModelCommand => this._switchViewModelCommand ?? (this._switchViewModelCommand = new RelayCommand(x => {
if (this.SelectedViewModel == RVM) {
this.SelectedViewModel = WVM;
return;
}
this.SelectedViewModel = RVM;
}));
public INotifyPropertyChanged SelectedViewModel {
get {
return this._selectedViewModel;
}
set {
if (Equals(value, this._selectedViewModel))
return;
this._selectedViewModel = value;
this.OnPropertyChanged();
}
}
public ReadViewModel RVM {
get; set;
}
public WriteViewModel WVM {
get; set;
}
public event PropertyChangedEventHandler PropertyChanged;
[NotifyPropertyChangedInvocator]
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null) {
this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
public class ReadViewModel : INotifyPropertyChanged {
public ReadViewModel(ICommand sayHelloCommand) {
this.HelloCommand = sayHelloCommand;
}
public ICommand HelloCommand {
get;
}
public event PropertyChangedEventHandler PropertyChanged;
[NotifyPropertyChangedInvocator]
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null) {
this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
public class WriteViewModel : INotifyPropertyChanged {
public WriteViewModel(ICommand sayHelloCommand) {
this.HelloCommand = sayHelloCommand;
}
public ICommand HelloCommand {
get;
}
public ICommand HelloMoonCommand => new RelayCommand(x => { MessageBox.Show("Hello Moon"); });
public event PropertyChangedEventHandler PropertyChanged;
[NotifyPropertyChangedInvocator]
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null) {
this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
XAML
<Window.DataContext>
<local:MainViewModel/>
</Window.DataContext>
<Grid Height="200">
<Grid.RowDefinitions>
<RowDefinition></RowDefinition>
<RowDefinition></RowDefinition>
</Grid.RowDefinitions>
<ContentControl Content="{Binding SelectedViewModel, UpdateSourceTrigger=PropertyChanged}">
<ContentControl.Resources>
<DataTemplate DataType="{x:Type local:ReadViewModel}">
<StackPanel>
<Button Content="Say Hello world" Command="{Binding HelloCommand}"></Button>
</StackPanel>
</DataTemplate>
<DataTemplate DataType="{x:Type local:WriteViewModel}">
<StackPanel>
<Button Content="Say Hello world" Command="{Binding HelloCommand}"></Button>
<Button Content="Say Hello Moon" Command="{Binding HelloMoonCommand}"></Button>
</StackPanel>
</DataTemplate>
</ContentControl.Resources>
</ContentControl>
<Button Content="Switch VM" Command="{Binding SwitchViewModelCommand}" Grid.Row="1"/>
</Grid>

You can pass in the MainViewModel as DataContext for your user control and set the data context of elements as Read/Create model
something like
<Grid> <!--using MainWindowViewModel as data context-->
<Grid DataContext="{Binding Path=CreateViewModel}"> <!--using CreateViewModel as data context-->
.....
</Grid>
<Grid>

WPF, Property does not return value to the binding

So, I have a project with a scrolling text (marqee) that rotates over a string array. And I want it to change the string value after 20 seconds of each animation iteration.
There is a problem though, the property(ScrollingText) that uses the INotifyPropertyChanged interface to bind to a textblock(using XAML) does not return after the first iteration. Even though it refreshes normally(in the set part), it does not return on the Getter part.... except for the first set in the default ctor.
MAIN CLASS:
class GetScrollingText : CommonBase
{
private string _scrollingtext = String.Empty;
DoubleAnimation Animation;
public GetScrollingText()
{
ScrollingText = GetScrollString();
}
public string ScrollingText
{
get
{
return _scrollingtext;
}
set
{
if (value != _scrollingtext)
{
_scrollingtext = value;
RaisePropertyChanged("ScrollingText");
}
}
} // INJECTS the string in the animated textblock {binding}.
public TextBlock scrollBlock { get; set; }
string GetScrollString()
{
.........
return scrolltext;
}
public void LeftToRightMarqee(double from, double to)
{
Animation = new DoubleAnimation();
Animation.From = from;
Animation.To = to;
Animation.Duration = new Duration(TimeSpan.FromSeconds(20));
Animation.Completed += animation_Completed;
scrollBlock.BeginAnimation(Canvas.LeftProperty, Animation);
}
void animation_Completed(object sender, EventArgs e)
{
ScrollingText = GetScrollString();
scrollBlock.BeginAnimation(Canvas.LeftProperty, Animation);
}
}
For some reason the animation_Completed Event only changes the value ScrollingText, but it does not invoke the Getter part therefore there is not a return to the {binding}.
XAML:
<Window
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:vm="clr-namespace:AnnouncingSys"
x:Class="AnnouncingSys.MainWindow"
x:Name="Window"
Width="1280" Height="720" MinHeight="566" MinWidth="710">
<Window.Resources>
<vm:GetScrollingText x:Key="ScrollingText"/>
</Window.Resources>
<Canvas x:Name="MainCanvas" ClipToBounds="True" Margin="0,0,0,0" Grid.Row="5" Background="Black" Grid.ColumnSpan="5" >
<TextBlock x:Name="ScrollBlock" TextWrapping="Wrap" VerticalAlignment="Top" Height="113" Width="5147" Canvas.Left="-1922" Text="{Binding ScrollingText, Source={StaticResource ScrollingText}}"/>
</Canvas>
</Window>
CODE BEHIND:
public partial class MainWindow : Window
{
GetScrollingText scrolling = new GetScrollingText();
public MainWindow()
{
InitializeComponent();
scrolling.scrollBlock = this.ScrollBlock;
scrolling.LeftToRightMarqee(2000, -3000);
}
}
And finally the helper class CommonBase:
public class CommonBase : INotifyPropertyChanged
{
protected CommonBase()
{
}
public event PropertyChangedEventHandler PropertyChanged;
protected void RaisePropertyChanged(string PropertyName)
{
PropertyChangedEventHandler handler = this.PropertyChanged;
if (handler != null)
{
PropertyChangedEventArgs e = new PropertyChangedEventArgs(PropertyName);
handler(this, e);
}
}
}
I have even put a breakpoint on the return block of the Getter but it only activates on the first: "ScrollingText = GetScrollString()". I mean, shouldn't it return each time the value is changed???

You are using two different instances of your GetScrollingText class, one in XAML as StaticResource, the other in code behind as the scrolling field in class MainWindow.
Instead of creating a StaticResource in XAML, you could just set the DataContext property of your MainWindow:
public partial class MainWindow : Window
{
GetScrollingText scrolling = new GetScrollingText();
public MainWindow()
{
InitializeComponent();
scrolling.scrollBlock = this.ScrollBlock;
scrolling.LeftToRightMarqee(2000, -3000);
DataContext = scrolling; // here
}
}
Now you would not explicitly set the binding's Source property, because the DataContext is used as default binding source:
<TextBlock ... Text="{Binding ScrollingText}"/>

Data binding a nested property to a listbox

I cannot get any display from my observable collection in a custom object bound to a ListBox. This works fine when I have a string collection in my view model, but no names display when I try to access the property through a custom object. I am not receiving any errors in the output window.
Here is my code:
Custom Object
public class TestObject
{
public ObservableCollection<string> List { get; set; }
public static TestObject GetList()
{
string[] list = new string[] { "Bob", "Bill" };
return new TestObject
{
List = new ObservableCollection<string>(list)
};
}
}
Xaml
<Window x:Class="TestWPF.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Grid>
<ListBox Height="100" HorizontalAlignment="Left" Margin="120,61,0,0" Name="listBox1" VerticalAlignment="Top" Width="120" ItemsSource="{Binding Path=TObj.List}" />
</Grid>
Xaml.cs
public partial class MainWindow : Window
{
private ModelMainWindow model;
public MainWindow()
{
InitializeComponent();
model = new ModelMainWindow();
this.DataContext = model;
this.Loaded += new RoutedEventHandler(MainWindow_Loaded);
}
public void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
this.model.Refresh();
}
}
ViewModel
public class ModelMainWindow : INotifyPropertyChanged
{
private TestObject tObj;
public event PropertyChangedEventHandler PropertyChanged;
public TestObject TObj
{
get
{
return this.tObj;
}
set
{
this.tObj = value;
this.Notify("Names");
}
}
public void Notify(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
public void Refresh()
{
this.TObj = TestObject.GetList();
}
}

Can't bind to private properties. Also the change notification targets the wrong property, change "Names" to "TObj". (Also i would recommend making the List property get-only (backed by a readonly field), or implementing INoptifyPropertyChanged so the changes cannot get lost)

Your List is private. Make it a public property otherwise WPF can't see it.

WPF C# binding code - why doesn't this simple example work?

I've attached some WPF C# binding code - why doesn't this simple example work? (just trying to understanding binding to a custom object). That is when clicking on the button to increase the counter in the model, the label isn't updated.
<Window x:Class="testapp1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Grid>
<Button Height="23" HorizontalAlignment="Left" Margin="20,12,0,0"
Name="testButton" VerticalAlignment="Top" Width="126"
Click="testButton_Click" Content="Increase Counter" />
<Label Content="{Binding Path=TestCounter}" Height="37"
HorizontalAlignment="Right" Margin="0,12,122,0"
Name="testLabel2" VerticalAlignment="Top"
BorderThickness="3" MinWidth="200" />
</Grid>
</Window>
namespace testapp1
{
public partial class MainWindow : Window
{
public TestModel _model;
public MainWindow()
{
InitializeComponent();
InitializeComponent();
_model = new TestModel();
_model.TestCounter = 0;
this.DataContext = _model;
}
private void testButton_Click(object sender, RoutedEventArgs e)
{
_model.TestCounter = _model.TestCounter + 1;
Debug.WriteLine("TestCounter = " + _model.TestCounter);
}
}
public class TestModel : DependencyObject
{
public int TestCounter { get; set; }
}
}
thanks

For this simple example, consider using INotifyPropertyChanged and not DependencyProperties!
UPDATE
If you do want to use DPs, use the propdp snippet in VS2010 or Dr WPF's snippets for VS2008?

TestCounter needs to be a DepenencyProperty
public int TestCounter
{
get { return (int)GetValue(TestCounterProperty); }
set { SetValue(TestCounterProperty, value); }
}
// Using a DependencyProperty as the backing store for TestCounter.
//This enables animation, styling, binding, etc...
public static readonly DependencyProperty TestCounterProperty =
DependencyProperty.Register
("TestCounter",
typeof(int),
typeof(TestModel),
new UIPropertyMetadata(0));

You can implement the INotifyPropertyChanged interface in the System.ComponentModel namespace. I usually implement a Changed method that can take a number of property names and check for the event not being set. I do that because sometimes I have multiple properties that depend on one value and I can call one method from all of my property setters.
For instance if you had a Rectangle class with Width and Height properties and an Area read-only property that returns Width * Height, you could put Changed("Width", "Area"); in the property setter for Width.
public class TestModel : INotifyPropertyChanged
{
int m_TestCounter;
public int TestCounter {
get {
return m_TestCounter;
}
set {
m_TestCounter = value;
Changed("TestCounter");
}
}
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
#endregion
void Changed(params string[] propertyNames)
{
if (PropertyChanged != null)
{
foreach (string propertyName in propertyNames)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
}
}