Develop Reference

Trying to access DataContext class from code-behind

I'm following this question on how to access your DataContext class from code-behind. Implemeting it the way it says doiesn't work for me (apparently I'm doing something wrong).
What I'm trying to do:
I have a button that is trying to read a Text property from a class that is binded to TextBox:
private void myButton_Click(object sender, RoutedEventArgs e)
{
var dataContext = myWindow.DataContext as myClass;
System.Windows.MessageBox.Show(dataContext.Text);
}
For some reason dataContext is always null.
XAML:
<Window x:Class="TestApp.MainWindow"
x:Name="myWindow"
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:TestApp"
mc:Ignorable="d"
Title="MainWindow" Height="200" Width="300">
<Window.Resources>
<local:myClass x:Key="myDataSource"/>
</Window.Resources>
<Window.DataContext>
<Binding Source="myDataSource"/>
</Window.DataContext>
<Grid Margin="0,0,0,2">
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition/>
</Grid.ColumnDefinitions>
<TextBox HorizontalAlignment="Stretch" Margin="5,5,5,5" TextWrapping="Wrap" Text="{Binding Path=Text, UpdateSourceTrigger=PropertyChanged}" VerticalAlignment="Stretch" HorizontalContentAlignment="Stretch" VerticalContentAlignment="Stretch" />
<Button x:Name="myButton" Content="Button" HorizontalAlignment="Stretch" Margin="5,5,5,5" Grid.Row="1" VerticalAlignment="Stretch" Click="myButton_Click"/>
</Grid>
</Window>
Code-behind:
namespace TestApp
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
//var dataContext = myWindow.DataContext as myClass;
}
private void myButton_Click(object sender, RoutedEventArgs e)
{
var dataContext = myWindow.DataContext as myClass;
System.Windows.MessageBox.Show(dataContext.Text);
}
}
public class myClass : INotifyPropertyChanged
{
private string text;
public string Text
{
get { return text; }
set {
text = value;
OnPropertyChanged("Text");
}
}
public event PropertyChangedEventHandler? PropertyChanged;
// Create the OnPropertyChanged method to raise the event
// The calling member's name will be used as the parameter.
protected void OnPropertyChanged([CallerMemberName] string name = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
}
}
}
The full code is also in this question (that was closed since it was "answered" in the question linked above, but the solution still doesn't work for me). What am I doing wrong?

The expression
<Window.DataContext>
<Binding Source="myDataSource"/>
</Window.DataContext>
binds the DataContext to the string "myDataSource". It should instead be
<Window.DataContext>
<Binding Source="{StaticResource myDataSource}"/>
</Window.DataContext>
You do however not have to declare the DataContext object as resource at all. Just write
<Window.DataContext>
<local:myClass/>
</Window.DataContext>
Also be aware that your conclusion that the "DataContext is always null" is incorrect. It is the result of the expression DataContext as myClass which was null, because DataContext did not contain an object of type myClass. In general, when you use the as operator, you should always check the result for null before accessing it. Or use the is operator like
if (DataContext is myClass dataContext)
{
MessageBox.Show(dataContext.Text);
}
You may also notice that using the myWindow field was redundant, since the code behind methods belong to the MainWindow class.

WPF update textblock from different window

I have a window called RolledPaper.xaml with a textblock called SequenceValue.
SequenceValue is defined in another window called CounterSettings.xaml by typing in a textbox called SequenceRequested. I would like to have SequenceValue be always in sinch with SequenceRequested.
I tryed and failed using the same datacontext for both windows.
here it is the code for RolledPaper.xaml:
<Window x:Class="Numbering3.View.RolledPaper"
WindowStyle="None"
ResizeMode="NoResize"
BorderBrush="LightGray"
BorderThickness="5"
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:Numbering3.View"
xmlns:vm="clr-namespace:Numbering3.ViewModel"
xmlns:cv="clr-namespace:Numbering3.Helper"
xmlns:vo="clr-namespace:Numbering3.ViewModel"
xmlns:sys="clr-namespace:System;assembly=mscorlib"
mc:Ignorable="d"
Title="RolledPaper" Height="750" Width="1218"
Background="{DynamicResource WindowBrush}"
DataContextChanged="Rolledpaper_DataContextChanged">
<Window.DataContext>
<vm:ViewModelRolledPaper/>
</Window.DataContext>
<TextBlock x:Name="SequenceValue" Grid.Column="3" HorizontalAlignment="Left" Height="19" Margin="72,310,0,0" TextWrapping="Wrap"
Background="White" VerticalAlignment="Top" Width="98" Text="{Binding _SequenceValueToShow.SequenceValuetoShow, Mode=TwoWay, FallbackValue=NNN, TargetNullValue=NNN, UpdateSourceTrigger=PropertyChanged}"/>
</Grid>
code behind:
public partial class RolledPaper : Window
{
public RolledPaper()
{
WindowStartupLocation = System.Windows.WindowStartupLocation.CenterScreen;
SaveKeeper.fromMain = false;
InitializeComponent();
this.MouseLeftButtonDown += delegate { this.DragMove(); };
DataContextChanged += new DependencyPropertyChangedEventHandler(Rolledpaper_DataContextChanged);
}
the CounterSetting window:
<Window x:Class="Numbering3.View.CounterSettings"
...
...
<Window.DataContext>
<ViewModelCounterSettings/>
</Window.DataContext>
<Grid Margin="0,-19,-0.4,0">
<TextBox x:Name="SequenceRequested" PreviewTextInput="SequenceValidationTextBox" MaxLength="10" HorizontalAlignment="Left" Height="23" Margin="154,324,0,0" TextWrapping="Wrap"
Text="{Binding Path=_SequenceValueToShow.SequenceValueToKeep, FallbackValue='NNN', TargetNullValue ='NNN', Mode=TwoWay, UpdateSourceTrigger=PropertyChanged }" VerticalAlignment="Top" Width="120" TextChanged="SequenceRequested_TextChanged" />
</Grid>
its code behind:
public partial class CounterSettings : Window
{
public CounterSettings()
{
InitializeComponent();
this.MouseLeftButtonDown += delegate { this.DragMove(); };
DataContextChanged += new DependencyPropertyChangedEventHandler(CounterSettings_DataContextChanged);
}
And the SequeneValue class:
public class SequenceValue : INotifyPropertyChanged
{
public string SequenceValueToKeep
{
get
{
return _sequenceValueToKeep=_sequenceProcessor.GetSequence();
}
set
{
if (_sequenceValueToKeep != value)
{
__sequenceValueToKeep = value;
RaisePropertyChanged("SequenceValueToKeep");
}
}
}
private void RaisePropertyChanged(string prop)
{
if (PropertyChanged != null)
{ PropertyChanged(this, new PropertyChangedEventArgs(prop)); }
}
public event PropertyChangedEventHandler PropertyChanged;
}
}
I need to put a value in the textbox of countersetting =>string Sequencevalue.SequenceValueToKeep is updated => textblock in RolledPaper window shows Sequencevalue.SequenceValueToKeep .
Thank you.

Solutio 1:
You can use one Static ViewModel to get the to to know each other.
in App.xaml
<Application.Resources>
...
<MainViewModel x:Key="mainViewModel" />
</Application.Resources>
and then in CounterSetting.xaml
<Window otherProperties=...
DataContext="{Binding ViewModelCounterSettings, Source={StaticResource mainViewModel}}"/>
and in RolledPaper.xaml
<Window otherProperties=...
DataContext="{Binding ViewModelRolledPaper, Source={StaticResource mainViewModel}}"/>
The MainViewModel does inits your two ViewModel and makes them accessible as properties and also gives the the same instance of SequenceValue
Solution 2:
Create an object of SequenceValue as an static resource, like
<Application.Resources>
...
<SequenceValue x:Key="sequenceValue"/>
</Application.Resources>
and set it as the DataContext of your TextBox and TextBlock
<TextBox DataContext="{StaticResource sequenceValue}"
Text="{Binding SequenceValueToKeep, FallbackValue='NNN', TargetNullValue='NNN', Mode=TwoWay, UpdateSourceTrigger=PropertyChanged }"
... />
<TextBlock DataContext="{StaticResource sequenceValue}"
Text="{Binding SequenceValuetoShow, FallbackValue=NNN, TargetNullValue=NNN, UpdateSourceTrigger=PropertyChanged}"

Data binding to the output of a method

I'm trying to display the output of a method in a WPF TextBox. I'm just trying a simple attempt, to print a single string 3 in a TextBox.
I'm trying to do it the following way, using an ObjectDataProvider:
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication1"
xmlns:system="clr-namespace:System;assembly=mscorlib"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<ObjectDataProvider x:Key="dataprovider" ObjectType="{x:Type system:String}" MethodName="GetValue">
</ObjectDataProvider>
</Window.Resources>
<Grid>
<TextBox Text="{Binding Source={StaticResource dataprovider}, Mode=OneWay}" HorizontalAlignment="Left" Height="23" Margin="201,168,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="120"/>
</Grid>
</Window>
And here's my code behind:
namespace WpfApplication1
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
}
public string GetValue()
{
return "3";
}
}
}
I'm getting no output. the TextBox is just blank. Where am I going wrong?

Instead of ObjectDataProvider create a property like this:
public string GetMethod
{
get
{
return GetValue();
}
}
And:
public MainWindow()
{
InitializeComponent();
DataContext = this;
}
Then in the XAML remove ObjectDataProvider and just:
<TextBox Text="{Binding GetMethod, Mode=OneWay}" HorizontalAlignment="Left"
Height="23" Margin="201,168,0,0" TextWrapping="Wrap"
VerticalAlignment="Top" Width="120"/>

How do you use a DataTemplate defined in Application.Resources?

What is the purpose of Application.Resources if my windows cannot access the resources defined within it?
This works, I get a window with a TextBox that says "Loki" inside of it...
App.xaml.cs:
public partial class App : Application
{
protected override void OnStartup(StartupEventArgs e)
{
base.OnStartup(e);
ViewModel.ViewModel1 oVM = new ViewModel.ViewModel1 { Name = "Loki" };
MainWindow oVW = new MainWindow { Content = oVM };
oVW.ShowDialog();
}
}
MainWindow.xaml
<Window x:Class="TableGenerator.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:vm="clr-namespace:TableGenerator.ViewModel"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<DataTemplate DataType="{x:Type vm:ViewModel1}">
<TextBox Text="{Binding Path=Name}" />
</DataTemplate>
</Window.Resources>
<ContentPresenter />
</Window>
But moving the DataTemplate to Application.Resources instead of Window.Resources does not work. When I run this I get a window, there is no TextBox at all, but there is text being displayed somehow that just says the name of my viewmodel class, "TableGenerator.ViewModel.ViewModel1".
App.xaml.cs is unchanged.
MainWindow.xaml changes to:
<Window x:Class="TableGenerator.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">
<ContentPresenter />
</Window>
App.xaml:
<Application x:Class="TableGenerator.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:vm="clr-namespace:TableGenerator.ViewModel">
<Application.Resources>
<DataTemplate DataType="{x:Type vm:ViewModel1}">
<TextBox Text="{Binding Path=Name}" />
</DataTemplate>
</Application.Resources>
</Application>
Why does it not look in Application.Resources to find my DataTemplate?

Add your datatemplate to a dictionary. It is required to have a default style which application resources should have it seems. Refer link for more explaination. datatemplate in app.xaml is not getting picked up without any styles?
On the creation of every object in XAML, if a default style is present (i.e. style w/ a key of Type) that style should be applied. As you can imagine there are several performance optimizations to make that (implied) lookup a light weight as possible.
One of them is that we don’t look inside Resource Dictionaries unless they are flagged as “containing default Styles”. There is a bug: if all your default styles are nested in merged dictionaries three levels deep (or deeper) the top dictionary does not get flagged so the search skips it. The work around is to put a default Style to something, anything, in the root Dictionary.
Then refer the below code.
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:vm="clr-namespace:SQ15Mar2015_Learning">
<DataTemplate DataType="{x:Type vm:ViewModel}">
<DockPanel>
<TextBox Text="{Binding Path=Name,UpdateSourceTrigger=PropertyChanged}">
</TextBox>
</DockPanel>
</DataTemplate>
<Application x:Class="SQ15Mar2015_Learning.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:vm="clr-namespace:SQ15Mar2015_Learning">
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="/Dictionary1.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
OR
<Application.Resources>
<DataTemplate DataType="{x:Type vm:ViewModel}">
<DockPanel>
<TextBox Text="{Binding Path=Name,UpdateSourceTrigger=PropertyChanged}">
</TextBox>
</DockPanel>
</DataTemplate>
<Style TargetType="{x:Type Rectangle}" />
</Application.Resources>
class ViewModel : INotifyPropertyChanged
{
private string myVar;
public string Name
{
get { return myVar; }
set
{
if (value != myVar)
{
myVar = value;
OnPropertyChanged("Name");
}
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged(string propName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propName));
}
}
}
public partial class App : Application
{
protected override void OnStartup(StartupEventArgs e)
{
base.OnStartup(e);
ViewModel oVM = new ViewModel { Name = "Loki" };
MainWindow oVW = new MainWindow();
oVW.DataContext = oVM;
oVW.ShowDialog();
}
}
<Window x:Class="SQ15Mar2015_Learning.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:vm="clr-namespace:SQ15Mar2015_Learning"
Title="MainWindow" Height="350" Width="525" >
<Grid>
<ContentControl Content="{Binding }" />
</Grid>
</Window>

UserControl is not displayed

EDIT: It seems like that I needed to change the height of RowDefinition. Thanks goes to Alvaro. However:
how do I reference different elements inside my usercontrol (change their properties), when I want to change them in MainWindow.xaml and MainWindow.xaml.cs?
I have two XAML files in the same namespace.
MainWindow.xaml
List.xaml
When I try to add List.xaml usercontrol to my mainwindow xaml (that is insert xaml from another file), it does not show up.
I insert usercontrol that exists in Lemosystem namespace and inside View folder.
xmlns:lemoview="clr-namespace:Lemosystem.View"
I add usercontrol to my MainWindow.xaml:
<lemoview:List/>
Nothing shows up. Here is my List XAML (code is the default):
<UserControl x:Class="Lemosystem.View.List"
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="300" d:DesignWidth="300">
<Grid>
<Label Content="What do you want to do today?" HorizontalAlignment="Left" Margin="10,96,-83,0" VerticalAlignment="Top" Height="44" Width="373" FontSize="24" FontWeight="Bold"/>
</Grid>
</UserControl>
I expect the label from my usercontrol to show up in MainWindow.xaml GUI, but it doesn't.
MainWindow.xaml
<Window x:Class="Lemosystem.View.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:lemocontroller="clr-namespace:Lemosystem.Controller"
xmlns:lemoview="clr-namespace:Lemosystem.View"
Title="{Binding Path=SystemName}" Height="603" Width="827"
ResizeMode="NoResize" WindowStartupLocation="Manual"
>
<Grid Name="Window" Margin="0,0,2,0">
<Grid.RowDefinitions>
<RowDefinition Height="0*"/>
<RowDefinition/>
</Grid.RowDefinitions>
...
<lemoview:List/>
...
</Grid>
</Window>
And how do I reference different elements inside my usercontrol (change their properties), when I want to change them in MainWindow.xaml and MainWindow.xaml.cs?

So, I created a project with :
List.Xaml
<UserControl x:Class="WpfApplication1.List"
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="300" d:DesignWidth="300">
<Grid>
<Label Content="{Binding TextToDisplay}"
HorizontalAlignment="Left"
Margin="10,96,-83,0"
VerticalAlignment="Top"
Height="44"
Width="373"
FontSize="24"
FontWeight="Bold" />
</Grid>
</UserControl>
ListViewModel.cs:
public class ListViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
private string textToDisplay;
public string TextToDisplay
{
get { return textToDisplay; }
set { textToDisplay = value; OnPropertyChanged("TextToDisplay"); }
}
public ListViewModel(string value)
{
TextToDisplay = value;
}
}
}
MainWindow.Xaml:
<Window x:Class="WpfApplication1.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"
xmlns:lemoview="clr-namespace:WpfApplication1">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="auto"></RowDefinition>
<RowDefinition Height="*"></RowDefinition>
</Grid.RowDefinitions>
<Button Content="Click Me" HorizontalAlignment="Center" Click="Button_OnClick"></Button>
<ListView Grid.Row="1" ItemsSource="{Binding MyList}"></ListView>
</Grid>
</Window>
MainWindow.xaml.cs:
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
DataContext = this;
MyList=new ObservableCollection<ListViewModel>();
}
private ObservableCollection<ListViewModel> myList;
public ObservableCollection<ListViewModel> MyList
{
get { return myList; }
set { myList = value; }
}
private void Button_OnClick(object sender, RoutedEventArgs e)
{
MyList.Add(new ListViewModel("MyValue"));
}
}
App.xaml:
<Application x:Class="WpfApplication1.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:wpfApplication1="clr-namespace:WpfApplication1"
StartupUri="MainWindow.xaml">
<Application.Resources>
<DataTemplate DataType="{x:Type wpfApplication1:ListViewModel}">
<wpfApplication1:List />
</DataTemplate>
</Application.Resources>
in App.xaml, I just defined the binding between List.xaml and ListViewModel.cs
the viewModel of MainWindow is itself.
after every Click on the button, a new ViewModel is created, added to the list with a defined value (you will need to modify this part, to set the value you want).
I hope it will help you ! it works for me.

How do you like to show something, if you put it il a row that has height ="0" ?
change it to :
<Window x:Class="Lemosystem.View.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:lemocontroller="clr-namespace:Lemosystem.Controller"
xmlns:lemoview="clr-namespace:Lemosystem.View"
Title="{Binding Path=SystemName}" Height="603" Width="827"
ResizeMode="NoResize" WindowStartupLocation="Manual"
>
<Grid Name="Window" Margin="0,0,2,0">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition/>
</Grid.RowDefinitions>
<lemoview:List/>
</Grid>