Develop Reference

ListBox display multiple selected items

I have here a ListBox which shall show serveral items where some of them are selected.
<ListBox IsEnabled="False" x:Name="SecondaryModelSelector" SelectionMode="Extended" SelectedItem="{Binding Path=DataContext.SelectedSecondaryModels, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ContentControl}, Mode=OneWay}" ItemsSource="{Binding Path=DataContext.AvailableModels, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ContentControl}}" >
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<Label Content="{Binding Name}" Margin="5,0,5,0"/>
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
The selected items are set in a ComboBox and thus the ListBox is disabled and Mode=OneWay to disable selection in the ListBox itself.
The Template looks like this:
<Style x:Key="MyListBoxItem" TargetType="ListBoxItem">
<Setter Property="SnapsToDevicePixels" Value="true" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ListBoxItem">
<Border x:Name="Border" Padding="2">
<ContentPresenter />
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsSelected" Value="true">
<Setter TargetName="Border" Property="Background" Value="Blue"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="MyListBoxBorder" TargetType="{x:Type ListBox}">
<Setter Property="ItemContainerStyle" Value="{StaticResource MyListBoxItem}"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ListBox">
<Border x:Name="OuterBorder">
<ScrollViewer Margin="0" Focusable="false">
<StackPanel Margin="2" IsItemsHost="True" />
</ScrollViewer>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
The problem is now, that I cannot display multiple selected items, i.e. the background does not turn blue. Showing only one selected item is no problem.
Works (but shows only one selected item...)
public Cpu SelectedSecondaryModels
{
get { return SelectedGlobalVariable.SecondaryModels.FirstOrDefault(); }
}
Does not work:
public ObservableCollection<Model> SelectedSecondaryModels
{
get { return new ObservableCollection<Model>(SelectedGlobalVariable.SecondaryModels); }
}
I have no errors, so how can I show multiple selected items?
I tried SelectedItems instead of SelectedItem instead, but this is read-only field.
When I allow to select items in the ListBox than the blue background appears, and binding throws errors as there is no setter method.

The SelectedItem property cannot be bound to a collection of several items to be selected.
And as you have already noticed, SelectedItems is a read-only property.
You could use an attached behaviour to work around this. Please refer to this blog post and this example for more information about how to do this.
You will also find some solutions here:
Bind to SelectedItems from DataGrid or ListBox in MVVM

Get trigger from element

So I have this problem. I want to data trigger a column visibility for a grid in a WPF, but I can't get to bind the property name from the view with the actual property from the ViewModel.
View looks like this:
<Page x:Class="ImageViewer.Pages.ImageViewPage">
<Grid.ColumnDefinitions>
<ColumnDefinition x:Name="SidePanelColumn">
<ColumnDefinition.Style>
<Style TargetType="ColumnDefinition">
<Setter Property="Width" Value="250"/>
<Setter Property="MinWidth" Value="250"/>
<Style.Triggers>
<DataTrigger Binding="{Binding}" x:Name="SidePanelColumnVisibility" Value="True">
<Setter Property="Width" Value="0"/>
<Setter Property="MinWidth" Value="0"/>
</DataTrigger>
</Style.Triggers>
</Style>
</ColumnDefinition.Style>
</ColumnDefinition>
The binding is done like this (for other properties):
this.OneWayBind(ViewModel, vm => vm.AppCursor, v => v.Cursor).DisposeWith(cleanup);
here we bind the property name AppCursor from the ViewModel with the property Cursor from the View.
I would like to do the same with SidePanelColumnVisibility. My problem is that I can't seem to be able to access SidePanelColumnVisibility from the View.
The biding should look something like
this.OneWayBind(ViewModel, vm => vm.SidebarVisible, v => v.SidePanelColumnVisibility).DisposeWith(cleanup);
this.SidePanelColumn.Style.Triggers when inspecting this element I can see my trigger, but can't find a way to get the trigger by it's name SidePanelColumnVisibility.
I am fairly new to WPF ( been doing it for a week). I for sure somewhere misunderstood something, any help would be great.
LE:
The column looks like this
<ListBox Grid.Column="0"
MinWidth="250"
x:Name="FrameListBox"
BorderThickness="0"
HorizontalContentAlignment="Stretch"
ScrollViewer.HorizontalScrollBarVisibility="Disabled"
ScrollViewer.VerticalScrollBarVisibility="Visible">
<ListBox.ItemsPanel>
<ItemsPanelTemplate>
<UniformGrid Columns="1"></UniformGrid>
</ItemsPanelTemplate>
</ListBox.ItemsPanel>
<ListBox.ItemTemplate>
<DataTemplate>
<Image MinWidth="200" Height="Auto" Width="Auto" Source ="{Binding}" Stretch="Uniform"></Image>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>

Using three different ComboBoxItem styles in one WPF ComboBox

So I'm trying to create a user control for an application I'm working on. It's basically a ToggleButton next to a ComboBox. I was able to pretty much mock the ComboBox portion of the user control up in VS2015 the way the designers want it, but I feel like the way I'm going about it is not exactly the best way.
First, here is a link to a screenshot of what it looks like:
https://www.dropbox.com/s/019f4xqgu8r4i0e/DropDown.png
To do this, I ended up creating 3 different ComboBoxItem styles. The first puts together a CheckBox, a TextBlock with the ContentPresenter, and a Rectangle. The second just has a Separator, and the last just has the TextBlock with the ContentPresenter. Here is my XAML, which is declared in the UserControl.Resources section:
<Style x:Key="cbTestStyle" TargetType="{x:Type ComboBoxItem}">
<Setter Property="SnapsToDevicePixels" Value="True"/>
<Setter Property="HorizontalAlignment" Value="Stretch"/>
<Setter Property="VerticalAlignment" Value="Stretch"/>
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ComboBoxItem">
<Border Name="Border"
Padding="5"
Margin="2"
BorderThickness="2"
CornerRadius="0"
BorderBrush="Transparent">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="20"/>
<ColumnDefinition Width="75"/>
<ColumnDefinition Width="15"/>
</Grid.ColumnDefinitions>
<CheckBox Grid.Column="0"
IsChecked="{Binding Path=IsSelected, RelativeSource={RelativeSource TemplatedParent}}"/>
<TextBlock Grid.Column="1"
TextAlignment="Left"
Foreground="Black">
<ContentPresenter/>
</TextBlock>
<Rectangle Grid.Column="2"
Stroke="Black"
Width="15"
Height="15"
Fill="{TemplateBinding Foreground}"/>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsHighlighted" Value="True">
<Setter TargetName="Border" Property="BorderBrush" Value="Gray"/>
<Setter TargetName="Border" Property="Background" Value="LightGray"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="cbSeparatorStyle" TargetType="ComboBoxItem">
<Setter Property="SnapsToDevicePixels" Value="True"/>
<Setter Property="HorizontalAlignment" Value="Stretch"/>
<Setter Property="VerticalAlignment" Value="Stretch"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate>
<Separator/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="cbResetStyle" TargetType="{x:Type ComboBoxItem}">
<Setter Property="SnapsToDevicePixels" Value="True"/>
<Setter Property="HorizontalAlignment" Value="Stretch"/>
<Setter Property="VerticalAlignment" Value="Stretch"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ComboBoxItem">
<Border x:Name="Border"
Padding="5"
Margin="2"
BorderThickness="2"
CornerRadius="0"
BorderBrush="Transparent">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="20"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<TextBlock Grid.Column="1">
<ContentPresenter/>
</TextBlock>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsHighlighted" Value="True">
<Setter TargetName="Border" Property="BorderBrush" Value="Gray"/>
<Setter TargetName="Border" Property="Background" Value="LightGray"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
I guess my first question would be, is this the best way to make my ComboBox look like the screenshot I have presented?
Of course, there are deeper issues that I have yet to address. Firstly, the cbTestStyle of ComboBoxItem I want to be able to populate dynamically. Databinding would be my obvious go-to, but with the separator and "Reset" styles at the end, I'm not sure how to do this. I currently have the ComboBoxItems "hard-coded" in XAML:
<ComboBox x:Name="cbTestSelect"
Height="34"
Width="18"
IsEnabled="False">
<ComboBoxItem Style="{StaticResource cbTestStyle}" Content="Test 1" Foreground="#7FFF0000" Selected="ComboBoxItem_Selected"/>
<ComboBoxItem Style="{StaticResource cbTestStyle}" Content="Test 2" Foreground="#7F00FF00" Selected="ComboBoxItem_Selected"/>
<ComboBoxItem Style="{StaticResource cbTestStyle}" Content="Test 3" Foreground="#7F0000FF" Selected="ComboBoxItem_Selected"/>
<ComboBoxItem Style="{StaticResource cbSeparatorStyle}"/>
<ComboBoxItem Style="{StaticResource cbResetStyle}" Content="Reset all"/>
</ComboBox>
In this example, I would ideally like to dynamically create the first three items and have the separator and "reset" items remain static. I'm still relatively new to WPF. I felt like trying to create this control in WinForms (which the application this user control would be used in is) would be a lot more complicated. Plus I'm trying to steer us towards using WPF more anyway.
Any help or links to other questions or tutorials online would be greatly appreciated.

Solution 1:
Use a CompositeCollection so that you can bring up your data items with DataBinding, and use regular XAML to define the hard-coded items:
<Window x:Class="WpfApplication31.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication31"
Title="MainWindow" Height="350" Width="525"
x:Name="view">
<Window.Resources>
<DataTemplate DataType="{x:Type local:DataItem}">
<StackPanel Orientation="Horizontal">
<CheckBox IsChecked="{Binding IsChecked}"/>
<TextBlock Text="{Binding Text}"/>
<Rectangle Stroke="Black" StrokeThickness="1"
Fill="{Binding Color}" Width="20"/>
</StackPanel>
</DataTemplate>
</Window.Resources>
<Grid>
<ComboBox VerticalAlignment="Center"
HorizontalAlignment="Center"
Width="100" x:Name="Combo">
<ComboBox.Resources>
<CompositeCollection x:Key="ItemsSource">
<CollectionContainer Collection="{Binding DataContext,Source={x:Reference view}}"/>
<Separator Height="10"/>
<Button Content="Clear All"/>
</CompositeCollection>
</ComboBox.Resources>
<ComboBox.ItemsSource>
<StaticResource ResourceKey="ItemsSource"/>
</ComboBox.ItemsSource>
</ComboBox>
</Grid>
</Window>
Code Behind:
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
var colors = new[] {"Red", "Green", "Blue", "Brown", "Cyan", "Magenta"};
this.DataContext =
Enumerable.Range(0, 5)
.Select(x => new DataItem
{
Text = "Test" + x.ToString(),
Color = colors[x],
IsChecked = x%2 == 0
});
}
}
Data Item:
public class DataItem
{
public bool IsChecked { get; set; }
public string Text { get; set; }
public string Color { get; set; }
}
Result:

Solution 2:
Using Expression Blend, you can get the XAML for the default Template for the ComboBox control, and modify this XAML to accomodate your extra visuals.
The XAML you get is rather long, and I'm not going to post it here. You will have to put that in a ResourceDictionary and reference that in the XAML where you define this ComboBox.
The relevant part you need to edit is the Popup:
<Popup x:Name="PART_Popup" AllowsTransparency="true" Grid.ColumnSpan="2" IsOpen="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" Margin="1" PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}" Placement="Bottom">
<Themes:SystemDropShadowChrome x:Name="shadow" Color="Transparent" MaxHeight="{TemplateBinding MaxDropDownHeight}" MinWidth="{Binding ActualWidth, ElementName=templateRoot}">
<Border x:Name="dropDownBorder" BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}" BorderThickness="1" Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}">
<DockPanel>
<Button Content="Clear All" DockPanel.Dock="Bottom"/>
<Separator Height="2" DockPanel.Dock="Bottom"/>
<ScrollViewer x:Name="DropDownScrollViewer">
<Grid x:Name="grid" RenderOptions.ClearTypeHint="Enabled">
<Canvas x:Name="canvas" HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
<Rectangle x:Name="opaqueRect" Fill="{Binding Background, ElementName=dropDownBorder}" Height="{Binding ActualHeight, ElementName=dropDownBorder}" Width="{Binding ActualWidth, ElementName=dropDownBorder}"/>
</Canvas>
<ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Contained" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</Grid>
</ScrollViewer>
</DockPanel>
</Border>
</Themes:SystemDropShadowChrome>
</Popup>
Notice that I added a DockPanel, the Button and a Separator.
Then you can bind your ItemsSource to the DataItem collection normally:
<ComboBox ItemsSource="{Binding}"
VerticalAlignment="Center"
HorizontalAlignment="Center"
Width="100"/>
Result:
Notice that this approach is a lot better than my previous solution, and other answers posted here, because it does not wrap the extra visuals in ComboBoxItems, and therefore you don't get the selection highlight for them, which is rather weird.

You could use a DataTemplateSelector with the DataTemplates defined in the XAML and some item type variable it the data you're binding to.
public class StyleSelector : DataTemplateSelector
{
public DataTemplate DefaultTemplate
{ get; set; }
public DataTemplate SeparatorTemplate
{ get; set; }
public DataTemplate ResetTemplate
{ get; set; }
public override DataTemplate SelectTemplate(object item, DependencyObject container)
{
var type = item as SomeType;
if (type != null)
{
switch (type.SomeItemTypeField)
{
case TypeENum.Separator: return SeparatorTemplate;
case TypeENum.Reset: return ResetTemplate;
default:
return DefaultTemplate;
}
}
return base.SelectTemplate(item, container);
}
}
Check out this more detailed example.

I think your best bet is to learn about DataTemplate and DataTemplateSelector.
Here is an blog post that will show you a simple example of using a DataTemplate.
The ComboBox Control
Essentially, you could bind your ComboBox to a collection of objects, and use a DataTemplateSelector to pick which template to use based on the type of object.

Rude tab keeps coming to the front

I have a TabControl that uses the following XAML.
<TabControl x:Class="MyApp.Tab.TabContainer"
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:MyApp.Tab"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300"
ItemsSource="{Binding}">
<TabControl.Resources>
<Style TargetType="TextBox">
<Setter Property="VerticalAlignment" Value="Stretch"></Setter>
<Setter Property="HorizontalAlignment" Value="Stretch"></Setter>
<Setter Property="AcceptsReturn" Value="True"></Setter>
<Setter Property="TextWrapping" Value="WrapWithOverflow"></Setter>
<Setter Property="MaxLines" Value="5000"></Setter>
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"></Setter>
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"></Setter>
</Style>
<SolidColorBrush x:Key="mouseOverColor" Color="Red"></SolidColorBrush>
<RadialGradientBrush x:Key="glowColor" Center="0.5,0.5" RadiusX="0.5" RadiusY="0.5" GradientOrigin="0.5,0.5">
<GradientStop Color="Red" Offset="0.5"></GradientStop>
</RadialGradientBrush>
<RadialGradientBrush x:Key="glowMask" Center="0.5,0.5" RadiusX="0.5" RadiusY="0.5" GradientOrigin="0.5,0.5">
<GradientStop Color="#80FFFFFF" Offset="0"></GradientStop>
<GradientStop Color="#80FFFFFF" Offset="0.6"></GradientStop>
<GradientStop Color="#00FFFFFF" Offset="1"></GradientStop>
</RadialGradientBrush>
<DataTemplate x:Key="NewTabHeaderTemplate" DataType="{x:Type local:AddNewTab}">
<DockPanel>
<TextBlock Name="TextBlock" VerticalAlignment="Center" Text="+" FontSize="18" MouseLeftButtonDown="TextBlock_OnMouseLeftButtonDown"></TextBlock>
</DockPanel>
</DataTemplate>
<DataTemplate x:Key="MapTabHeaderTemplate" DataType="local:ACTabItemTabItem">
<DockPanel>
<Button Name="btn_TabDelete" DockPanel.Dock="Right" Margin="5,0,0,0" Padding="0" Click="btn_TabDelete_Click" CommandParameter="{Binding RelativeSource={RelativeSource AncestorType={x:Type TabItem}}, Path=Name}">
<Image Source="icon_grey_cross.gif" Height="9" Width="9" Margin="0,0,0,0"></Image>
<Button.Template>
<ControlTemplate TargetType="Button">
<Border x:Name="bdr_main" Margin="4" BorderThickness="0" >
<ContentPresenter VerticalAlignment="Center" HorizontalAlignment="Center" Margin="4,4,4,4" ContentSource="Content" />
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter TargetName="bdr_main" Property="Background" Value="{StaticResource glowColor}"></Setter>
<Setter TargetName="bdr_main" Property="OpacityMask" Value="{StaticResource glowMask}"></Setter>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Button.Template>
</Button>
<TextBlock VerticalAlignment="Center" Text="{Binding RelativeSource={RelativeSource AncestorType={x:Type TabItem}}, Path=Header}"></TextBlock>
</DockPanel>
</DataTemplate>
</TabControl.Resources>
public class AddNewTab : TabItem {}
During the constructor I do this to produce a blank tab with a simple '+' in the header.
_tabItems = new ObservableCollection<ACTabItem>();
try
{
AddNewTab tabAdd = new AddNewTab();
tabAdd.HeaderTemplate = this.FindResource("NewTabHeaderTemplate") as DataTemplate;
_tabItems.Add(tabAdd);
}
this.DataContext = _tabItems;
When the user clicks the '+' I create a new TabItem and add it to the Items collection. I then attempt to bring the new tab to the front. (ImplementTab() simply creates a new TabItem with some values preset by the contents of 'td').
//code up here just collects some data from user to populate 'td'
TabItem newTab = ImplementTab(td);
if (newTab != null)
{
_tabItems.Add((ACTabItem)newTab);
newTab.Focus();
}
If I watch the this_SelectionChanged() event handler I see that the Focus() call does cause a change in SelectedIndex to the index of the newly added tab, but I also see that after the method the new tab code is in completes (right after the Focus()), SelectionChanged is thrown again, but this time with a SelectedIndex of '0' (the original blank tab with the '+' header).
Clicking on a tab throws SelectionChanged only once and I get the expected results.
I am simply trying to have the TabControl focus on a newly added TabItem... how did I mess that up?

I feel that you don't have to inherit your AddNewTab from TabItem if you do databind. It should be just any class, inheriting it from TabItem makes it confusing since TabControl will create a new TabItem to wrap around this data item so all the methods like Focus() on the tab you created will have no effect.
To select the last tab just set the SelectedIndex instead of calling Focus.
if (TabControl1.Items.Count > 0)
TabControl1.SelectedIndex = TabControl1.Items.Count - 1;

Custom ListBox in WPF

I am trying to create a custom ListBox control in WPF for a chat Messenger. I am using an ellipse to show the online/offline user. The ellipse is to be displayed on left and some text in center of the ListBoxItem.
I want to set the ellipse fill propert to red/green based on some variable.
This is what I have done :
<ListBox Name="myList" HorizontalAlignment="Left" Height="232" Margin="117,74,0,0" VerticalAlignment="Top" Width="207">
<ListBox.ItemTemplate>
<DataTemplate>
<DockPanel>
<Ellipse Name="ellipse" Fill="Red" DockPanel.Dock="Left">
<Ellipse.Triggers>
<Trigger Property="{Binding Online}" Value="True">
<Setter TargetName="ellipse" Property="Ellipse.Fill" Value="Green"/>
</Trigger>
</Ellipse.Triggers>
</Ellipse>
<TextBlock Text="{Binding text}"></TextBlock>
</DockPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
and in the code :
myList.Items.Add(new { text="Hello",Online="True" });
I am getting an error as
Cannot find the static member 'FillProperty' on the type 'ContentPresenter'.
What am I doing wrong here?

Obviously this is wrong: Property="{Binding Online}"
Also you should use a Style for triggers, no need to set TargetName, and you need to take precedence into consideration, and use a Setter for the default value.

you are actually misleading WPF with some of these concerns.
Binding A property on trigger will not work. you have to use DataTrigger insteed of Triggers.
Implementing Trigger on the Fly for any control. most of the times not work. So go with Styles.
While you are creating Ellipse in template make sure you have created enough size for it. So that can be visible to users.
try this.
<Window.Resources>
<Style x:Key="elstyle" TargetType="Ellipse">
<Setter Property="Height" Value="5"/>
<Setter Property="Width" Value="5"/>
<Setter Property="Fill" Value="Red"/>
<Style.Triggers>
<DataTrigger Binding="{Binding Online}" Value="true">
<Setter Property="Fill" Value="Green"/>
</DataTrigger>
</Style.Triggers>
</Style>
</Window.Resources>
<Grid>
<ListBox x:Name="myList" HorizontalAlignment="Left" Height="232" Margin="117,74,0,0" VerticalAlignment="Top" Width="207">
<ListBox.ItemTemplate>
<DataTemplate>
<DockPanel>
<Ellipse Name="ellipse" Margin="5" DockPanel.Dock="Left" Style="{DynamicResource elstyle}">
</Ellipse>
<TextBlock Text="{Binding Name}"></TextBlock>
</DockPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</Grid>
code behind .
public MainWindow()
{
Random r = new Random();
InitializeComponent();
for (int i = 0; i < 10; i++)
{
myList.Items.Add(new { Name = "Name" + i.ToString(), Online = Convert.ToBoolean(r.Next(-1, 1)) });
}
}

You need to set the initial colour via a Setter and not in XAML. For more information and see this question: Change the color of ellipse when mouse over

there are a few issues in your XAML
set the size of your Ellipse
you need to use Style instead of Ellipse.Triggers
set your Fill color in your Style if you wane be able to change it in XAML by some condition
here an working example for your problem
<DataTemplate>
<!--<DockPanel> juste because i like StackPanel-->
<StackPanel Orientation="Horizontal">
<!--<Ellipse Name="ellipse" Fill="Red" DockPanel.Dock="Left">-->
<Ellipse Name="ellipse" Width="15" Height="15">
<!--<Ellipse.Triggers>-->
<Ellipse.Style>
<Style TargetType="Ellipse">
<Setter Property="Fill" Value="Red"/>
<Style.Triggers>
<!--<Trigger Property="{Binding Online}" Value="True">-->
<DataTrigger Binding="{Binding Online}" Value="True">
<Setter Property="Fill" Value="LimeGreen"/>
</DataTrigger>
</Style.Triggers>
</Style>
</Ellipse.Style>
</Ellipse>
<TextBlock Text="{Binding text}"/>
</StackPanel>
</DataTemplate>