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I use the MVVM Pattern but now I have a problem with the ObservableCollection in my WorkspaceViewModel.

I want to create from a "NavigationView" a TabItem in my "WorkspaceView". This is not the problem with Commands and an EventAggregator.
Then I had the problem the TabItem lost all informations because after TabItem_Changed the ViewModel was recreated. This was fixed with an Extended TabControl. Here

ViewModel: WorkspaceViewModel

public class WorkspaceViewModel : ViewModelBase.ViewModelBase

 public WorkspaceViewModel()
 
 this.TabItems = new ObservableCollection<ViewModelBase.ViewModelBase>();
 CreateTabItemEvent.Instance.Subscribe(CreateTabItem);
 

 #region Properties

 private ViewModelBase.ViewModelBase _SelectedTabItem;
 public ViewModelBase.ViewModelBase SelectedTabItem
 
 get return _SelectedTabItem; 
 set SetProperty(ref _SelectedTabItem, value); 
 

 private ObservableCollection<ViewModelBase.ViewModelBase> _TabItems;
 public ObservableCollection<ViewModelBase.ViewModelBase> TabItems
 
 get return _TabItems; 
 set SetProperty(ref _TabItems, value); 
 

 #endregion

 #region Methodes

 private void CreateTabItem(string pObjectName)
 
 //create TabItem and add it to the list
 ViewModelBase.ViewModelBase newTabItem = null;

 //Test Objects from Buttonnames
 if (pObjectName.EndsWith("1"))
 
 newTabItem = new CustomerViewModel();
 
 else if (pObjectName.EndsWith("2"))
 
 newTabItem = new ContactViewModel();
 

 if (newTabItem != null)
 
 this.TabItems.Add(newTabItem); 
 this.SelectedTabItem = newTabItem;

 //Test Refresh - not work because list is "old"
 RaisePropertyChanged(nameof(this.TabItems));
 
 

 #endregion

View: WorkspaceView

 <UserControl.Resources>
 <DataTemplate DataType="x:Type viewmodel:CustomerViewModel">
 <view:CustomerView/>
 </DataTemplate>
 <DataTemplate DataType="x:Type viewmodel:ContactViewModel">
 <view:ContactView/>
 </DataTemplate>

 <Style TargetType="x:Type tabControlEx:TabControlEx">
 <Setter Property="Template">
 <Setter.Value>
 <ControlTemplate TargetType="x:Type TabControl">
 <Grid Background="TemplateBinding Background" ClipToBounds="True" KeyboardNavigation.TabNavigation="Local" SnapsToDevicePixels="True">
 <Grid.ColumnDefinitions>
 <ColumnDefinition x:Name="ColumnDefinition0" />
 <ColumnDefinition x:Name="ColumnDefinition1" Width="0" />
 </Grid.ColumnDefinitions>
 <Grid.RowDefinitions>
 <RowDefinition x:Name="RowDefinition0" Height="Auto" />
 <RowDefinition x:Name="RowDefinition1" Height="*" />
 </Grid.RowDefinitions>
 <DockPanel Margin="2,2,0,0" LastChildFill="False">
 <TabPanel x:Name="HeaderPanel" Margin="0,0,0,-1" VerticalAlignment="Bottom" Panel.ZIndex="1" DockPanel.Dock="Left"
 IsItemsHost="True" KeyboardNavigation.TabIndex="1" />
 </DockPanel>
 <Border x:Name="ContentPanel" Grid.Row="1" Grid.Column="0"
 Background="TemplateBinding Background"
 BorderBrush="TemplateBinding BorderBrush"
 BorderThickness="TemplateBinding BorderThickness"
 KeyboardNavigation.DirectionalNavigation="Contained" KeyboardNavigation.TabIndex="2" KeyboardNavigation.TabNavigation="Local">
 <Grid x:Name="PART_ItemsHolder" Margin="TemplateBinding Padding" SnapsToDevicePixels="TemplateBinding SnapsToDevicePixels" />
 </Border>
 </Grid>
 </ControlTemplate>
 </Setter.Value>
 </Setter>
 </Style>
</UserControl.Resources>

<Grid>
 <Grid.RowDefinitions>
 <RowDefinition Height="Auto"></RowDefinition>
 <RowDefinition></RowDefinition>
 </Grid.RowDefinitions>

 <TextBlock Grid.Row="0" Text="Workspace Control"></TextBlock>
 <tabControlEx:TabControlEx x:Name="tbWorkspace" Grid.Row="1" 
 ItemsSource="Binding TabItems" SelectedItem="Binding SelectedTabItem"
 IsSynchronizedWithCurrentItem="True" >

 <TabControl.ItemContainerStyle>
 <Style TargetType="TabItem">
 <Setter Property="Header" Value="Binding Title"/>
 </Style>
 </TabControl.ItemContainerStyle>

 </tabControlEx:TabControlEx>
</Grid>

ViewModel: CustomerViewModel

public class CustomerViewModel : ViewModelBase.ViewModelBase

 public CustomerViewModel()
 
 this.SendMessageCommand = new DelegateCommand<string>(SendMessage);
 this.Title = "Customer";
 

 #region Commands

 private ICommand _SendMessageCommand;
 public ICommand SendMessageCommand
 
 get return _SendMessageCommand; 
 set SetProperty(ref _SendMessageCommand, value); 
 

 #endregion

 #region Methodes

 private void SendMessage(string pMessage)
 
 //Test to change the Title on the TabItem
 this.Title = pMessage;
 this.EditModus = false;
 MessageSentEvent.Instance.Publish(pMessage);
 

 #endregion

Now if I click on the button which is use the Command "SendMessageCommand" the Title is changed but the TabItem Header is not changed. The Title is every time "Customer" and the EditMode = true.
After this I look at the ObservableCollection in the WorkspaceViewModel... There is no changes. It has the "Title = Customer" and "EditMode = true"

So why the changes from the CustomerViewModel not apply the the WorkspaceViewModel.ObservableCollection?

Can someone help me?

Sample Project

EDIT:

ViewModel: ViewModelBase

 public class ViewModelBase : BindableBase

 public ViewModelBase()
 
 this.Title = String.Empty;
 this.EditModus = true;
 

 #region Properties

 private string _Title;
 public string Title
 
 get return _Title; 
 set SetProperty(ref _Title, value); 
 

 private bool _EditModus;
 public bool EditModus
 
 get return _EditModus; 
 set SetProperty(ref _EditModus, value); 
 

 #endregion

all ViewModels inherits from ViewModelBase and BindableBase from Prism 7 has INotifyPropertyChanged

EDIT 2: SOLUTION

I found the error. If I create a new TabItem, I create a new ViewModel too. But in the XAML I create a new ViewModel too. So the View hasnt the same ViewModel as the ObservableCollection in the WorkspaceViewModel.
So I delete the DataContext in the XAML and now it works.

<UserControl x:Class="CustomerModul.View.CustomerView"
 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:CustomerModul.View"
 xmlns:vm="clr-namespace:CustomerModul.ViewModel"
 mc:Ignorable="d" 
 d:DesignHeight="450" d:DesignWidth="800">
<UserControl.DataContext>
 <vm:CustomerViewModel/>
</UserControl.DataContext>

<Grid>
 <Grid.RowDefinitions>
 <RowDefinition></RowDefinition>
 <RowDefinition></RowDefinition>
 </Grid.RowDefinitions>

 <CheckBox Grid.Row="0" IsChecked="Binding EditModus">EditModus aktiv</CheckBox>
 <Button Grid.Row="1" Command="Binding SendMessageCommand" CommandParameter="Gespeichert" Content="Save"></Button>
</Grid>