WPF multiple Instance of ViewModel in Usercontrol and Window - wpf

Question

This is my first question on stackoverflow.
I have a Usercontrol that contains a viewModel as a staticresource. Than I have a Window that contains this UserModel. I want to send commands from the Window to the viewmodel that is used by the usercontrol. So I bound the Viewmodel class to the Usercontrol and to the Window as a static Resource. But the viewmodell ist instanciated 2 times. Once from the Window and once from the usercontrol. What am I doing wrong?
Here is the xaml of the usercontrol:
<UserControl x:Class="ScanVM.ScanView"
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:t="clr-namespace:ScanVM"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="501.305">
<UserControl.Resources>
<t:ScanVM x:Key="ABC"/>
</UserControl.Resources>
<DockPanel DataContext="{Binding Source={StaticResource ABC}}">
<Grid ShowGridLines="True" Margin="10">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="1*" />
<ColumnDefinition Width="5*" />
<ColumnDefinition Width="1*" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="1*" />
<RowDefinition Height="10*" />
<RowDefinition Height="1*" />
</Grid.RowDefinitions>
<Label Content="Geräte:" Grid.Column="0" Grid.Row="1" HorizontalAlignment="Right" Margin="0,10,10,0"/>
<DataGrid ItemsSource="{Binding Path=ScanIDs, Mode=OneWay}" Grid.Column="1" HorizontalAlignment="Left" Margin="10,10,0,0" Grid.Row="1" VerticalAlignment="Top" AutoGenerateColumns="false" Height="152" >
<DataGrid.Columns>
<DataGridTextColumn Binding="{Binding ScanIds}" ClipboardContentBinding="{x:Null}" Header="Bezeichnung" MinWidth="200"/>
</DataGrid.Columns>
</DataGrid>
</Grid>
</DockPanel>
This is the code of my window:
<Window x:Class="ScanViewer.MainWindow"
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:ribbon="clr-namespace:System.Windows.Controls.Ribbon;assembly=System.Windows.Controls.Ribbon"
xmlns:self="clr-namespace:ScanViewer"
xmlns:t="clr-namespace:ScanVM;assembly=ScanVM"
mc:Ignorable="d"
Title="Scan-Viewer" Height="682.225" Width="694">
<Window.Resources>
<t:ScanVM x:Key="ABC"/>
</Window.Resources>
<Window.CommandBindings>
<CommandBinding
Command="self:MainWindow.ExitCommand"
CanExecute="ExitCommand_CanExecute"
Executed="ExitCommand_Execute"/>
</Window.CommandBindings>
<DockPanel>
<ribbon:Ribbon DockPanel.Dock="Top">
<Ribbon.ApplicationMenu>
<RibbonApplicationMenu Visibility="Collapsed">
<RibbonApplicationMenuItem Header="Beenden"/>
<RibbonApplicationMenu.FooterPaneContent>
<RibbonButton Label="Exit"/>
</RibbonApplicationMenu.FooterPaneContent>
</RibbonApplicationMenu>
</Ribbon.ApplicationMenu>
<RibbonTab Header="Start">
<RibbonGroup Header="Programm">
<RibbonButton Label="Beenden" Margin="0,5,0,0" Command="self:MainWindow.ExitCommand" />
</RibbonGroup>
<RibbonGroup Header="Scannen">
<RibbonButton Label="Daten Scannen" Margin="0,5,0,0" Command="{Binding ScanCmd, Source={StaticResource ABC}}"/>
</RibbonGroup>
</RibbonTab>
</ribbon:Ribbon>
<Grid DockPanel.Dock="Top" Height="464">
<t:ScanView DataContext="{Binding Source={StaticResource ABC}}"/>
</Grid>
</DockPanel>
And this is the code of my Viewmodel:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data;
using System.Windows.Input;
using System.Threading;
using MVVMBase;
namespace ScanVM
{
public class ScanVM : ViewModelBase
{
private ICommand _doWorkCmd;
private ICommand _scanCmd;
private bool scanAllowed = false;
public List<int> ScanIDs
{
get;
set;
}
public ScanVM()
{
_doWorkCmd = new DelegateCommand((param) => DoWork());
_scanCmd = new DelegateCommand((param) => Scan());
}
public bool NWScanAllowed(object executeAllowed)
{
return scanAllowed;
}
public ICommand WorkCmd
{
get { return _doWorkCmd; }
}
public ICommand ScanCmd
{
get
{
return _scanCmd;
}
}
private void Scan()
{
scanAllowed = true;
}
private void DoWork()
{
}
}
}

Answer 1

The answer depends on what you want to do with the shared object.
You can either move ABC to App.xaml inside <Application.Resources> tag to share it throughout your project.
Or you can remove the one in user control and use the ABC in window's resources.
You will need a valid reference path to address the ABC in window resource from user control in this case.
which brings us to these two situations:
The valid path is possible to find:
So let's say if your user control is a part of the mentioned window (and if the window's DataContext is set to ABC) then inside the user control you will reference it like this:
<DockPanel DataContext="{Binding RelativeSource={RelativeSource AncestorType={x:Type Window}}, Path=DataContext}">
The valid path is impossible to find and you just want the static resource. If you insist on doing it this way it is better to just move the resource to App.xaml than to try to tweak this out.