MVVMLight ViewModelLocator on a UserControl

Question

Is it possible to use the MVVMLight ViewModelLocator on a UserControl. I have added it to my user control in the same way as is done on the MainWindow, but i get an error/popup in VS2010 stating "Cannot find resource named 'Locator'. Resource names are case sensitive."

Has anyone tried this?

Code i have thus far is pretty much a standard MVVMLight WPF starter application...

UserControl

<UserControl x:Class="NavTest3.PersonControl"
         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" 
         Height="116" MinWidth="250" Width="300"
         DataContext="{Binding Person, Source={StaticResource Locator}}"
         >

<!---->
<UserControl.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="Skins/MainSkin.xaml" />
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</UserControl.Resources>

App.xaml contains..

<Application.Resources>
    <!--Global View Model Locator-->
    <vm:ViewModelLocator x:Key="Locator"
                         d:IsDataSource="True" />
</Application.Resources>

so the issue is with setting "DataContext="{Binding Person, Source={StaticResource Locator}}" on the userControl."

As mentioned, doing this will mean every instance of this user control will share the same ViewModel, but I want to start with understanding this issue before moving on..

can you post your locator code that pertains to this particular section? — ecathell

Ehsan Zargar Ershadi Ehsan Zargar Ershadi · Accepted Answer · 2011-06-14T07:58:06

Yes you can, you need to create a static resource in your user control

<UserControl x:Class="MvvmLight1.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" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300"

             xmlns:vm="clr-namespace:MvvmLight1.ViewModel"
             >

    <UserControl.Resources>
        <vm:ViewModelLocator x:Key="Locator" d:IsDataSource="True" />
    </UserControl.Resources>
    <Grid>

    </Grid>
</UserControl>

but IMO it's not a good idea to use MVVM Light ViewModelLocator for UserControles because it is a static property and when you are going to instantiate multiple instances of your user control there are going to have the same common ViewModel so they all act same and this is not what we want for a UserControl in case you decide to use it once in your entire project.

to get around this problem you need to modify the ViewModelLocator by making all the properties Non static for instance :

 public class ViewModelLocator
    {
        //         v--- You got to comment this out
        private /*static*/ MainViewModel _main;

        public ViewModelLocator()
        {            
            CreateMain();
        }

        public /*static*/ MainViewModel MainStatic
        {
            get
            {
                if (_main == null)
                {
                    CreateMain();
                }

                return _main;
            }
        }

        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Performance",
            "CA1822:MarkMembersAsStatic",
            Justification = "This non-static member is needed for data binding purposes.")]
        public MainViewModel Main
        {
            get
            {
                return MainStatic;
            }
        }

        public /*static*/ void ClearMain()
        {
            _main.Cleanup();
            _main = null;
        }

        public /*static*/ void CreateMain()
        {
            if (_main == null)
            {
                _main = new MainViewModel();
            }
        }

        public /*static*/ void Cleanup()
        {
            ClearMain();
        }
    }

MVVMLight ViewModelLocator on a UserControl

3 Answers