Onload binding error at runtime in MVVM

Question

I'm trying to use the MVVM Light framework with an "OnLoad" view function, so that when the form is loaded a function from the ViewModel is executed.

I have achieved that, following the examples of several links:

https://stackoverflow.com/a/3409801/2794484 http://aslamhadi.com/add-onload-event-in-wpf-mvvm/ https://social.msdn.microsoft.com/Forums/vstudio/en-US/b4d2afe6-4c28-44e5-98a5-b7ba30fec220/how-to-capture-window-loaded-event-in-view-model-when-using-mvvm?forum=wpf

My view has:

<i:Interaction.Triggers>
    <i:EventTrigger EventName="Loaded">
        <GalaSoft_MvvmLight_Command:EventToCommand Command="{Binding LoadWindowCommand, Mode=OneWay}" />
    </i:EventTrigger>
</i:Interaction.Triggers>

And my ViewModel:

    public TestViewModel()
    {
        LoadWindowCommand = new RelayCommand(OnLoadWindowCommand);
    }

    public RelayCommand LoadWindowCommand { get; private set; }
    private void OnLoadWindowCommand()
    {
        //put your code here
    }

It works, but using the methods of any of the links, raises me a binding error at runtime:

System.Windows.Data Information: 10 : Cannot retrieve value using the binding and no valid fallback value exists; using default instead. BindingExpression:Path=LoadWindowCommand; DataItem=null; target element is 'EventToCommand' (HashCode=4875788); target property is 'Command' (type 'ICommand')

How can I avoid this error?

---

Update, I have updated all my code as your recommendations.

MainView:

<Window x:Class="MvvmLight1.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:GalaSoft_MvvmLight_Command="clr-namespace:GalaSoft.MvvmLight.Command;assembly=GalaSoft.MvvmLight.Extras.WPF4"
       xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
        xmlns:ignore="http://www.ignore.com"
        xmlns:local="clr-namespace:MvvmLight1.ViewModel"
        mc:Ignorable="d ignore"
        Title="MVVM Light Application" Height="300" Width="300">
    <Window.Resources>
        <local:ViewModelLocator x:Key="Locator" d:IsDataSource="True" />
    </Window.Resources>
    <Window.DataContext>
        <!--Move DataContext binding there-->
        <Binding Path="Main" Source="{StaticResource Locator}"/>
    </Window.DataContext>
    <Grid>
        <i:Interaction.Triggers>
            <i:EventTrigger EventName="Loaded">
                <GalaSoft_MvvmLight_Command:EventToCommand Command="{Binding LoadWindowCommand, Mode=OneWay}" />
            </i:EventTrigger>
        </i:Interaction.Triggers>
    </Grid>
</Window>

ViewModel:

class MainWindowViewModel : ViewModelBase
{
    public MainWindowViewModel()
    {
        LoadWindowCommand = new RelayCommand(OnLoadWindowCommand);
    }

    public RelayCommand LoadWindowCommand { get; private set; }
    private void OnLoadWindowCommand()
    {
        Debug.WriteLine("Loaded!");
    }
}

ViewModelLocator

class ViewModelLocator
{
    public ViewModelLocator()
    {
        Main = new MainWindowViewModel();
    }

    public MainWindowViewModel Main { get; private set; }
}

And I have commented the following lines in the App.xaml:

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

But in my "Output" Windows I continue seeing:

....
System.Windows.Data Information: 10 : Cannot retrieve value using the binding and no valid fallback value exists; using default instead. BindingExpression:Path=LoadWindowCommand; DataItem=null; target element is 'EventToCommand' (HashCode=44967810); target property is 'Command' (type 'ICommand')
'MvvmLight1.vshost.exe' (Administrado (v4.0.30319)): se cargó 'C:\windows\Microsoft.Net\assembly\GAC_MSIL\PresentationFramework.Aero\v4.0_4.0.0.0__31bf3856ad364e35\PresentationFramework.Aero.dll', se omitió la carga de símbolos. Se optimizó el módulo y se habilitó la opción 'Solo mi código'.
'MvvmLight1.vshost.exe' (Administrado (v4.0.30319)): se cargó 'C:\windows\Microsoft.Net\assembly\GAC_MSIL\UIAutomationProvider\v4.0_4.0.0.0__31bf3856ad364e35\UIAutomationProvider.dll', se omitió la carga de símbolos. Se optimizó el módulo y se habilitó la opción 'Solo mi código'.
'MvvmLight1.vshost.exe' (Administrado (v4.0.30319)): se cargó 'C:\windows\Microsoft.Net\assembly\GAC_MSIL\UIAutomationTypes\v4.0_4.0.0.0__31bf3856ad364e35\UIAutomationTypes.dll', se omitió la carga de símbolos. Se optimizó el módulo y se habilitó la opción 'Solo mi código'.
Loaded!

The only difference I can see with your code, is in the namespaces references of the MainWindow:

xmlns:mvvm_light_cmd="clr-namespace:GalaSoft.MvvmLight.Command;assembly=GalaSoft.MvvmLight.Platform"

And mine:

xmlns:GalaSoft_MvvmLight_Command="clr-namespace:GalaSoft.MvvmLight.Command;assembly=GalaSoft.MvvmLight.Extras.WPF4"

If I add your namespace it returns and error trying to resolve "GalaSoft.MvvmLight.Platform"

Any suggestion?

Answer 1

Ok, I've got it. Problem is that you are trying to get the Locator resource before it is initialize. Just put your DataContext binding after resources section. Here is the code:

MainWindow.xaml

<Window x:Class="MvvmLight1.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:GalaSoft_MvvmLight_Command="clr-namespace:GalaSoft.MvvmLight.Command;assembly=GalaSoft.MvvmLight.Extras"
       xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
        xmlns:ignore="http://www.ignore.com"
        xmlns:local="clr-namespace:MvvmLight1.ViewModel"
        mc:Ignorable="d ignore"
        Title="MVVM Light Application" Height="300" Width="300">
    <Window.Resources>
        <local:ViewModelLocator x:Key="Locator" d:IsDataSource="True" />
    </Window.Resources>
    <Window.DataContext>
        <!--Move DataContext binding there-->
        <Binding Path="Main" Source="{StaticResource Locator}"/>
    </Window.DataContext>
    <Grid>
        <i:Interaction.Triggers>
            <i:EventTrigger EventName="Loaded">
                <GalaSoft_MvvmLight_Command:EventToCommand Command="{Binding LoadWindowCommand, Mode=OneWay}" />
            </i:EventTrigger>
        </i:Interaction.Triggers>
    </Grid>
</Window>

MainViewModel.cs

namespace MvvmLight1.ViewModel
{
    public class MainViewModel : ViewModelBase
    {
        public MainViewModel()
        {
            LoadWindowCommand = new RelayCommand(OnLoadWindowCommand);
        }

        public RelayCommand LoadWindowCommand { get; private set; }
        private void OnLoadWindowCommand()
        {
            Debug.WriteLine("Loaded!");
        }
    }
}

ViewModelLocator.cs

namespace MvvmLight1.ViewModel
{
    public class ViewModelLocator
    {

        public ViewModelLocator()
        {
            ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);
            SimpleIoc.Default.Register<MainViewModel>();
        }

        public MainViewModel Main
        {
            get
            {
                return ServiceLocator.Current.GetInstance<MainViewModel>();
            }
        }
    }
}