C#•8mo ago

✅ WPF - how to correctly call a method in the ViewModel when an event occurs in the View?

I want to call a method in the ViewModel when an event happens in the View. That is, when the View is loaded, I want it to execute a Authentication method in the ViewModel:

public partial class MainWindow : UserControl
{
    internal MainWindowViewModel MainWindowVM;

    private async Task MyToolWindow_LoadedAsync(object sender, RoutedEventArgs e)
    {
        await MainWindowVM.AuthenticateUserAsync();
    }
}

internal class MainWindowViewModel: ViewModelBase
{
    internal bool IsAuthenticated { get; set; } = false;

    private GistClientService GistClientService = new GistClientService(); 
    
    public MainWindowViewModel()
    {
    }

    internal async Task AuthenticateUserAsync()
    {
        IsAuthenticated = await GistClientService.AuthenticateAsync();
    }
}

public partial class MainWindow : UserControl
{
    internal MainWindowViewModel MainWindowVM;

    private async Task MyToolWindow_LoadedAsync(object sender, RoutedEventArgs e)
    {
        await MainWindowVM.AuthenticateUserAsync();
    }
}

internal class MainWindowViewModel: ViewModelBase
{
    internal bool IsAuthenticated { get; set; } = false;

    private GistClientService GistClientService = new GistClientService(); 
    
    public MainWindowViewModel()
    {
    }

    internal async Task AuthenticateUserAsync()
    {
        IsAuthenticated = await GistClientService.AuthenticateAsync();
    }
}

I get Error CS0407 'Task MainWindow.MyToolWindow_LoadedAsync(object, RoutedEventArgs)' has the wrong return type at compile time.When coding MyToolWindow_LoadedAsync, I followed the intellisence warning about naming, uing void and not awaiting to end up with how it looks preently. I know it's me - what am I doing wrong?

6 Replies

dancepanda42•8mo ago

Simple answer, a event handler has no return type. The method should look like this: private async void MyToolWindow_LoadedAsync(object sender, RoutedEventArgs e) But the right way with a viewmodel is the MVVM Pattern. https://learn.microsoft.com/de-de/archive/msdn-magazine/2009/february/patterns-wpf-apps-with-the-model-view-viewmodel-design-pattern

THE MODEL-VIEW-VIEWMODEL (MVVM) DESIGN PATTERN FOR WPF

stigzlerOP•8mo ago

thanks for the reply. private async void gives the intellisense warning of VSTHRD100 Avoid async void methods Also, yeah I'm trying to get to grips with MVVM and have watched lots of videos. Putting it into practice is another issue! Should the right approach involve commands somehow? I know roughly how to tie viewmodel commands to button clicks etc, but can you just do the same for UiEvents? If so, how do I do that, as UIElements have the Command attribute, but UIEvents don't?

sibber•8mo ago

gui event handlers are the exception where you can use async void because you have no other option you can also use behaviors to avoid having to manually call the method in the codebehind and just bind the event directly to a command but this is fine too its not a big deal if youre not doing it often also you should use an mvvm library like CommunityToolkit.Mvvm avoids a shit ton of boilerplate

stigzlerOP•8mo ago

I will always take advice from someone who uses phrases such as "a shit ton" Thanks, 👍

sibber•8mo ago

haha :) $close

MODiX•8mo ago

Use the /close command to mark a forum thread as answered

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✅ WPF - how to correctly call a method in the ViewModel when an event occurs in the View?