EventToCommandBehavior in .Net9 – It does not work anymore?

If you’re migrating your .NET MAUI app to newer versions (especially version 9 or later) and have encountered the EventToCommandBehavior error that previously worked flawlessly, you’re not alone! A subtle but crucial change in how BindingContext is handled for Behaviors can cause some headaches. But don’t worry, this post will guide you through the change, explain what happened, and, most importantly, show you how to solve the problem.

The Scenario:

Imagine you have code like this, which used the EventToCommandBehavior to execute a command in your ViewModel when the page loaded:

<ContentPage.Behaviors>
    <toolkit:EventToCommandBehavior
        EventName="Loaded"
        Command="{Binding AtualizaDadosCommand}" />
</ContentPage.Behaviors>

In previous versions of .NET MAUI, this probably worked without issues. The BindingContext of the Behavior was implicitly inherited from the parent element (the page, in this case), which was already correctly set to your ViewModel.

The Change:

In recent .NET MAUI versions, this implicit inheritance of the BindingContext for Behaviors has been removed or altered. This means that the EventToCommandBehavior can no longer find your AtualizaDadosCommand command in the ViewModel, resulting in failures or the command simply not being executed.

Why the Change?

The exact reasons for the change may vary, but they are generally related to improvements in the .NET MAUI architecture, performance optimizations, and possibly to make the behavior of Behaviors more consistent and predictable. By making the BindingContext explicit, the framework offers more control and avoids ambiguities.

The Solution: Explicitly Setting the BindingContext

The solution is relatively simple, but crucial: you need to set the BindingContext of the EventToCommandBehavior explicitly. Here’s how:

<ContentPage.Behaviors>
    <toolkit:EventToCommandBehavior
        EventName="Loaded"
        Command="{Binding AtualizaDadosCommand}"
        BindingContext="{Binding Source={x:Reference this}, Path=BindingContext}" />
</ContentPage.Behaviors>

Understanding the Solution:

1. BindingContext="{Binding Source={x:Reference this}, Path=BindingContext}": This is the magic line!
   • Source={x:Reference this}: Defines the source of the BindingContext as the page itself.
   • Path=BindingContext: Specifies that the value of the BindingContext to be used is the BindingContext of the page.

In other words, we are telling the EventToCommandBehavior: “Use the same BindingContext that the page has.” And since the page’s BindingContext is already set to your ViewModel (through the line), the Behavior will now have access to your AtualizaDadosCommand command.

Complete Example:

<ContentPage
    x:Class="YourNamespace.YourPage"
    xmlns:toolkit="http://schemas.microsoft.com/dotnet/2022/maui/toolkit"
    x:Name="this">

    <ContentPage.BindingContext>
        <local:YourViewModel />
    </ContentPage.BindingContext>

    <ContentPage.Behaviors>
        <toolkit:EventToCommandBehavior
            EventName="Loaded"
            Command="{Binding AtualizaDadosCommand}"
            SourceObject="{x:Reference this}"
            BindingContext="{Binding Source={x:Reference this}, Path=BindingContext}" />
    </ContentPage.Behaviors>
</ContentPage>

Conclusion:

The change in how BindingContext is handled for Behaviors in .NET MAUI may have caused some confusion, but the solution is simple and straightforward. By explicitly setting the BindingContext, you ensure that your Behaviors work as expected. We hope this post has clarified the change and helped you resolve the issue quickly. If you have any questions, leave a comment below!