Can't pass bound view model as command parameter

Question

<CollectionView.ItemTemplate>
                <DataTemplate x:DataType="data:PhotoItemViewModel">
                    <Border StrokeThickness="1"
                            Padding="0">
                        <VerticalStackLayout>
                            <Grid>
                                <Image
                                    Source="{Binding ThumbnailImageSource}"
                                    Aspect="AspectFill"
                                    HeightRequest="120"
                                    WidthRequest="120">
                                    <Image.Behaviors>
                                        <toolkit:TouchBehavior
                                            
                                            LongPressCommand="{Binding x:DataType=ContentPage, Source={x:Reference Page}, Path=BindingContext.LongPressCommand}"
                                            LongPressCommandParameter="{Binding .}"/>
                                    </Image.Behaviors>
                                </Image>
                                
                                <CheckBox
                                    IsChecked="{Binding IsSelected}"
                                    IsVisible="{Binding x:DataType=ContentPage,Source={x:Reference Page}, Path=BindingContext.IsSelectionMode}"
                                    VerticalOptions="Start"
                                    HorizontalOptions="End"
                                    Margin="5"
                                    BackgroundColor="White"
                                    Opacity="0.8"/>
                            </Grid>
                        </VerticalStackLayout>
                    </Border>
                </DataTemplate>
            </CollectionView.ItemTemplate>
        </CollectionView>

Here I'm using a collection view to display a list of images, with the support of long press gesture on each image, implemented using CommunityToolkit.Maui's TouchBehavior. And my view model's LongPressCommand command is initialized with this method:

  private void OnPhotoLongPressed(PhotoItemViewModel photo)
    {
        IsSelectionMode = true;
        // photo.IsSelected = true;
    }

Now the issue is the photo passed from the command is always null.

Whilst this isn't directly what you asked for, if you consider deriving from Image, say, MyImage and add the TouchBehavior to your subclass then your component will have access to everything including the BindingContext. — Stephen Quan
– Stephen Quan, Commented Jul 10 at 9:08

Nico · Accepted Answer · 2025-09-09 03:11:22Z

1

Turns out there're just couple of things to add in order to have my original code to work:

Add BindingContext assignment to the behavior;
Set x:DataType to the BindingContext;
Use x:Reference to refer to the parent Image control and set its BindingContext to the behavior.

Hope this could be added to the documentation as this is quite a common scenario.

answered Sep 9 at 3:11

Nico

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Comments

Ben · Accepted Answer · 2025-07-13 13:52:08Z

0

".NET MAUI does not set the BindingContext of a behavior"

Name the Grid:

<Grid x:Name="grid"

Add a BindingContext reference in LongPressCommandParameter:

LongPressCommandParameter="{Binding Source={x:Reference grid}, Path=BindingContext, x:DataType=Grid}"

The same as above in codebehind:

First, add Loadedand Unloaded in xaml:

<Grid x:Name="grid" Loaded="AddBehavior" Unloaded="ClearBehaviors">

Codebehind (.xaml.cs) (with your viewmodel named ViewModel and LongPressCommand named LongPressCommand):

AddBehavior:

if (sender is Grid grid)
{
    var behavior = new TouchBehavior();
    behavior.SetBinding(TouchBehavior.LongPressCommandProperty, static (Page p) => (p.BindingContext as ViewModel).LongPressCommand, source: page);
    behavior.SetBinding(TouchBehavior.LongPressCommandParameterProperty, static (Grid g) => g.BindingContext, source: grid);

    grid.Behaviors.Add(behavior);
}

ClearBehaviors:

if (sender is Grid grid)
{
    foreach (var behavior in grid.Behaviors)
    {
        behavior.RemoveBinding(TouchBehavior.LongPressCommandParameterProperty);
        behavior.RemoveBinding(TouchBehavior.LongPressCommandProperty);
    }
    grid.Behaviors.Clear();
}

This avoids the XamlC warning XC0045: Binding: Property "LongPressCommand" not found on "System.Object".

edited Jul 13 at 13:52

answered Jul 11 at 9:46

Ben

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6 Comments

Nico Jul 12 at 9:48

Care to explain why my code didn't work?

Ben Jul 13 at 13:54

Please see learn.microsoft.com/en-us/dotnet/communitytoolkit/maui/… or learn.microsoft.com/en-us/dotnet/maui/fundamentals/…

Nico Jul 16 at 2:23

I read the official document before posting here. Unfortunately it doesn't provide examples with command parameters.

S.Will Jul 17 at 21:10

I could be mistaken but the reason it didn't work is because your Image object is bound to ThumbnailImageSource not PhotoItemViewModel. Not sure if MAUI has the concept of ancestors but you could have also used that when binding to go up one level so that "Binding ." was binding to PhotoItemViewModel instead of ThumbnailImageSource, which could not be converted to PhotoItemViewModel. If you change your previous code to have the method accept type ThumbnailImageSource you might get a value. Hope that clears it up for you

Nico Jul 19 at 14:24

ThumbnailImageSource is the name of a property of PhotoItemViewModel. Technically speaking, using Binding . is supposed to pass the entire bound object, which is the view model, to the backend command.

Nico Sep 6 at 4:55

The grid is inside the ItemTemplate, meaning there'll be multiple grids, so I assume I can't name it and reference it as if there were only 1 grid on the page.

Stephen Quan · Accepted Answer · 2025-10-09 00:00:05Z

Refactoring Image into MyImage would have reduced the binding complexity of this problem.

// MyImage.cs
public partial class MyImage : Image
{
    [BindableProperty] public partial ICommand? LongPressedCommand { get; set; }
    [BindableProperty] public partial object? LongPressedCommandParameter { get; set; }
    public MyImage()
    {
        var behavior = new TouchBehavior();
        behavior.SetBinding(
            TouchBehavior.LongPressCommandProperty, static (MyImage i) => i.LongPressedCommand,
            BindingMode.OneWay,
            source: this);
        behavior.SetBinding(
            TouchBehavior.LongPressCommandParameterProperty, static (MyImage i) => i.LongPressedCommandParameter,
            BindingMode.OneWay,
            source: this);
        this.Behaviors.Add(behavior);
    }
}

<ContentPage
    x:Class="StackOverflow.Maui.App.SO79696605.MainPage"
    xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    xmlns:so79696605="clr-namespace:StackOverflow.Maui.App.SO79696605"
    x:DataType="so79696605:MainPage">
    <CollectionView ItemsSource="{Binding Photos}">
        <CollectionView.ItemTemplate>
            <DataTemplate x:DataType="so79696605:PhotoItemViewModel">
                <Border Padding="0" StrokeThickness="1">
                    <VerticalStackLayout>
                        <Grid>
                            <so79696605:MyImage
                                Aspect="AspectFill"
                                HeightRequest="120"
                                LongPressedCommand="{Binding ImageLongPressedCommand, Source={RelativeSource AncestorType={x:Type so79696605:MainPage}}, x:DataType='so79696605:MainPage'}"
                                LongPressedCommandParameter="{Binding .}"
                                Source="{Binding ThumbnailImageSource}"
                                WidthRequest="120" />

                            <CheckBox
                                Margin="5"
                                BackgroundColor="White"
                                HorizontalOptions="End"
                                IsChecked="{Binding IsSelected, Mode=TwoWay}"
                                Opacity="0.8"
                                VerticalOptions="Start" />
                        </Grid>
                    </VerticalStackLayout>
                </Border>
            </DataTemplate>
        </CollectionView.ItemTemplate>
    </CollectionView>
</ContentPage>

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