XAML Binding warnings when using commands and AncestorType

Question

I have a small MAUI program using CollectionView - and per item, there are buttons attached with a command (like changeitem)

The app is working. But I am getting warnings at compile and runtime about data binding.

            <CollectionView.ItemTemplate>
            <DataTemplate x:DataType="model:CheckItem">
                <Grid ColumnDefinitions="*,Auto" ColumnSpacing="{StaticResource StdSpacing}">
                    <Grid
                        Grid.Column="1"
                        Margin="{StaticResource StdMargin}"
                        ColumnDefinitions="auto,auto"
                        ColumnSpacing="{StaticResource StdSpacing}">
                        <Button
                            Grid.Column="0"
                            Command="{Binding Source={RelativeSource AncestorType={x:Type viewmodel:OverviewViewModel}}, Path=ChangeItemCommand}"
                            CommandParameter="{Binding .}"
                            Style="{x:StaticResource GoogleFont}"
                            Text="{x:Static font:GoogleIconFont.Edit}" />
                    </Grid>
                </Grid>
            </DataTemplate>
        </CollectionView.ItemTemplate>

The commands are located in the ViewModel viewmodel:OverviewViewModel whereas the data model for an item is located in model:CheckItem.

The ContenPage starts with

x:Class="BicycleCheckList.OverviewPage"
xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:font="clr-namespace:BicycleCheckList.Utils"
xmlns:local="clr-namespace:BicycleCheckList.Resources.Strings"
xmlns:model="clr-namespace:BicycleCheckList.Models"
xmlns:utils="clr-namespace:BicycleCheckList.Utils"
xmlns:viewmodel="clr-namespace:BicycleCheckList.ViewModels"
Title="{Binding Title}"
x:DataType="viewmodel:OverviewViewModel">

On Compile time, I am getting a warning: XC0045 Binding: Property "ChangeItemCommand" not found on "BicycleCheckList.Models.CheckItem".

The binding of the command Command="{Binding Source={RelativeSource AncestorType={x:Type viewmodel:OverviewViewModel}}, Path=ChangeItemCommand}" does not find the command.

Why?

The (toy) app is also available on Github

How can you set the Datatype on the binding directly? Command="{Binding Source={RelativeSource AncestorType={x:Type viewmodel:OverviewViewModel}}, Path=ChangeItemCommand}" Setting the data type on the button itself silents the compile time warning. But then when I want to change the item with the button, I am getting a runtime error, since the command's paramter is now null — Money
– Money, Commented Nov 17, 2024 at 13:32
Literally by specifying x:DataType=viewmodel:OverviewViewModel as a property: Command="{Binding Source={RelativeSource AncestorType={x:Type viewmodel:OverviewViewModel}}, Path=ChangeItemCommand, x:DataType=viewmodel:OverviewViewModel}" — Jon Skeet
– Jon Skeet, Commented Nov 17, 2024 at 13:33
I've deleted my earlier comments as they were based on not understanding the two modes on offer. I'm delighted to say this question didn't just help solve your problem, but mine too :) — Jon Skeet
– Jon Skeet, Commented Nov 17, 2024 at 13:40

Jon Skeet · Accepted Answer · 2024-11-17 13:37:48Z

0

While it looks somewhat redundant, you can specify the x:DataType of the binding. I believe all you need to do is change your Button declaration to:

<Button
    Grid.Column="0"
    Command="{Binding x:DataType=viewmodel:OverviewViewModel, Source={RelativeSource AncestorType={x:Type viewmodel:OverviewViewModel}}, Path=ChangeItemCommand}"
    CommandParameter="{Binding .}"
    Style="{x:StaticResource GoogleFont}"
    Text="{x:Static font:GoogleIconFont.Edit}" />

(I would normally put x:DataType at the end, but then it wouldn't be visible without scrolling...)

answered Nov 17, 2024 at 13:37

Jon Skeet

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2 Comments

Money Over a year ago

Command="{Binding Source={RelativeSource AncestorType={x:Type viewmodel:OverviewViewModel}}, Path=ChangeItemCommand, x:DataType=viewmodel:OverviewViewModel}"

is the answer. Man ..., it is so simple. I always put the data type outside the the quotes. That was wrong.

Money Over a year ago

And another hint. There is no code completion in XAML VS 2022 for the x:DataType in a Binding.

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