Prism DelegateCommand's CanExecute getting null parameter after RaiseCanExecuteChanged

Question

I'm trying to write a ListView, where for each item there is a button bound to a DelegateCommand. I want this command's CanExecute to be based on a boolean property of an item. ItemSource is an ObservableCollection and I'm populating it using an async method. The problem is that when RaiseCanExecuteChanged() is fired, CanExecute receives null as parameter.After that, when I use another UI bound command to RaiseCanExecuteChanged(), the method behaves properly.

Here's part of the view:

<ListView x:Name="root" ItemsSource="{Binding olVersions}" IsSynchronizedWithCurrentItem="true" HorizontalContentAlignment="Stretch">
    <ListView.ItemTemplate>
        <DataTemplate>
            <Grid>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="*" />
                    <ColumnDefinition Width="Auto" />
                    <ColumnDefinition Width="Auto" />
                </Grid.ColumnDefinitions>

                <TextBlock Text="{Binding Path=sFullName}" Grid.Column="0" VerticalAlignment="Center"></TextBlock>
                <Button Grid.Column="1" Content="Run" CommandParameter="{Binding}" Command="{Binding ElementName=root, Path=DataContext.cRunVersionCommand}"></Button>
                <Button Grid.Column="2" Content="Download" CommandParameter="{Binding}" Command="{Binding ElementName=root, Path=DataContext.cDownloadVersionCommand}"></Button>
            </Grid>
        </DataTemplate>
    </ListView.ItemTemplate>
</ListView>

Here's the ViewModel:

public class VersionsTabViewModel : BindableBase
{

    public ObservableCollection<CVersion> olVersions { get; private set; }

    public DelegateCommand<CVersion> cRunVersionCommand { get; private set; }
    public DelegateCommand<CVersion> cDownloadVersionCommand { get; private set; }

    private IVersionManager c_version_manager;

    public VersionsTabViewModel(IVersionManager cVersionManager)
    {
        olVersions = new ObservableCollection<CVersion>();
        cRunVersionCommand = new DelegateCommand<CVersion>(v_try_run_version, b_can_run);
        cDownloadVersionCommand = new DelegateCommand<CVersion>(v_try_download_version);
        c_version_manager = cVersionManager;
        v_read_available_versions();
    }

    private async void v_read_available_versions()
    {
        List<CVersion> l_versions = await c_version_manager.lGetVersions();
        List<CVersion> l_versions_sorted = l_versions.OrderByDescending(cVersion => cVersion.cSemanticVersion).ToList();
        olVersions.Clear();
        olVersions.AddRange(l_versions_sorted);

        cRunVersionCommand.RaiseCanExecuteChanged();
    }

    private void v_try_run_version(CVersion cVersionToRun)
    {
        MessageBox.Show($"Run {cVersionToRun.sFullName}", "Run"); //TODO
    }

    private void v_try_download_version(CVersion cVersionToDownload)
    {
        MessageBox.Show($"Download {cVersionToDownload.sFullName}", "Download"); //TODO
        cRunVersionCommand.RaiseCanExecuteChanged();
    }

    private bool b_can_run(CVersion cVersion)
    {
        return cVersion?.bIsInstalled ?? false;
    }
}

score 1 · Accepted Answer · answered Jul 15 '20 at 16:13

It seems that this is a general issue with command bindings, there is a related question, here. In order to make it work in your case, you have to change your ElementName binding to root for the Command property from

Command="{Binding ElementName=root, Path=DataContext.cRunVersionCommand}"

to a relative source binding to the parent ListView control like this.

Command="{Binding RelativeSource={RelativeSource AncestorType={x:Type ListView}}, Path=DataContext.cRunVersionCommand}"

Thank you very much. It solved it. I've seen the answer you linked earlier today, but it somehow didn't click for me then. — Szymon Woźniak, Jul 15 '20 at 16:24

Prism DelegateCommand's CanExecute getting null parameter after RaiseCanExecuteChanged

1 Answers1