DataGrid Exception

Kevin Marois

I have a DataGrid that allows users to define CSV file columns for a UI I'm creating. It's bound to an ObservableCollection. I have CanUserAddRows = true. Apparantly setting this to True exposes some strange bug. The grid's first column has a button:

    <DataGrid Grid.Row="2"
              Grid.Column="0"
              ItemsSource="{Binding ColumnInfos}"
              HorizontalGridLinesBrush="LightGray"
              VerticalGridLinesBrush="LightGray"
              AutoGenerateColumns="False"
              CanUserAddRows="True"
              CanUserDeleteRows="True"
              CanUserReorderColumns="False"
              CanUserResizeColumns="False"
              CanUserSortColumns="False"
              BorderBrush="SteelBlue"
              BorderThickness="1"
              HeadersVisibility="Column"
              HorizontalScrollBarVisibility="Auto"
              VerticalScrollBarVisibility="Auto"
              VerticalAlignment="Stretch"
              Margin="5,0,5,5">

        <DataGrid.Resources>
            <local:BindingProxy x:Key="proxy" Data="{Binding IsColumnWidthVisibile}"/>
        </DataGrid.Resources>

        <DataGrid.Columns>

            <DataGridTemplateColumn>
                <DataGridTemplateColumn.CellTemplate>
                    <DataTemplate>
                        <Button Command="{Binding DeleteColumnCommand, RelativeSource={RelativeSource FindAncestor, 
                                                                        AncestorLevel=1, AncestorType={x:Type Window}}}"
                                CommandParameter="{Binding}"
                                Margin="2"
                                Height="18"
                                Width="18"/>
                    </DataTemplate>
                </DataGridTemplateColumn.CellTemplate>
            </DataGridTemplateColumn>

            <DataGridTextColumn Header="Column Name" 
                                Binding="{Binding ColumnName}"
                                Width="\*"/>

            <DataGridTextColumn Header="Width"
                                Binding="{Binding ColumnWidth}"
                                Width="65"
                                Visibility="{Binding Data, Conve

Richard Deeming

The problem is in your RelayCommand<T> class, which indiscriminately tries to cast the command parameter to the specified type. Your DeleteCommand is an instance of RelayCommand<ColumnInfoEntity>, so it will try to cast the parameter to the ColumnInfoEntity. But for the "new item" row, the parameter will be passed the CollectionView.NewItemPlaceholder[^] value, which will be an MS.Internal.NamedObject instance. Hence the cast will fail, and your code will crash. Change the RelayCommand<T> to test whether the parameter is the expected type:

private bool CanExecuteCore(T parameter)
{
return _canExecute is null || _canExecute(parameter);
}

public bool CanExecute(object parameter)
{
return parameter is T param && CanExecuteCore(param);
}

public void Execute(object parameter)
{
if (parameter is T param && CanExecuteCore(param))
{
_execute(param);
}
}

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

Kevin Marois

That did it. Thank you!

In theory, theory and practice are the same. But in practice, they never are.” If it's not broken, fix it until it is. Everything makes sense in someone's mind.

Kevin Marois

OK, so that works fine now. Now I added an event trigger to the grid:

When I run this, I get the exception

System.InvalidOperationException: 'Items collection must be empty before using ItemsSource.'

It's thrown in the class ItemCollection.cs. The ItemSource is set to an ObservableCollection which is loaded in the CTOR. This is why I hate the DataGrid. I commited this change. Any thoughts on this?

In theory, theory and practice are the same. But in practice, they never are.” If it's not broken, fix it until it is. Everything makes sense in someone's mind.

Richard Deeming

You've added the <i:EventTrigger> as a direct child of the grid. Looking at the documentation[^], it should be wrapped in an <i:Interation.Triggers> element instead.

<i:Interaction.Triggers>
    <i:EventTrigger EventName="AddingNewItem">
        <i:InvokeCommandAction Command="{Binding NewItemAddedCommand}" CommandParameter="{Binding}"/>
    </i:EventTrigger>
</i:Interaction.Triggers>

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

Kevin Marois

duh. Too many late nights Thanks

In theory, theory and practice are the same. But in practice, they never are.” If it's not broken, fix it until it is. Everything makes sense in someone's mind.