I’m facing an issue in my WPF application regarding synchronizing the column widths of a DataGrid and its row details. While I’ve managed to achieve one-to-one adaptive alignment of column widths, I also need to combine the widths of multiple columns.
I’m using a multi-value converter to calculate the total width of the DataGrid columns and applying it to Row Details.

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my first table:

<UserControl x:Class="TableUserControl">
    <DataGrid x:Name="DTGrid" ItemsSource="{Binding ItemsViewCollection, UpdateSourceTrigger=PropertyChanged}">
        <DataGrid.Columns>
            <DataGridTextColumn Header="Col1" Binding="{Binding Date}" x:Name="DateElement"/>
            <DataGridTextColumn Header="Col2" Binding="{Binding Id}" x:Name="IdElement"/>
            <DataGridTextColumn Header="Col3" Binding="{Binding Info}" x:Name="InfoElement"/>
        </DataGrid.Columns>
        <DataGrid.RowDetailsTemplate>
            <DataTemplate>
                <DockPanel>
                    <uc:DetailsTableUserControl ItemsSource="{Binding EditHistoryStore}" />
                </DockPanel>
            </DataTemplate>
        </DataGrid.RowDetailsTemplate>
    </DataGrid> 
</UserControl>

my second table:

 
<UserControl x:Class="DetailsTableUserControl">
    <UserControl.Resources> 
        <converters:WidthConverter x:Key="WidthConverter"/>
        <DiscreteObjectKeyFrame x:Key="DateElementColProxy" Value="{Binding ElementName=DateElement}"/>     
        <DiscreteObjectKeyFrame x:Key="IdElementColProxy" Value="{Binding ElementName=IdElement}"/>      
        <DiscreteObjectKeyFrame x:Key="InfoElementColProxy" Value="{Binding ElementName=InfoElement}"/>
    </UserControl.Resources>

    <DataGrid x:Name="DetailsTable" ItemsSource="{Binding ItemsSource,RelativeSource={RelativeSource AncestorType=UserControl}}">
        <DataGrid.Columns>
            <DataGridTextColumn Header="Col1" Binding="{Binding IdDetails}" x:Name="IdDetailsElement" 
                Width="{Binding Path=Value.Width, Mode=TwoWay, Source={StaticResource IdElementColProxy}}"/>

                <DataGridTextColumn Header="Col2_3" Binding="{Binding InfoDetails}" x:Name="AboutElement">
                    <DataGridTextColumn.Width>  
                        <MultiBinding Converter="{StaticResource WidthConverter}">
                            <Binding Source="{StaticResource DateElementColProxy}" Path="Value.Width" Mode="TwoWay"/>
                            <Binding Source="{StaticResource InfoElementColProxy}" Path="Value.Width" Mode="TwoWay"/>
                        </MultiBinding> 
                    </DataGridTextColumn.Width> 
                </DataGridTextColumn> 

            </DataGrid.Columns> 
        </DataGrid> 
    </UserControl>

my converter with eror :
InvalidCastException: Unable to cast object of type ‘MS.Internal.NamedObject’ to type ‘System.Double’.

 public class WidthConverter : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            double totalWidth = 0;
            foreach (double Width in values)
            {
                if (Width is double)
                    totalWidth += Width;
            }
            return new DataGridLength(totalWidth);
        }
        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
        {
            return null;
        }
    }

How can I modify the converter so that it correctly calculates the size of the multi columns?