Why the column is not resize with the multi value converter?

Question

I want to use a multi value converter to set the dinamically the width of a column.

For testing purposes, the multivalue converter just set 20 as width, a very low value to see if the value is used.

public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
//I have tried both ways, DataGridLength and return 20
    //return new DataGridLength(20, DataGridLengthUnitType.Pixel);
    return 20;
}

In this code I tried to use return directly 20 and also I have tried to return a DataGridLength, in both cases, it doesn't work.

I have tried a simple xaml code that I set the width directly (no using the converter), in the DataGridTextColumn.ElementStyle, in this way.

                <DataGridTextColumn.ElementStyle>
                    <Style TargetType="{x:Type TextBlock}">
                        <Setter Property="Width" Value="20"/>
                    </Style>
                </DataGridTextColumn.ElementStyle>
            </DataGridTextColumn>

In this case, it works, if I set the width of the textblock in the elemet style.

So next step I have tried using the converter, in this way:

            <DataGridTextColumn.ElementStyle>
                <Style TargetType="{x:Type TextBlock}">
                    <Setter Property="Width">
                        <Setter.Value>
                            <MultiBinding Converter="{StaticResource MyMultiValueConverter}" >
                                <Binding Source="{x:Reference ProxyElement}" Path="DataContext.Property1" />
                                <Binding Source="{x:Reference ProxyElement}" Path="DataContext.Property2" />
                            </MultiBinding>
                        </Setter.Value>
                    </Setter>
                </Style>
            </DataGridTextColumn.ElementStyle>
        </DataGridTextColumn>

The converter is rised, but the value 20 that is returned by the converter it doesn't used, so the column is not resized.

How could I use a multivalue converter to set the width of a column?

Thanks.

Answer 1

Hi,
I think, your ProxyElement doesn't detect PropertyChanged. My demo works fine, try it.

XAML:

<Window x:Class="WpfApp1.Window074"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp074"
        mc:Ignorable="d"
        Title="Window074" Height="450" Width="800">
  <Window.Resources>
    <local:ViewModel x:Key="vm"/>
    <local:MyMultiValueConverter x:Key="MyMultiValueConverter"/>
  </Window.Resources>
  <StackPanel DataContext="{StaticResource vm}">
    <TextBox Text="{Binding WidthValue}"/>
    <CheckBox IsChecked="{Binding WidthDefault}" Content="Default Width"/>
    <DataGrid ItemsSource="{Binding View}" AutoGenerateColumns="False">
      <DataGrid.Columns>
        <DataGridTextColumn Header="Col1" Binding="{Binding SomeString}">
          <DataGridTextColumn.ElementStyle>
            <Style TargetType="{x:Type TextBlock}">
              <Setter Property="Width">
                <Setter.Value>
                  <MultiBinding Converter="{StaticResource MyMultiValueConverter}">
                    <Binding Path="WidthValue" Source="{StaticResource vm}"/>
                    <Binding Path="WidthDefault" Source="{StaticResource vm}"/>
                  </MultiBinding>
                </Setter.Value>
              </Setter>
            </Style>
          </DataGridTextColumn.ElementStyle>
        </DataGridTextColumn>
      </DataGrid.Columns>
    </DataGrid>
  </StackPanel>
</Window>

Code:

using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Globalization;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Data;

namespace WpfApp074
{
  public class ViewModel : INotifyPropertyChanged
  {
    public ViewModel()
    {
      ObservableCollection<Data> col = new ObservableCollection<Data>();
      for (int i = 1; i < 10; i++) col.Add(new Data() { SomeString = $"Row {i}" });
      cvs.Source = col;
    }
    private CollectionViewSource cvs = new CollectionViewSource();
    public ICollectionView View { get => cvs.View; }

    private double _widthValue = 20;
    public double WidthValue
    {
      get => this._widthValue;
      set { this._widthValue = value; OnPorpertyChanged(); }
    }

    private bool _widthDefault = false;
    public bool WidthDefault
    {
      get => this._widthDefault;
      set { this._widthDefault = value; OnPorpertyChanged(); }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    internal void OnPorpertyChanged([CallerMemberName] string propName = "") => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));

  }

  public class Data
  {
    public string SomeString { get; set; }
  }

  public class MyMultiValueConverter : IMultiValueConverter
  {
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
      double w = 100;
      if (!(bool)values[1]) double.TryParse(values[0].ToString(), out w);
      return w;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
      throw new NotImplementedException();
    }
  }    
}

Answer 2

Thanks for your example, it is good to know another way to do it.

ABout the proxy, it works, because when I debug and set a breakpoint in the converter, the 2 properties have the value that I expect. So it is true that I seems I have a problem with the converter, but I think that is not because of the proxy.

Also, the converter is rised and in the test I return just the value 20, so it desn't use the parameters of the multibinding.

NOTE: when I try to use this:

<local:ViewModel x:Key="vm"/>

I get an error that says it doesn't exist in the namespace local.

I am using a usercontrol, not a windows, could it be this the reason of the error?

NOTE: I have created a simple project to recreate the problem and it is true that to bindings doesn't make to fire the converter. I will try your solution to see.

Thanks so much.