Using bindings to control column order in a DataGrid

Question

Problem

I have a WPF Toolkit DataGrid, and I'd like to be able to switch among several preset column orders. This is an MVVM project, so the column orders are stored in a ViewModel. The problem is, I can't get bindings to work for the DisplayIndex property. No matter what I try, including the sweet method in this Josh Smith tutorial, I get:

The DisplayIndex for the DataGridColumn with Header 'ID' is out of range. DisplayIndex must be greater than or equal to 0 and less than Columns.Count. Parameter name: displayIndex. Actual value was -1.

Is there any workaround for this?

I'm including my test code below. Please let me know if you see any problems with it.

---

ViewModel code

public class MainViewModel
{
    public List<Plan> Plans { get; set; }
    public int IdDisplayIndex { get; set; }
    public int NameDisplayIndex { get; set; }
    public int DescriptionDisplayIndex { get; set; }

    public MainViewModel()
    {
        Initialize();
    }

    private void Initialize()
    {
        IdDisplayIndex = 1;
        NameDisplayIndex = 2;
        DescriptionDisplayIndex = 0;
        Plans = new List<Plan>
        {
            new Plan { Id = 1, Name = "Primary", Description = "Likely to work." },
            new Plan { Id = 2, Name = "Plan B", Description = "Backup plan." },
            new Plan { Id = 3, Name = "Plan C", Description = "Last resort." }
        };
    }
}

---

Plan Class

public class Plan
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
}

---

Window code - this uses Josh Smith's DataContextSpy

<Window
    x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:WpfApplication1"
    xmlns:mwc="http://schemas.microsoft.com/wpf/2008/toolkit"
    Title="Main Window" Height="300" Width="300">
    <Grid>
        <mwc:DataGrid ItemsSource="{Binding Plans}" AutoGenerateColumns="False">
            <mwc:DataGrid.Resources>
                <local:DataContextSpy x:Key="spy" />
            </mwc:DataGrid.Resources>
            <mwc:DataGrid.Columns>
                <mwc:DataGridTextColumn
                    Header="ID" 
                    Binding="{Binding Id}"
                    DisplayIndex="{Binding Source={StaticResource spy}, Path=DataContext.IdDisplayIndex}" />
                <mwc:DataGridTextColumn
                    Header="Name"
                    Binding="{Binding Name}"
                    DisplayIndex="{Binding Source={StaticResource spy}, Path=DataContext.NameDisplayIndex}" />
                <mwc:DataGridTextColumn
                    Header="Description"
                    Binding="{Binding Description}"
                    DisplayIndex="{Binding Source={StaticResource spy}, Path=DataContext.DescriptionDisplayIndex}" />
            </mwc:DataGrid.Columns>
        </mwc:DataGrid>
    </Grid>
</Window>

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Note: If I just use plain numbers for DisplayIndex, everything works fine, so the problem is definitely with the bindings.

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Update 5/1/2010

I was just doing a little maintenance on my project, and I noticed that when I ran it, the problem I discuss in this post had returned. I knew that it worked last time I ran it, so I eventually narrowed the problem down to the fact that I had installed a newer version of the WPF Toolkit (Feb '10). When I reverted to the June '09 version, everything worked fine again. So, I'm now doing something I should have done in the first place: I'm including the WPFToolkit.dll that works in my solution folder and checking it into version control. It's unfortunate, though, that the newer toolkit has a breaking change.

Answer 1

Set FallbackValue=<idx> in your DisplayIndex binding, where <idx> is your column's index. For example:

DisplayIndex="{Binding Source={StaticResource spy}, Path=DataContext.IdDisplayIndex, FallbackValue=0}" />