How to set ItemsPanelTemplate to a dynamically created Grid in code behind

Question

I've got this UserControl defined in XAML and would like to set the ItemsPanelTemplate dynamically in my code behind class (not in the XAML like in the example):

<UserControl>
    <ItemsControl x:Name="Items">
        <ItemsControl.ItemsPanel>
            <ItemsPanelTemplate>
                <Grid> <!-- I want to add this Grid definition in code behind -->
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition />
                    </Grid.ColumnDefinitions>
                    <Grid.RowDefinitions>
                        <RowDefinition />
                    </Grid.RowDefinitions>
                </Grid>
            </ItemsPanelTemplate>
       </ItemsControl.ItemsPanel>
    </ItemsControl>
</UserControl>

I tried something like

this.Items.ItemsPanel.Template = new Grid();

but failed miserably. Any help?

Background: I only know the number of grid columns and rows at runtime.

Solution 1

You need to create an ItemsPanelTemplate and set it's VisualTree to a FrameworkElementFactory (deprecated) which creates the Grid, or use the XamlReader to parse a XAML-string which specifies the template.

This question contains usage examples of both methods (albeit for a different template property).

An easier method to manipulate the panel at runtime is outlined in this question.

Solution 2

You can do as you want by creating MannualCode in code behind as: 1. Create a Method as following which will return a ItemsPanelTemplate

     private ItemsPanelTemplate GetItemsPanelTemplate()
    {
        string xaml = @"<ItemsPanelTemplate   xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation' xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
                            <Grid>
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition />
                                </Grid.ColumnDefinitions>
                                <Grid.RowDefinitions>
                                    <RowDefinition />
                                </Grid.RowDefinitions>
                            </Grid>
                    </ItemsPanelTemplate>";
        return XamlReader.Parse(xaml) as ItemsPanelTemplate;
    }

1. Now add this template in your Listbox ItemsPanel as:

      MyListBox.ItemsPanel = GetItemsPanelTemplate();
   

This is working fine for me. Hope this will help.

Keep Coding....:)

Solution 3

In case that you still have some work to do with the elements, you should take the following (extended) code:

First we need a helper in order to get the element:

// --------------------------------------------------------------------
// This function fetches the WrapPanel from oVisual.
private WrapPanel m_FetchWrapPanel (Visual oVisual)
{
  // WrapPanel to be returned
  WrapPanel oWrapPanel = null;
  // number of childs of oVisual
  int iNumberChilds = VisualTreeHelper.GetChildrenCount (oVisual);
  // and running through the childs
  int i = 0;
  while ( ( i < iNumberChilds ) && ( oWrapPanel == null ) )
  { // fetching visual
    Visual oVisualChild = 
      ( VisualTreeHelper.GetChild (oVisual, i) as Visual );
    if ( ( oVisualChild is WrapPanel ) is true )
    { // found
       oWrapPanel = ( oVisualChild as WrapPanel );
    }
    else
    { // checking the childs of oVisualChild 
      oWrapPanel = m_FetchWrapPanel (oVisualChild);
    };
    // checking next child
    i++;
  };
  // returning WrapPanel
  return (oWrapPanel);
}

Now we create the Panel (or something):

// --------------------------------------------------------------------
private void m_SettingTemplate ()
{
  // the online doc recommends to parse the template
  string xaml = 
    @"<ItemsPanelTemplate
          xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'
          xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
        <WrapPanel ItemWidth=""150"" MaxWidth=""150""/>
      </ItemsPanelTemplate>";
  // assigning the template
  oMyListView.ItemsPanel = ( System.Windows.Markup.XamlReader.Parse (xaml) as ItemsPanelTemplate );
  // fetching the WrapPanel
  WrapPanel oWrapPanel = m_WrapPanelAusVisualHolen (oMyListView);
  Debug.Assert (oWrapPanel != null);
  if ( oWrapPanel != null )
  { // adjusting the size of the WrapPanel to the ListView
    Binding oBinding = new Binding ("ActualWidth");
    oBinding.Source = oMyListView;
    oWrapPanel.SetBinding (WrapPanel.MaxWidthProperty, oBinding);
  };
}

Comments

[nabulke]

Ok, I have to admit I'm a WPF beginner and I totally don't understand your answer (my fault). I was hoping I could create the grid (new Grid) and just set the Template to the dynamically created Grid (like hinted at in my question). Could you perhaps show an example?

[H.B.]

@nabulke: Did you have a look at the linked question?

[nabulke]

Checking the examples right now. Thanks for adding. I hoped there would be an easier way to set the number of grid rows/columns at runtime though.

[H.B.]

Well, if you want to define it from scratch this is the way to do it, if you just want to edit it (you can add a Grid without definitions) this question here might be what you are looking for.

[nabulke]

The solution posted in the link you provided works great: I added a loaded event handler that adds the cols/rows to the grid, which is defined in XAML. Thanks for your help.