WPF & C#: Create an override for a datagrid within a user control?

Question

I have a large WPF application where I have a datagrid within a user control and I need to create an override for OnCreateAutomationPeer. I am having trouble doing that and the event never seems to fire. In my code-behind I have something similar to

public partial class DocChecklistView :  UserControl, IDataModuleView {     

        protected override System.Windows.Automation.Peers.AutomationPeer OnCreateAutomationPeer()
        {                
            return null;
        }

        public CDocumentChecklistView() {
            InitializeComponent();
        }
}

The XAML is pretty standard with code like

<UserControl>
 <Grid>
        <toolkit:DataGrid  ItemsSource="{Binding Source={StaticResource DocumentsVS}}" AutoGenerateColumns="False" CanUserAddRows="False" CanUserDeleteRows="False"
                FontSize="16" Name="_dgDocuments" Style="{StaticResource EklektosDataGridStyle}" . . . >
</UserControl>

And in the above, the toolkit:DataGrid is set to the namespace for the WPFToolkit. The DataGrid works as designed, I've just never done an override within a user control and the code I have above never fires - a breakpoint there is never reached.

Any thoughts?

BTW I'm trying to solve the problem presented in this question: http://stackoverflow.com/questions/4017786/wpf-recursive-call-to-automation-peer-api-is-not-valid — Unknown Coder, Oct 22 '12 at 15:51
So, what issue you are facing? Just that breakpoint is not hitting? — Rohit Vats, Oct 22 '12 at 16:58
Thats right - I dont think Ive implemented the override correctly — Unknown Coder, Oct 22 '12 at 17:18

score 7 · Accepted Answer · edited Feb 24 '13 at 18:12

You have override the method correctly. In case you want to override the OnCreateAutomationPeer of your dataGrid, you have to subclass the DataGrid-

public class MyDataGrid : DataGrid
{
    protected override AutomationPeer OnCreateAutomationPeer()
    {
        return null;
    }
}

And in xaml, use your custom dataGrid

<local:MyDataGrid x:Name="dataGrid"/>

And in constructor of your UserControl -

public CDocumentChecklistView()
{
    InitializeComponent();
    AutomationPeer a = UIElementAutomationPeer.CreatePeerForElement(dataGrid);
}

You need to ask for AutomationPeer to hit the breakpoint. Isn't it how you desired it?

This is what you missing - UIElementAutomationPeer.CreatePeerForElement(dataGrid);

score 2 · Answer 2 · edited Feb 24 '13 at 18:13

You need to override the OnCreateAutomationPeer method in a self defined DataGrid control, like so:

public partial class MyDataGrid : DataGrid
{
    protected override System.Windows.Automation.Peers.AutomationPeer OnCreateAutomationPeer()
    {
        return null;
    }
}

Then add it to your UserControl, like so (just drag and drop using Visual Studio after building the project):

<UserControl xmlns:YourApplicationNamespace="clr-namespace:YourApplicationNamespace"  x:Class="YourApplicationNamespace.DocChecklistView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <Grid Margin="0,1,0,-1">
        <YourApplicationNamespace:MyDataGrid x:Name="myDataGrid1" />  
    </Grid>
</UserControl>

You are specifying that you need to override the OnCreateAutomationPeer-method. This is only to be done if your application is making use of UIA (UI Automation), and also, if that part of the application is subject to UIA (from what I can understand). If not, then you don't even need to override the OnCreateAutomationPeer-method. However, if your application is restricted to UIA in all of its features then you need to make sure you've also implemented the following five steps (please read this documentation for more information on this) for your Custom Control:

Expose the control to UIA by overriding OnCreateAutomationPeer
Provide correct property values by overriding Core methods
Enable the client to interact with the control using methods
Implement Pattern providers
Raise events

If you want to get the automation peer object of a control (you can use this object to get information about a control’s characteristics and features and to simulate interactive use) you can either get it once it has been created using the FromElement-method and if not, you can use the CreatePeerForElement-method.

AutomationPeer automationPeer = UIElementAutomationPeer.CreatePeerForElement(checklistView.myDataGrid1);
            automationPeer = UIElementAutomationPeer.FromElement(checklistView.myDataGrid1);

Whatever UIA approach (testing code or building applications that provide accessibility features or other) your using, you need to make sure that the requirements have been made for the Custom Control to support UIA ánd that the AutomationPeer-object has been created so the Automation code can use it to get information about the control’s characteristics and features or to simulate interactive use.

score 1 · Answer 3 · edited Feb 24 '13 at 18:13

The Override seems correct, you just have to create an AutomationPeer in order to get a breakpoint to hit:

XAML:

<local:DocChecklistView x:Name="DocChecklistView" Initialized="DocChecklistView_Initialized"/>

In CodeBehind to XAML above:

private void DocChecklistView_Initialized(object sender, EventArgs e)
{
   var peer = UIElementAutomationPeer.CreatePeerForElement(DocChecklistView);
}

And if you want to override the OnCreateAutomationPeer for the toolkit:DataGrid inside your UserControl you must subclass the toolkit:DataGrid as a CustomControl which you use inside you UserControl

WPF & C#: Create an override for a datagrid within a user control?

3 Answers3