Error: 'Invalid cross-thread access' while using the Silverlight Client Object Model

Question

Hi,

I'm trying to fill a datagrid with data from a list but I keep getting error:  'Invalid cross-thread access'. 

Steps:

1. Get ID from a item selected in a ComboBox from list (Solution Frameworks)
2. Use the ID field above to find items with this ID in another list (Solution Framework_Mapping)
3. Bind the returned listitems from Step 2 to a datagrid

Here's what my code looks like:

private void OfferingComboBox_SelectionChanged(object sender, System.Windows.Controls.SelectionChangedEventArgs e)
 {
  object strSolutionFrameworkSelected;
  ListItem offerings = (ListItem)OfferingComboBox.SelectedItem;
  strSolutionFrameworkSelected = offerings["Offering_x0020_Name"];
  var context = new ClientContext(ApplicationContext.Current.Url);
  context.Load(context.Web);
  List lst = context.Web.Lists.GetByTitle("Solution Frameworks");
  context.Load(lst);
  var query = new Microsoft.SharePoint.Client.CamlQuery();
  var strQuery = "<View><Query><Where><Eq><FieldRef Name='OID_x003a_Offering_x0020_Name' /><Value Type='Lookup'>" + strSolutionFrameworkSelected + "</Value></Eq></Where></Query></View>";
  query.ViewXml = strQuery;
  _projectItems = lst.GetItems(query);
  context.Load(_projectItems);
  context.ExecuteQueryAsync(OnSucceededListenerGetOfferingDetails, null);
 }

 private void OnSucceededListenerGetOfferingDetails(object sender, ClientRequestEventArgs args)
 {
  sfID = _projectItems[0].Id;
  BindData();
 }

 private void BindData()
 {
  var context = new ClientContext(ApplicationContext.Current.Url);
  context.Load(context.Web);
  List lst = context.Web.Lists.GetByTitle("Solution Framework_Mapping");
  context.Load(lst);
  var query = new Microsoft.SharePoint.Client.CamlQuery();
  var strQuery = "<View><Query><Where><Eq><FieldRef Name='SFID' /><Value Type='Lookup'>" + sfID + "</Value></Eq></Where></Query></View>";
  query.ViewXml = strQuery;
  _projectItems = lst.GetItems(query);
  context.Load(_projectItems);
  context.ExecuteQueryAsync(OnSucceededListenerGetOfferings2, null); 
 }

 private void OnSucceededListenerGetOfferings2(object sender, ClientRequestEventArgs args)
 {
  var list = new List<Project>();
  foreach (var li in _projectItems)
  {
  list.Add(new Project
  {
   OID = li["ID"].ToString()
  });
  }
  dataGrid1.ItemsSource = list;
 }

After the code steps over:

context.ExecuteQueryAsync(OnSucceededListenerGetOfferingDetails, null);

it then jumps to:

dataGrid1.ItemsSource = list;

throwing the error.  Could point me to a reference or example where I can learn about the cross-thread access error?

Thank you!

Answer 1

You need to use the Dispatcher to get to the UI thread.

Try this: ( note I did not run this so you might need to play with the syntax)

 private void OnSucceededListenerGetOfferings2(object sender, ClientRequestEventArgs args)
{

Deployment.Current.Dispatcher.BeginInvoke(UpdateList);

}

private void UpdateList()

{

private void UpdateList
 var list = new List<Project>();
  foreach (var li in _projectItems)
  {
  list.Add(new Project
  {
   OID = li["ID"].ToString()
  });
  }
  dataGrid1.ItemsSource = list;
}

---

Darrin Bishop Darrin Bishop Group, Inc.

Answer 2

Darrin,

That was right on!  I will have to do some reading about the 'Dispatcher'.

Appreciate your help!

-Andres