Tutorial :Silverlight DataBinding cross thread issue


I have an Image control with it's source bound to a property on an object(string url to an image). After making a service call, i update the data object with a new URL. The exception is thrown after it leaves my code, after invoking the PropertyChanged event.

The data structure and the service logic are all done in a core dll that has no knowledge of the UI. How do I sync up with the UI thread when i cant access a Dispatcher?

PS: Accessing Application.Current.RootVisual in order to get at a Dispatcher is not a solution because the root visual is on a different thread(causing the exact exception i need to prevent).

PPS: This only is a problem with the image control, binding to any other ui element, the cross thread issue is handled for you.


System.Windows.Deployment.Current.Dispatcher.BeginInvoke(() => {...});  

Also look here.


Have you tried implementing INotifyPropertyChanged?


The property getter for RootVisual on the Application class has a thread check which causes that exception. I got around this by storing the root visual's dispatcher in my own property in my App.xaml.cs:

public static Dispatcher RootVisualDispatcher { get; set; }    private void Application_Startup(object sender, StartupEventArgs e)  {      this.RootVisual = new Page();      RootVisualDispatcher = RootVisual.Dispatcher;  }  

If you then call BeginInvoke on App.RootVisualDispatcher rather than Application.Current.RootVisual.Dispatcher you shouldn't get this exception.


I ran into a similar issue to this, but this was in windows forms:

I have a class that has it's own thread, updating statistics about another process, there is a control in my UI that is databound to this object. I was running into cross-thread call issues, here is how I resolved it:

Form m_MainWindow; //Reference to the main window of my application  protected virtual void OnPropertyChanged(string propertyName)  {    if(PropertyChanged != null)      if(m_MainWindow.InvokeRequired)        m_MainWindow.Invoke(          PropertyChanged, this, new PropertyChangedEventArgs(propertyName);      else        PropertyChanged(this, new PropertyChangedEventArgs(propertyName);  }  

This seems to work great, if anyone has suggestions, please let me know.


When ever we want to update UI related items that action should happen in the UI thread else you will get an invalid cross thread access exception

Deployment.Current.Dispatcher.BeginInvoke( () =>    {    UpdateUI(); // DO the actions in the function Update UI    });    public void UpdateUI()    {    //to do :Update UI elements here    }  


The INotifyPropertyChanged interface is used to notify clients, typically binding clients, that a property value has changed.

For example, consider a Person object with a property called FirstName. To provide generic property-change notification, the Person type implements the INotifyPropertyChanged interface and raises a PropertyChanged event when FirstName is changed.

For change notification to occur in a binding between a bound client and a data source, your bound type should either:

Implement the INotifyPropertyChanged interface (preferred).

Provide a change event for each property of the bound type.

Do not do both.


using System;  using System.Collections.Generic;  using System.ComponentModel;  using System.Drawing;  using System.Runtime.CompilerServices;  using System.Windows.Forms;    // Change the namespace to the project name.   namespace TestNotifyPropertyChangedCS  {      // This form demonstrates using a BindingSource to bind       // a list to a DataGridView control. The list does not       // raise change notifications. However the DemoCustomer type        // in the list does.       public partial class Form1 : Form      {          // This button causes the value of a list element to be changed.           private Button changeItemBtn = new Button();            // This DataGridView control displays the contents of the list.           private DataGridView customersDataGridView = new DataGridView();            // This BindingSource binds the list to the DataGridView control.           private BindingSource customersBindingSource = new BindingSource();            public Form1()          {              InitializeComponent();                // Set up the "Change Item" button.              this.changeItemBtn.Text = "Change Item";              this.changeItemBtn.Dock = DockStyle.Bottom;              this.changeItemBtn.Click +=                  new EventHandler(changeItemBtn_Click);              this.Controls.Add(this.changeItemBtn);                // Set up the DataGridView.              customersDataGridView.Dock = DockStyle.Top;              this.Controls.Add(customersDataGridView);                this.Size = new Size(400, 200);          }            private void Form1_Load(object sender, EventArgs e)          {              // Create and populate the list of DemoCustomer objects               // which will supply data to the DataGridView.              BindingList<DemoCustomer> customerList = new BindingList<DemoCustomer>();              customerList.Add(DemoCustomer.CreateNewCustomer());              customerList.Add(DemoCustomer.CreateNewCustomer());              customerList.Add(DemoCustomer.CreateNewCustomer());                // Bind the list to the BindingSource.               this.customersBindingSource.DataSource = customerList;                // Attach the BindingSource to the DataGridView.               this.customersDataGridView.DataSource =                  this.customersBindingSource;            }            // Change the value of the CompanyName property for the first            // item in the list when the "Change Item" button is clicked.          void changeItemBtn_Click(object sender, EventArgs e)          {              // Get a reference to the list from the BindingSource.              BindingList<DemoCustomer> customerList =                  this.customersBindingSource.DataSource as BindingList<DemoCustomer>;                // Change the value of the CompanyName property for the                // first item in the list.              customerList[0].CustomerName = "Tailspin Toys";              customerList[0].PhoneNumber = "(708)555-0150";          }        }        // This is a simple customer class that        // implements the IPropertyChange interface.       public class DemoCustomer : INotifyPropertyChanged      {          // These fields hold the values for the public properties.           private Guid idValue = Guid.NewGuid();          private string customerNameValue = String.Empty;          private string phoneNumberValue = String.Empty;            public event PropertyChangedEventHandler PropertyChanged;            // This method is called by the Set accessor of each property.           // The CallerMemberName attribute that is applied to the optional propertyName           // parameter causes the property name of the caller to be substituted as an argument.           private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")          {              if (PropertyChanged != null)              {                  PropertyChanged(this, new PropertyChangedEventArgs(propertyName));              }          }            // The constructor is private to enforce the factory pattern.           private DemoCustomer()          {              customerNameValue = "Customer";              phoneNumberValue = "(312)555-0100";          }            // This is the public factory method.           public static DemoCustomer CreateNewCustomer()          {              return new DemoCustomer();          }            // This property represents an ID, suitable           // for use as a primary key in a database.           public Guid ID          {              get              {                  return this.idValue;              }          }            public string CustomerName          {              get              {                  return this.customerNameValue;              }                set              {                  if (value != this.customerNameValue)                  {                      this.customerNameValue = value;                      NotifyPropertyChanged();                  }              }          }            public string PhoneNumber          {              get              {                  return this.phoneNumberValue;              }                set              {                  if (value != this.phoneNumberValue)                  {                      this.phoneNumberValue = value;                      NotifyPropertyChanged();                  }              }          }      }  }  

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