Silverlight DataBinding cross thread issue

Question

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.

Answer 1

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.

Example:

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();
                }
            }
        }
    }
}

Answer 2

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

}

Answer 3

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

Also look here.

Answer 4

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.

Answer 5

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.

Answer 6

Have you tried implementing INotifyPropertyChanged?