Problem with Silverlight DataBinding

Question

I have an Image control with its source bound to an object property (url string for the image). After calling the service, I update the data object with the new URL. An exception is thrown after it leaves my code, after calling the PropertyChanged event.

The data structure and service logic are executed in a core DLL that does not know the user interface. How can I synchronize with the UI thread when I cannot access the dispatcher?

PS: Accessing Application.Current.RootVisual to access the dispatcher is not a solution because the root visual is in a different thread (resulting in an exception that needs to be excluded).

PPS: this is only an image management issue, snapping to any other ui element, the cross-flow issue is handled for you.

Answer 1

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

Also look here.

Answer 2

Have you tried implementing INotifyPropertyChanged ?

Answer 3

The getter property for RootVisual in the Application class has a thread check that throws this exception. I got around this by storing the root visual 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 in App.RootVisualDispatcher and not Application.Current.RootVisual.Dispatcher, you should not get this exception.

Answer 4

I ran into a similar problem, but it was in the form of windows:

I have a class that has its own thread that updates statistics about another process, there is a binding to this object in my user interface. I ran into cross-calling issues, here's how I solved 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 just fine, if anyone has any suggestions please let me know.

Answer 5

When we want to update UI-related elements that an action should happen in the UI thread, 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 6

The INotifyPropertyChanged interface INotifyPropertyChanged used to notify clients, usually associating clients, with a property value change.

For example, consider a Person object with the FirstName property. To provide a general notification of property changes, the Person type implements the INotifyPropertyChanged interface and raises the PropertyChanged event when FirstName changes.

To notify of a change in the binding between the associated client and the data source, your associated type must either:

Implement the INotifyPropertyChanged interface ( INotifyPropertyChanged ).

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

Do not use 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 = "[phone]"; } } // 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 = "[phone]"; } // 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(); } } } } }