Grid.children.clear thread error xmpp wpf - wpf

Question

I am calling a method that has a Grid.Children.Clear() functionality. When calling it from different methods it works well. But when I call my the method from an xmpp_onmessage method. I am experiencing an error. “The calling thread cannot access this object because a different thread owns it.”
Here is the method that containts Grid.Children.Clear()the :
private void ConstructChatView(Boolean isChat)
{
System.Uri resourceUri = new System.Uri("Public/Images/chat_green-textarea.png", UriKind.Relative);
StreamResourceInfo streamInfo = Application.GetResourceStream(resourceUri);
System.Uri resourceUri2 = new System.Uri("Public/Images/chat_green-textarea-tail.png", UriKind.Relative);
StreamResourceInfo streamInfo2 = Application.GetResourceStream(resourceUri2);
System.Uri resourceUri3 = new System.Uri("Public/Images/chat_blue-textarea.png", UriKind.Relative);
StreamResourceInfo streamInfo3 = Application.GetResourceStream(resourceUri3);
System.Uri resourceUri4 = new System.Uri("Public/Images/chat_blue-textarea-tail.png", UriKind.Relative);
StreamResourceInfo streamInfo4 = Application.GetResourceStream(resourceUri4);
BitmapFrame temp = BitmapFrame.Create(streamInfo.Stream);
var brush = new ImageBrush();
brush.ImageSource = temp;
BitmapFrame temp2 = BitmapFrame.Create(streamInfo2.Stream);
BitmapFrame temp3 = BitmapFrame.Create(streamInfo3.Stream);
var brush2 = new ImageBrush();
brush2.ImageSource = temp3;
BitmapFrame temp4 = BitmapFrame.Create(streamInfo4.Stream);
int ctr = 0;
chatGrid.Children.Clear();
if (isChat == true)
{
for (int i = 0; i < _messageView.Count; i++)
{
if ((!_messageView.ElementAt(i).Message.ToString().Trim().Equals("")))
{
RowDefinition chatGridRow1 = new RowDefinition();
RowDefinition chatGridRow2 = new RowDefinition();
RowDefinition chatGridRow3 = new RowDefinition();
chatGrid.RowDefinitions.Add(chatGridRow1);
chatGrid.RowDefinitions.Add(chatGridRow2);
chatGrid.RowDefinitions.Add(chatGridRow3);
if (_messageView.ElementAt(i).IsMe == true)
{
TextBlock Message = new TextBlock();
Message.Foreground = Brushes.White;
Message.Padding = new Thickness(10, 10, 10, 10);
Message.HorizontalAlignment = HorizontalAlignment.Right;
Message.Margin = new Thickness(0, 0, 5, 0);
Message.Background = brush2;
Message.TextWrapping = TextWrapping.Wrap;
Message.Text = _messageView.ElementAt(i).Message;
Grid.SetRow(Message, ctr);
Grid.SetColumn(Message, 0);
ctr++;
Image Bluetail = new Image();
Bluetail.Source = temp4;
Bluetail.HorizontalAlignment = HorizontalAlignment.Right;
Bluetail.Height = 10;
Bluetail.Width = 20;
Bluetail.Margin = new Thickness(0, -(0.7), 10, 0);
Grid.SetRow(Bluetail, ctr);
ctr++;
Label Sender = new Label();
Sender.Foreground = Brushes.White;
Sender.Margin = new Thickness(0, 0, 0, 10);
Sender.HorizontalAlignment = HorizontalAlignment.Right;
Sender.Content = "Sent By : " + _messageView.ElementAt(i).Name.ToString() + " " + _messageView.ElementAt(i).DateCreated.ToString();
Grid.SetRow(Sender, ctr);
Grid.SetColumn(Sender, 0);
ctr++;
chatGrid.Children.Add(Message);
chatGrid.Children.Add(Bluetail);
chatGrid.Children.Add(Sender);
}
else
{
TextBlock Message = new TextBlock();
Message.Foreground = Brushes.White;
Message.Padding = new Thickness(10, 10, 10, 10);
Message.HorizontalAlignment = HorizontalAlignment.Left;
Message.Margin = new Thickness(5, 0, 0, 0);
Message.Background = brush;
Message.TextWrapping = TextWrapping.Wrap;
Message.Text = _messageView.ElementAt(i).Message;
Grid.SetRow(Message, ctr);
Grid.SetColumn(Message, 0);
ctr++;
Image Greentail = new Image();
Greentail.Source = temp2;
Greentail.HorizontalAlignment = HorizontalAlignment.Left;
Greentail.Height = 10;
Greentail.Width = 20;
Greentail.Margin = new Thickness(10, -(0.7), 5, 0);
Grid.SetRow(Greentail, ctr);
ctr++;
Label Sender = new Label();
Sender.Foreground = Brushes.White;
Sender.Margin = new Thickness(0, 0, 0, 10);
Sender.HorizontalAlignment = HorizontalAlignment.Left;
Sender.Content = "Sent By : " + _messageView.ElementAt(i).Name.ToString() + " " + _messageView.ElementAt(i).DateCreated.ToString();
Grid.SetRow(Sender, ctr);
Grid.SetColumn(Sender, 0);
ctr++;
chatGrid.Children.Add(Message);
chatGrid.Children.Add(Greentail);
chatGrid.Children.Add(Sender);
}
}
}
}
else
{
for (int i = 0; i < _messageView.Count; i++)
{
if (_messageView.ElementAt(i).IsMe == true && (!_messageView.ElementAt(i).Message.ToString().Trim().Equals("")))
{
RowDefinition chatGridRow1 = new RowDefinition();
RowDefinition chatGridRow2 = new RowDefinition();
RowDefinition chatGridRow3 = new RowDefinition();
chatGrid.RowDefinitions.Add(chatGridRow1);
chatGrid.RowDefinitions.Add(chatGridRow2);
chatGrid.RowDefinitions.Add(chatGridRow3);
TextBlock Message = new TextBlock();
Message.Foreground = Brushes.White;
Message.Margin = new Thickness(0, 10, 300, 0);
Message.Padding = new Thickness(10, 10, 10, 10);
Message.HorizontalAlignment = HorizontalAlignment.Left;
Message.Background = brush;
Message.TextWrapping = TextWrapping.Wrap;
Message.Text = _messageView.ElementAt(i).Message;
Grid.SetRow(Message, ctr);
Grid.SetColumn(Message, 0);
ctr++;
Image Greentail = new Image();
Greentail.Source = temp2;
Greentail.HorizontalAlignment = HorizontalAlignment.Left;
Greentail.Height = 10;
Greentail.Width = 20;
Greentail.Margin = new Thickness(5, -(0.7), 0, 0);
Grid.SetRow(Greentail, ctr);
ctr++;
Label Sender = new Label();
Sender.Foreground = Brushes.White;
Sender.Margin = new Thickness(0, 0, 0, 10);
Sender.Content = "Sent By : " + _messageView.ElementAt(i).Name.ToString() + " " + _messageView.ElementAt(i).DateCreated.ToString();
Grid.SetRow(Sender, ctr);
Grid.SetColumn(Sender, 0);
ctr++;
chatGrid.Children.Add(Message);
chatGrid.Children.Add(Greentail);
chatGrid.Children.Add(Sender);
}
}
}
//for (int i = 0; i < _messageView.Count; i++)
//{
// if (_messageView.ElementAt(i).IsMe == true && (!_messageView.ElementAt(i).Message.ToString().Trim().Equals("")))
// {
// }
//}
ctr = 0;
scrollView.ScrollToEnd();
}
Any ideas? thanks

Answer 1

Most UI elements may only be modified in the UI thread. As your event handler is apparently invoked in a different thread, you have to use the Dispatcher to invoke your code in the UI thread.
private void ConstructChatView(Boolean isChat)
{
Dispatcher.Invoke((Action)(() => chatGrid.Children.Clear()));
}
EDIT: You may also pass more code to the Invoke call by using an anonymous method:
private void ConstructChatView(Boolean isChat)
{
Dispatcher.Invoke((Action)(() =>
{
// more code here
}));
}
Of course you may also put a bunch of code in another method and pass that to the Invoke call:
private void ConstructChatView(Boolean isChat)
{
Dispatcher.Invoke((Action)(() => ConstructChatViewInUI(isChat)));
}
private void ConstructChatViewInUI(Boolean isChat)
{
...
}