Textbox doesn't keep selection on binded list update

Question

In my ViewModel i've got an ObservableCollection of string that contains all messages received from ethernet:

public ObservableCollection<string> Messages { get; set; }

And i binded it to a texbox in the view with a converter:

<TextBox Text="{Binding Messages, Converter={StaticResource ListToStringConverter}}" HorizontalAlignment="Center"/>

my converter is a simple

string finalStr;
foreach(var v in Messages) 
{ 
    finalStr += v + "\n";
}
return finalStr;

When i select some text the selection disapear when a new message is added to my Messages.

Any ideas how to keep the selection?

Answer 1

You could prevent this with some handling of the SelectionChanged and TextChanged events.

<TextBox Text="{Binding Messages, 
                        Converter={StaticResource ListToStringConverter}}"
         HorizontalAlignment="Center"
         SelectionChanged="TextBox_SelectionChanged"
         TextChanged="TextBox_TextChanged" />

Then, in the handlers:

private int selectionStart;
private int selectionLength;

private void TextBox_SelectionChanged(object sender, RoutedEventArgs e)
{
    var textBox = sender as TextBox;
    selectionStart = textBox.SelectionStart;
    selectionLength = textBox.SelectionLength;
}

private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
{
    if (selectionLength > 0)
    {
        var textBox = sender as TextBox;
        textBox.SelectionStart = selectionStart;
        textBox.SelectionLength = selectionLength;
    }
}

This would be best made in a Behavior with Attached Properties, and not in code-behind, but you get the idea.