Why does text of TextPointer vary depending on whether BackgroundProperty is set or not?

Question

After more than a little experimentation, I am kind of seeing double when trying to reliably get the position and text of a FlowDocument TextPointer. Problem is the text I get from the TextPointer varies depending on whether the BackgroundProperty of the text has been set or not.

Is this correct? If so, how do I return the same text at the same offset in the line whether the text has a BackgroundProperty applied to it or not?

In the code sample posted I fill a RichTextBox with 50 lines of text. Then I use GetLineStartPosition and GetPositionAtOffset to set a TextPointer to the start of line 3. Use GetTextInRun to get the text and the return value is an empty string.  Then I set a TextRange to the 6 characters of the line and set the BackgroundProperty of that range. The result is only the first 4 chars of the line are highlighted. And when I use GetPositionAtOffset again to get a TextPointer to the first char of the line, GetTextInRun now returns the text of the first 4 chars of the line.

It is all pretty confusing. How do I reliably get the text of a FlowDocument. And how to reliably be able to set the background property of all the characters on the line?

thanks,

<Window x:Class="ColumnSelect.Windows.Window2"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Window2" Height="300" Width="400">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"></RowDefinition>
            <RowDefinition Height="auto"></RowDefinition>
        </Grid.RowDefinitions>
        <RichTextBox x:Name="rbText"
                     AcceptsReturn="True"
                     VerticalScrollBarVisibility="Auto"
                     />
        <StackPanel Orientation="Horizontal" Grid.Row="1">
            <Button x:Name="but1" Margin="10" Click="but1_Click" >Get Text, then Highlight</Button>
            <Button x:Name="but2" Margin="10" Click="but2_Click">Get Text, then Highlight, then clear highlighting</Button>
        </StackPanel>
    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;

namespace ColumnSelect.Windows
{
  /// <summary>
  /// Interaction logic for Window2.xaml
  /// </summary>
  public partial class Window2 : Window
  {
    public Window2()
    {
      InitializeComponent();
      Loaded += Window2_Loaded;
    }

    void Window2_Loaded(object sender, RoutedEventArgs e)
    {
      FlowDocument flowDoc = new FlowDocument();

      var para = new Paragraph();
      para.FontSize = 28.0;
      para.FontFamily = new FontFamily("Lucida console");

      for (int cx = 0; cx < 50; ++cx)
      {
        Run run = new Run("line:" + cx.ToString());
        para.Inlines.Add(run);
        para.Inlines.Add(new LineBreak());

      }
      flowDoc.Blocks.Add(para);
      rbText.Document = flowDoc;
    }

    private void but1_Click(object sender, RoutedEventArgs e)
    {
      var flowDoc = rbText.Document;

      // position pointer to line 3.
      var lp1 = flowDoc.ContentStart.GetLineStartPosition(0);
      lp1 = lp1.GetLineStartPosition(3);
      TextRange trSave = null;

      // position pointer at column 0 of line 3. Text of pointer returns blanks. Why?
      {
        var tp1 = lp1.GetPositionAtOffset(0);
        var s1 = tp1.GetTextInRun(LogicalDirection.Forward);
        Debug.WriteLine("Before setting background property. Text at offset 0 of line 3:" + s1 +
          " Length:" + s1.Length);
      }

      // create textRange from offset 0 to offset 5 of line 3. Then apply a background
      // property value to the range. Only the first 4 characters are highlighted.
      // Expecting 6 chars to be highlighted. From offset 0 to offset 5 of the line.
      {
        var tp1 = lp1.GetPositionAtOffset(0);
        var tp2 = lp1.GetPositionAtOffset(5);
        var tr = new TextRange(tp1, tp2);
        tr.ApplyPropertyValue(TextElement.BackgroundProperty, Brushes.DarkRed);

        // save the textRange so the backround property can be cleared later.
        trSave = tr;
      }

      // now that the text is highlighted, get the Text of the TextPointer starting
      // at offset 0. The text of the text pointer now shows a value. But only the first 
      // 4 chars. In the code just run before setting the background property, the text
      // was blank.
      {
        var tp1 = lp1.GetPositionAtOffset(0);
        var s1 = tp1.GetTextInRun(LogicalDirection.Forward);
        Debug.WriteLine("After setting background property. Text at offset 0 of line 3:" + s1);
      }

      Debug.WriteLine("---------------------");
    }

    private void but2_Click(object sender, RoutedEventArgs e)
    {
      var flowDoc = rbText.Document;

      // position pointer to line 3.
      var lp1 = flowDoc.ContentStart.GetLineStartPosition(0);
      lp1 = lp1.GetLineStartPosition(3);
      TextRange trSave = null;

      // position pointer at column 0 of line 3. Text of pointer returns blanks. Why?
      {
        var tp1 = lp1.GetPositionAtOffset(0);
        var s1 = tp1.GetTextInRun(LogicalDirection.Forward);
        Debug.WriteLine("Before setting background property. Text at offset 0 of line 3:" + s1);
      }

      // create textRange from offset 0 to offset 5 of line 3. Then apply a background
      // property value to the range. Only the first 4 characters are highlighted.
      // Expecting 6 chars to be highlighted. From offset 0 to offset 5 of the line.
      {
        var tp1 = lp1.GetPositionAtOffset(0);
        var tp2 = lp1.GetPositionAtOffset(5);
        var tr = new TextRange(tp1, tp2);
        tr.ApplyPropertyValue(TextElement.BackgroundProperty, Brushes.DarkRed);

        // save the textRange so the backround property can be cleared later.
        trSave = tr;
      }

      // now that the text is highlighted, get the Text of the TextPointer starting
      // at offset 0. The text of the text pointer now shows a value. But only the first 
      // 4 chars. In the code just run before setting the background property, the text
      // was blank.
      {
        var tp1 = lp1.GetPositionAtOffset(0);
        var s1 = tp1.GetTextInRun(LogicalDirection.Forward);
        Debug.WriteLine("After setting background property. Text at offset 0 of line 3:" + s1);
      }

      // clear the background property. Get the text at offset 0 of the line again. The
      // text now shows as the full 6 characters of the line.
      {
        trSave.ApplyPropertyValue(TextElement.BackgroundProperty, null);
        var tp1 = lp1.GetPositionAtOffset(0);
        var s1 = tp1.GetTextInRun(LogicalDirection.Forward);
        Debug.WriteLine("After background is cleared. Text at offset 0 of line 3:" + s1);
      }

      Debug.WriteLine("---------------------");
    }
  }
}

Answer 1

Hi Steve Richter,

The cause of your issue it "GetLineStartPosition" give the position which is not surrounded by any text, it is not a insert position, so you could not get any text by

"GetTextInRun(LogicalDirection.Forward);"

and

" var tp1 = lp1.GetPositionAtOffset(0);
   var tp2 = lp1.GetPositionAtOffset(5);"

will be first "4" insert text.

You could use "GetInsertionPosition" method to fix your issue, refer to below code snippet:

lp1 = lp1.GetLineStartPosition(3).GetInsertionPosition(LogicalDirection.Backward);

Best regards,

---

Sheldon _Xiao[MSFT]
MSDN Community Support | Feedback to us
Microsoft
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