在richtextbox wpf中更改特定文本的颜色

Question

I am working on function, that will change back color of tags in text of RichTextBox (blah blah blah < i>This will be changed< /i> blah blah blah). I tried to make some code, but it is highlighting correctly only first tag. The second tag will be highlited a few chars before. Please, how could I get this work?

Actual state - Function colorize correctly only first tag, others are colored characters before.

State I want - Have colored only tags and it's content in RichTextBox.

Problem part:

        TextPointer text = txbPlainText.Document.ContentStart;
        while (text.GetPointerContext(LogicalDirection.Forward) != TextPointerContext.Text)
        {
          text = text.GetNextContextPosition(LogicalDirection.Forward);
        }
        TextPointer start = text.GetPositionAtOffset(tagStartIndex);
        TextPointer end = text.GetPositionAtOffset(i + tagBuilder.Length);

        TextRange textRange = new TextRange(start, end);
        textRange.ApplyPropertyValue(TextElement.BackgroundProperty, new SolidColorBrush(Color.FromRgb(220, 204, 163)));

How it looks like: Image with problem

This is whole function:

private void ColorizeTags()
{
  string tagString = string.Empty;
  int tagStartIndex = 0;
  char[] txbChars = GetTxbText().ToCharArray();

  for (int i = 0; i < txbChars.Count(); i++)
  {
    char actualChar = txbChars[i];

    if (actualChar == '<' && txbChars[i + 1] != '/')
    {
      StringBuilder tagBuilder = new StringBuilder();
      foreach (string tag in TagList)
      {
        for (int x = i; x < (i + tag.Length); x++)
        {
          if (x > txbChars.Count())
          {
            break;
          }

          tagBuilder.Append(txbChars[x]);
        }

        if (tagBuilder.ToString() == tag)
        {
          tagString = tagBuilder.ToString();
          tagStartIndex = i;
          break;
        }
      }
    }
    else if (actualChar == '<' && txbChars[i + 1] == '/')
    {
      if (string.IsNullOrWhiteSpace(tagString))
      {
        continue;
      }

      string endTag = tagString.Insert(tagString.IndexOf('<') + 1, "/");
      StringBuilder tagBuilder = new StringBuilder();

      for (int c = i; c < (i + endTag.Length); c++)
      {
        tagBuilder.Append(txbChars[c]);
      }

      if (tagBuilder.ToString() == endTag)
      {
        TextPointer text = txbPlainText.Document.ContentStart;
        while (text.GetPointerContext(LogicalDirection.Forward) != TextPointerContext.Text)
        {
          text = text.GetNextContextPosition(LogicalDirection.Forward);
        }
        TextPointer start = text.GetPositionAtOffset(tagStartIndex);
        TextPointer end = text.GetPositionAtOffset(i + tagBuilder.Length);

        TextRange textRange = new TextRange(start, end);
        textRange.ApplyPropertyValue(TextElement.BackgroundProperty, new SolidColorBrush(Color.FromRgb(220, 204, 163)));
        tagString = string.Empty;
        continue;
      }
    }
  }
}

private string GetTxbText()
{
  return new TextRange(txbPlainText.Document.ContentStart, txbPlainText.Document.ContentEnd).Text;
}

Answer 1

I searched and found the problem. The thing is when I was getting the position offset of the start and the end, value of the current textpointer has changed.

And second problem was, that my TextPointer text ignored whitespaces, solution below.

Solution: Don't get position offset straightly.

Before:

...
TextPointer text = txbPlainText.Document.ContentStart;
while (text.GetPointerContext(LogicalDirection.Forward) != TextPointerContext.Text)
{
  text = text.GetNextContextPosition(LogicalDirection.Forward);
}
TextPointer start = text.GetPositionAtOffset(tagStartIndex);
TextPointer end = text.GetPositionAtOffset(i + tagBuilder.Length);
...

After:

...
TextPointer text = txbPlainText.Document.ContentStart;
TextPointer start = GetTextPointAt(text, tagStartIndex);
TextPointer end = GetTextPointAt(text, endIndex);
...

private static TextPointer GetTextPointAt(TextPointer from, int pos)
{
  TextPointer ret = from;
  int i = 0;

  while ((i < pos) && (ret != null))
  {
    if ((ret.GetPointerContext(LogicalDirection.Backward) == TextPointerContext.Text) || (ret.GetPointerContext(LogicalDirection.Backward) == TextPointerContext.None))
      i++;

    if (ret.GetPositionAtOffset(1, LogicalDirection.Forward) == null)
      return ret;

    ret = ret.GetPositionAtOffset(1, LogicalDirection.Forward);
  }

  return ret;
}

Source of answer