如何将所选文本的开始和结束索引转换为精确的TextRange？

Question

我可以在RichTextBox中获得所选文本的确切开始和结束索引，但是你如何反过来呢？使用实际的开始和结束索引（字符位置）再次选择文本作为范围以对其执行某些格式化 - 例如突出显示背景。

示例 - 使用鼠标选择文本并应用突出显示不重叠

private void textBox_PreviewMouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
    if(this.textBox.Selection.Text.Length > 0)
        highlightFromSelection(this.textBox, new SolidColorBrush(Colors.Yellow));
}

public void highlightFromSelection(RichTextBox textBox, SolidColorBrush color)
{
    TextPointer docStart = textBox.Document.ContentStart;

    TextPointer selectionStart = textBox.Selection.Start;
    TextPointer selectionEnd = textBox.Selection.End;

    TextRange start = new TextRange(docStart, selectionStart);
    TextRange end = new TextRange(docStart, selectionEnd);

    int indexStart = start.Text.Length;
    int indexEnd = end.Text.Length;

    MessageBox.Show("start: " + indexStart + " end: " + indexEnd);

    ApplyHighlight(textBox, indexStart, indexEnd, color);
}

public void ApplyHighlight(RichTextBox textBox, int startIndex, int endIndex, SolidColorBrush color)
{
    TextPointer docStart = textBox.Document.ContentStart;

    //This code highlights the wrong part of text - its the closest I could get 
    //i.e. the highlighting does not overlap exactly with the original selection made

    //TextPointer startPointer = docStart.GetPositionAtOffset(startIndex, LogicalDirection.Forward);
    //TextPointer endPointer = docStart.GetPositionAtOffset(endIndex, LogicalDirection.Backward);
    //TextRange range = new TextRange(startPointer, endPointer);
    //range.ApplyPropertyValue(TextElement.BackgroundProperty, new SolidColorBrush(Colors.Yellow));
}

修改 我只能使用highlightFromSelection（）方法确定的绝对字符位置。