How to programmatically replace some content of WPF RichTextBox without loosing the formatting?

Question

Question: How can we programmatically replace some text in a WPF RichTextBox without loosing its formatting? In the following code I am clearly not doing something right. My online search gives some relevant suggestions but they seem to be using Winform where RichTextBox has rtf property - such as this one.

Following code correctly replaces text abcd with rstu inside a WPF RichTexBox but it looses the formatting of the RichTextBox as shown in the two images below:

//rtbTest is the name of the RichTextBox
TextRange textRange = new TextRange(rtbTest.Document.ContentStart, rtbTest.Document.ContentEnd);
string oldText = textRange.Text;
string newText = oldText.Replace("abcd", "rstu");
textRange.Text = newText;

Screenshot of RichTextBox BEFORE replacing abcd with rstu:

Screenshot of RichTextBox AFTER replacing abcd with rstu:

As we can see the formatting is lost. The list shown below is not really a formatted numbered list, it probably is just unformatted text (like 1. Item 1 etc.)

Answer 1

It is losing the formatting because you are storing the RTF into a string and the string doesn't keep RTF formatting.

You can preserve it as following,

TextRange textRange = new TextRange(rtbTest.Document.ContentStart, rtbTest.Document.ContentEnd);
string rtf;
using (var memoryStream = new MemoryStream())
{
    textRange.Save(memoryStream, DataFormats.Rtf);
    rtf = ASCIIEncoding.Default.GetString(memoryStream.ToArray());
}

rtf = rtf.Replace("abcd", "rstu");


MemoryStream stream = new MemoryStream(ASCIIEncoding.Default.GetBytes(rtf));
rtbTest.SelectAll();
rtbTest.Selection.Load(stream, DataFormats.Rtf);