There
is no direct method or property in RichTextBox that can extract the
document or contents of a RichTextBox to a string. To convert the
contents to a string, we first need to read the contents of a
RichTextBox in a TextRange object and use TextRange.Text property to
convert it to a string.
The following code snippet reads a RichTextBox contents from start to end and converts to a string.
string ConvertRichTextBoxContentsToString(RichTextBox rtb)
{
TextRange textRange = new TextRange(rtb.Document.ContentStart,
rtb.Document.ContentEnd);
return textRange.Text;
}