Edit specific line from code on WPF RichTextBox - wpf

Question

How can I edit specific line in RichTextBox from code?
I add lines to RichTextBox
FlowDocument mcFlowDoc = new FlowDocument();
Paragraph para = new Paragraph();
para.Inlines.Add(new Run("I am a RichTextBox control line 1\n"));
para.Inlines.Add(new Run("I am a RichTextBox control line 2\n"));
para.Inlines.Add(new Run("I am a RichTextBox control line 3\n")
{
Foreground = Brushes.Red
});
mcFlowDoc.Blocks.Add(para);
RichTextBox1.Document = mcFlowDoc;
my XAML
<RichTextBox Margin="5" Name="RichTextBox1" FontSize="16" VerticalAlignment="Top"
Width="500" Height="220">
</RichTextBox>
Now I want to add a button while clicking on it (click event) the second line will change to "I finally got to do it"

Answer 1

Replace a line like this:
private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
{
string lineToReplace = "I am a RichTextBox control line 2";
string newLine = "I finally got to do it";
TextRange text = new TextRange(RichTextBox1.Document.ContentStart, RichTextBox1.Document.ContentEnd);
TextPointer current = text.Start.GetInsertionPosition(LogicalDirection.Forward);
while (current != null)
{
string textInRun = current.GetTextInRun(LogicalDirection.Forward);
if (!string.IsNullOrWhiteSpace(textInRun))
{
int index = textInRun.IndexOf(lineToReplace);
if (index != -1)
{
TextPointer selectionStart = current.GetPositionAtOffset(index, LogicalDirection.Forward);
TextPointer selectionEnd = selectionStart.GetPositionAtOffset(lineToReplace.Length, LogicalDirection.Forward);
TextRange selection = new TextRange(selectionStart, selectionEnd);
selection.Text = newLine;
selection.ApplyPropertyValue(TextElement.FontWeightProperty, FontWeights.Bold);
RichTextBox1.Selection.Select(selection.Start, selection.End);
RichTextBox1.Focus();
}
}
current = current.GetNextContextPosition(LogicalDirection.Forward);
}
}

Answer 2

If your text structure is always the same you can simply find required Run and change it in click handler like this:
var par = (Paragraph) RichTextBox1.Document.Blocks.FirstOrDefault();
var run = (Run) par.Inlines.Skip(1).First();
run.Text = "I finally got to do it\n";

Answer 3

Quick and dirty:
private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
{
TextRange textRange = new TextRange(
// TextPointer to the start of content in the RichTextBox.
RichTextBox1.Document.ContentStart,
// TextPointer to the end of content in the RichTextBox.
RichTextBox1.Document.ContentEnd
);
// get the lines of text
string[] lines =textRange.Text.Split(new[] { Environment.NewLine } , StringSplitOptions.RemoveEmptyEntries);
// get the second line
lines[1] = "I finally got to do it" + Environment.NewLine;
// build a string from the string array
StringBuilder builder = new StringBuilder();
foreach (string value in lines)
{
builder.Append(value);
}
// test the text in the RichTextbox
Paragraph para = new Paragraph();
para.Inlines.Add(new Run(builder.ToString()));
RichTextBox1.Document.Blocks.Clear();
RichTextBox1.Document.Blocks.Add(para);
}
and add line breaks properly to your document:
FlowDocument mcFlowDoc = new FlowDocument();
Paragraph para = new Paragraph();
para.Inlines.Add(new Run("I am a RichTextBox control line 1"));
para.Inlines.Add(Environment.NewLine);
para.Inlines.Add(new Run("I am a RichTextBox control line 2"));
para.Inlines.Add(Environment.NewLine);
para.Inlines.Add(new Run("I am a RichTextBox control line 3")
{
Foreground = Brushes.Red
});
mcFlowDoc.Blocks.Add(para);
RichTextBox1.Document = mcFlowDoc;