Lines = richTextBox.Lines.ToList(); int lineToRemove = 0; Lines.RemoveAt(lineToRemove); richTextBox.Lines = Lines.ToArray();
Here is what the above code is Doing:
1. Convert the lines in the RichTextBox to a List.
2. Remove the first line from the List.
3. Convert the List back to an array and assign it to the RichTextBox.