Write to a textFile without substituting text 1

[Programming2007]

Hello I am trying to write to a textfile at a certian position. The problem is that when I do that what I write to the file writes over the existing data. How do I do a write that is like an insert which just inserts the values into the textFile without replacing them. Below is my code:

FileStream fs = new FileStream(cabFile, FileMode.OpenOrCreate, FileAccess.ReadWrite);
StreamWriter sw = new StreamWriter(fs,Encoding.UTF8);
StreamReader streamReader = new StreamReader(fs);
str_rawFileData = streamReader.ReadToEnd();

RecordHeader = str_rawFileData.Substring(0, startIndex);
fs.Position = RecordHeader.Length;
sw.Write("|");

 

[earthandfire]

Create a new file, read from the old file up to the insertion point, write this to the new file. Write the text to be inserted to the new file. Read the remaining text from the old file and write this to the new file. Delete the old file and rename the new file with the old file's name.

Hope this helps.