how to insert row in first line of text file?

Question

I have a test file that contains

1,2,3
2,3,4
5,6,7

I want to insert this into the first line: A,B,C

So that I get:

Answer 1

I think this answer is easier and faster:

public static void WriteToFile(string Path, string Text)
{
    string content = File.ReadAllText(Path);
    content = Text + "\n" + content;      
    File.WriteAllText(Path, content);
}

Then you can call it:

WriteToFile("yourfilepath", "A,B,C");