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

@Jake: StringBuilder is better and easiest

public static void WriteToFile(string path, string text)
{
        StringBuilder stringBuilder = new StringBuilder();

        stringBuilder.AppendLine("a,b,c");
        stringBuilder.Append(File.ReadAllText(path)).AppendLine();
        File.WriteAllText(path, stringBuilder.ToString()); 
}

Edit: But StringBuilder has a buffer limit: Maximum Capacity 2147483647