How to insert a Line of data in the middle of the Existiong Text File?

class InsertMidLine11
    {
        static string path = "@/../../test1.txt"; //refers to bin folder 
        static string data = "Line5";
        static int lineno = 10;

        static void Main(string[] args)
        {
            InsertIntoFile(path,data,lineno);
        }

        public static void InsertIntoFile(string Path, string Data, int LineNumber)
        {
            StringBuilder result = new StringBuilder();
            FileStream fs = new FileStream(Path, FileMode.Open, FileAccess.ReadWrite);
            int lines = 0;
            using (StreamReader sr = new StreamReader(fs))
            {
                string line;
                while ((line = sr.ReadLine()) != null)
                {
                    if (lines == (LineNumber - 1))
                    {
                        result.Append(Data);
                        result.Append(" | ");
                    }
                    result.Append(line);
                    result.Append(" | ");
                    lines++;
                }
            }
            string fullstr = result.ToString();
            string[] text = fullstr.Split('|');
            using (StreamWriter sw = new StreamWriter(Path))
            {
                for (int i = 0; i < text.Length; i++)
                {
                    string item = text[i].Trim(); 
                    sw.WriteLine(item);
                    Console.WriteLine(item);
                }
            }
        } 
    }

That results:

Some Line1
Some Line2
Some Line3
Some Line4
Some Line5
Some Line6

Here i have to insert a Line "Some Line10" infront of the Line "Some Line5" that Looks as:

Some Line1
Some Line2
Some Line3
Some Line4
Some Line10
Some Line5
Some Line6


Note: first create test1.txt along above data and place this in the current project's bin folder.