اضافه کردن متن به یک فایل txt موجود در ویندوز

سلام
چطوری میشه به یه فایل txt که درسیستم وجود داره یه متن اضافه کرد؟( بدون اینکه متن قبلی پاک بشه)

مثال 1:

// Compose a string that consists of three lines.

string lines = "First line.\r\nSecond line.\r\nThird line.";



// Write the string to a file.

System.IO.StreamWriter file = new System.IO.StreamWriter("c:\\test.txt");

file.WriteLine(lines);



file.Close();

مثال 2:

 class WriteTextFile

    {

        static void Main()

        {



            // These examples assume a "C:\Users\Public\TestFolder" folder on your machine.

            // You can modify the path if necessary.



            // Example #1: Write an array of strings to a file.

            // Create a string array that consists of three lines.

            string[] lines = {"First line", "Second line", "Third line"};

            System.IO.File.WriteAllLines(@"C:\Users\Public\Tes  tFolder\WriteLines.txt", lines);





            // Example #2: Write one string to a text file.

            string text = "A class is the most powerful data type in C#‎. Like structures, " +

                           "a class defines the data and behavior of the data type. ";

            System.IO.File.WriteAllText(@"C:\Users\Public\Test  Folder\WriteText.txt", text);



            // Example #3: Write only some strings in an array to a file.

            using (System.IO.StreamWriter file = new System.IO.StreamWriter(@"C:\Users\Public\TestFolde  r\WriteLines2.txt"))

            {

                foreach (string line in lines)

                {

                    if (line.Contains("Second") == false)

                    {

                        file.WriteLine(line);

                    }

                }

            }



            // Example #4: Append new text to an existing file

            using (System.IO.StreamWriter file = new System.IO.StreamWriter(@"C:\Users\Public\TestFolde  r\WriteLines2.txt", true))

            {

                file.WriteLine("Fourth line");

            }  

        }

    }

    /* Output (to WriteLines.txt):

        First line

        Second line

        Third line



     Output (to WriteText.txt):

        A class is the most powerful data type in C#‎. Like structures, a class defines the data and behavior of the data type.



     Output to WriteLines2.txt after Example #3:

        First line

        Third line



     Output to WriteLines2.txt after Example #4:

        First line

        Third line

        Fourth line

     */

  System.IO.File.AppendAllText("path","text");

یه راه ساده داره، اول اون فایل رو تویه یه چیزی ذخیره کن
بعدش به اون یکی متغییر ات اضافش کن! :)