read & write text file

// 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();

//In order to append an existing file, set the Boolean parameter to true, as follows:
System.IO.StreamWriter file =   new System.IO.StreamWriter("c:\\test.txt", true);

//Read from File
string allText = System.IO.Files.ReadAllText("C:\\test.txt");
//There are some way to read from file (ReadAllText, ReadAllLines,..). You should check it yourself