Writing to a file

using System.IO;

StreamWriter writer = new StreamWriter("test.txt");
writer.WriteLine("hello world");
writer.Close();

Reading from a file

Reading a single line:

using System.IO;

StreamReader reader = new StreamReader("test.txt");
string line = reader.ReadLine();
Console.WriteLine(line);

reader.Close();

From a file of known length:

using System.IO;

StreamReader reader = new StreamReader("test.txt");
string line;

for (int i = 0; i < 3; i++) {   // 3 is the # of lines in the file
    line = reader.ReadLine();
    Console.WriteLine(line);
} // end for

reader.Close();

From a file of unknown length:

using System.IO;

StreamReader reader = new StreamReader("test.txt");
while (reader.EndOfStream == false) { // this means "read until the end of the file"
  string line = reader.ReadLine();
  Console.WriteLine(line);
} // end while

reader.Close();