PURPOSE OF STREAM WRITER IN C#.NET:-
StreamWriter writes text files. It enables easy and efficient text output. It is best placed in a using-statement to ensure it is removed from memory when no longer needed. It provides several constructors and many methods.
We first declare and initialize a new StreamWriter instance in a using construct. Please note how the System.IO namespace is included at the top of the file. The keyword using means different things in different places.
Example:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;

namespace StreamReadWrite
{
    class Program
    {
        static void Main(string[] args)
        {
            // Get the directories currently on the C drive.
            DirectoryInfo[] cDirs = new DirectoryInfo(@"c:\").GetDirectories();

            // Write each directory name to a file. 
            using (StreamWriter sw = new StreamWriter("CDriveDirs.txt"))
            {
                foreach (DirectoryInfo dir in cDirs)
                {
                    sw.WriteLine(dir.Name);

                }
            }

            // Read and show each line from the file. 
            string line = "";
            using (StreamReader sr = new StreamReader("CDriveDirs.txt"))
            {
                while ((line = sr.ReadLine()) != null)
                {
                    Console.WriteLine(line);
                }
            }
        }
    }
}