StreamWriter Class is more popular in File Handling and it is very helpful in writing text data in the file.

using System.IO;
using System;

namespace CSharpFilesAndStreams
{
	class KtoD 
	{
		static void Main() 
		{
			string str;
			FileStream fout;

			// First, open the file stream.
			try {
				fout = new FileStream("test.txt", FileMode.Create);
			}
			catch(IOException exc) {
				Console.WriteLine("Error Opening File:\n" + exc.Message);
				return ;
			}

			// Wrap the file stream in a StreamWriter.
			StreamWriter fstr_out = new StreamWriter(fout);
			try {
				Console.WriteLine("Enter text ('stop' to quit).");
				do {
					Console.Write(": ");
					str = Console.ReadLine();
					if(str != "stop") {
					str = str + "\r\n"; // add newline
					fstr_out.Write(str);
				}
				} while(str != "stop");
			} 
			catch(IOException exc) {
				Console.WriteLine("I/O Error:\n" + exc.Message);
			} 
			finally {
				fstr_out.Close();
			}
		}
	}
}