using System.IO;
using System.Text;

var path = @"C:\tmp\file.txt"; 

//first method
var lines = new[] { "Line 1", "Line 2" };
File.WriteAllLines(path, lines, Encoding.UTF8);

//second method
string text = "Line 1\nLine 2";
File.WriteAllText(path, text, Encoding.UTF8);

//third method
text = "Line 1\nLine 2";

var file = new StreamWriter(path, false, Encoding.UTF8);
file.WriteLine(text);
file.Close();