string[] lines = { "First line", "Second line", "Third line" };

System.IO.File.WriteAllLines( @"C:\Users\Public\TsetFolder\WriteLines.txt", lines );

string text = "A class is the most powerful data type in C#. Like a structure, " + "a class defines the data and behavior of the data type. ";

System.IO.File.WriteAllText(@"C:\Users\Public\TestFolder\WriteText.txt", text);

using( System.IO.StreamWriter file = new System.IO.StreamWriter( @"C:\Users\Public\TestFolder\WriteLines2.txt" ) ){
    foreach( string line in lines ){
        if ( !line,Contains("Second") ){
            file.WriteLine(line);
        }
    }

}    

using( System.IO.StreamWriter file = new System.IO.StreamWriter( @"C:\Users\Public\TestFolder\WriteLines2.txt", true ) ){
        file.WriteLine( "Fourth line" );

}