(C# .NET) data ประมาณ 1 ล้านกว่า row จะ write ลง text file ได้อย่างไรบ้างครับ?

public static void ToCSV(string fileWRITE, string fileREAD)
{
    usint(var commas = new StreamWriter(fileWRITE))
    using(var file = new StreamReader("yourFile.txt"))
    {
        var line = file.ReadLine();

        while( line != null )
        { 
            string q = (y.Substring(0,15)+","+y.Substring(15,1)+","+y.Substring(16,6)+","+y.Substring(22,6)+ ",NULL,NULL,NULL,NULL");
            commas.WriteLine(q);
            line = file.ReadLine();
        }
    } 
}