//if you want to make excel visible

    excapp.Visible = true;

    //create a blank workbook

    var workbook = excapp.Workbooks.Add(NsExcel.XlWBATemplate.xlWBATWorksheet);

    //or open one - this is no pleasant, but yue're probably interested in the first parameter

    string workbookPath = "C:\test.xls";

    var workbook = excapp.Workbooks.Open(workbookPath,

        0, false, 5, "", "", false, Excel.XlPlatform.xlWindows, "",

        true, false, 0, true, false, false);

    //Not done yet. You have to work on a specific sheet - note the cast

    //You may not have any sheets at all. Then you have to add one with NsExcel.Worksheet.Add()

    var sheet = (NsExcel.Worksheet)workbook.Sheets[1]; //indexing starts from 1

    //do something usefull: you select now an individual cell

    var range = sheet.get_Range("A1", "A1");

    range.Value2 = "test"; //Value2 is not a typo

    //now the list

    string cellName;

    int counter = 1;

    foreach (var item in list)

    {

        cellName = "A" + counter.ToString();

        var range = sheet.get_Range(cellName, cellName);

        range.Value2 = item.ToString();

        ++counter;

    }

    //you've probably got the point by now, so a detailed explanation about workbook.SaveAs and workbook.Close is not necessary

    //important: if you did not make excel visible terminating your application will terminate excel as well - I tested it

    //but if you did it - to be honest - I don't know how to close the main excel window - maybee somewhere around excapp.Windows or excapp.ActiveWindow

}

using System.IO;

using(StreamWriter sw = File.CreateText("list.csv"))

{

  for(int i = 0; i < l.Count; i++)

  {

    sw.WriteLine(l[i]);

  }

}