c# - Data table to multiple CSV files -

i'm fetching records sql server 2012 table. i've used datatable. want write fetched records multiple csv files.

i've used stringbuilder write column headers , fields csv file (single csv file).

but, new requirement first filter records based on columns , write multiple files.

1. suppose i've 5 columns col1, col2, col3, col4 , col5.
2. i need write col1,col2 , col5 once csv file , col1,col3 , col4 csv file.

3. i want include column - col0 first column added through code , not present in database. column derived col3 , col5. this:

   dt.columns.add("col0", typeof(system.string)).setordinal(0);

   row["col0"] = row["col3"] + "_" + row["col5"];

4. issue point 3 can't derive col0 unless i'm fetching both rows - col3 , col5 datatable (dt). it's giving me error

system.argumentexception: column 'col3' not belong table selected.

please let me know how proceed?

code:

public static void createcsvfile(datatable dt, string csvpath, string csvfilename, int filesize) {     stringbuilder firstline = new stringbuilder();     stringbuilder records = new stringbuilder();      int num = 0;     int length = 0;      dt.columns.add("update", typeof(system.string)).setordinal(0);     dt.columns.add("key", typeof(system.string)).setordinal(1);      ienumerable<string> columnnames = dt.columns.cast<datacolumn>().select(column => column.columnname);     firstline.append(string.join(",", columnnames));     records.appendline(firstline.tostring());     length += records.tostring().length;      int lastindex = dt.rows.count;     foreach (var row1 in dt.rows.cast<datarow>().select((r, i) => new { row = r, index = }))     {         datarow row = row1.row;         row["update"] = "update";         row["key"] = row["name"] + "_" + row["code"];          //putting field values in double quotes         ienumerable<string> fields = row.itemarray.select(field =>             string.concat("\"", field.tostring().replace("\"", "\"\""), "\""));          records.appendline(string.join(",", fields));         length += records.tostring().length;          if (length > filesize)         {             //create new file             num++;             file.writealltext(csvpath + csvfilename + datetime.now.tostring("yyyymmddhhmmss") + num.tostring("_000") + ".csv", records.tostring());             records.clear();             length = 0;             records.appendline(firstline.tostring());             length += records.tostring().length;         }         else if (row1.index == lastindex - 1 && length != 0 && length < filesize)         {             //create new file             num++;             file.writealltext(csvpath + csvfilename + datetime.now.tostring("yyyymmddhhmmss") + num.tostring("_000") + ".csv", records.tostring());             records.clear();             length = 0;             records.appendline(firstline.tostring());             length += records.tostring().length;         }     } }