Question

I have a page with a button that when I click, it retrieves data from a database and stores it into a datatable and binds it to a GridView. This datatable is stored in a Session variable. I also have a button that exports specified columns from the datatable to an excel file, but when I click the export button a second time, I get the following error:

Object reference not set to an instance of an object.

It seems to happen at this line:

dtExport.Columns[‘Business’].ColumnName = ‘Licensee’;

I think I know why, the original column is called ‘Business’, but when I export, I want to export the column header as Licensee, so I change the ColumnName to ‘Licensee’; however when I call Export a second time, it is looking for dtExport.Columns[‘Business’] again which it does not find, so it throws an error. Do I just need to check if the column as already been renamed to resolve this or is there another way?

Here is the code that does the Export:

private void ExportExcel() {     DataTable dtExport = Session['dtSearchResults'] as DataTable;      dtExport.Columns['Business'].ColumnName = 'Licensee';      List<int> columnSelect = new List<int>();      columnSelect.Add(dtExport.Columns['Licensee'].Ordinal);     columnSelect.Add(dtExport.Columns['Name'].Ordinal);     columnSelect.Add(dtExport.Columns['Address'].Ordinal);     columnSelect.Add(dtExport.Columns['City'].Ordinal);     columnSelect.Add(dtExport.Columns['State'].Ordinal);     columnSelect.Add(dtExport.Columns['Zip'].Ordinal);      int[] ColList = columnSelect.ToArray();      GridViewExportUtil.ExportDetails(dtExport,                                       ColList,                                      GridViewExportUtil.ExportFormat.Excel,                                      string.Format('{0}_{1}-{2}-{3}{4}',                                      'SearchResults',                                      DateTime.Now.Month,                                       DateTime.Now.Day,                                      DateTime.Now.Year,                                       '.xls'));     } 

When you change a ColumnName, does it persit, even if you get a new DataTable from Session again?

Answer 1

I am guessing the value is by reference, and when you change the Business column name it is changed in Session as well. I would try this:

DataTable dtExport = (Session['dtSearchResults'] as DataTable).Copy();