Why am I losing the Session here?

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();

Answer 2

You're not getting a new DataTable you are getting the same one that you originally generated. Any changes you make to it will remain to be seen by subsequent uses.

In this case I would rename the column back to its original name after the export.

Answer 3

It's an awful solution, but... add

dtExport.Columns["Licensee"].ColumnName = "Business";

at the end of your method to restore the initial situation. And add some error/existence checking.