High RAM Usage On Postback Caused By Datatable

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C#5.0

memory

I basically have a listbox that has postcode areas i.e : AE,CW,GU etc etc.

The user selects this and then a postback occurs - an sql statement is builts and a database query operation is performed and the results are returned to a datatable called tempdata.

So far so good. I then need to loop through this datatable and copy the records to my main viewstate datatable which is the datasource for google maps api.

DataTable tempstore = GetData(querystring, "");

//check tempstore has rows otherwise add defaultcust as default otherwise map will be blank

if (tempstore.Rows.Count == 0)
{
    tempstore = GetData("WHERE CUSTCODE=='CD344'", "");

    infoalert.Visible = true;

    infoalert.InnerHtml = "No Results Returned For Selection";
}

foreach (DataRow row in tempstore.Rows)
{

    dtpc.ImportRow(row);
    

} 
dtpc.AcceptChanges();

So my main datatable can grow and grow as users add more postcodes. However when it gets to around 500 rows or so I get a huge memory spike only on postback and then it settles back down.

My ram usage goes from 2gb to 3gb and if even more postcodes is selected it maxes the memory and crashes my pc.

If I remove the:

dtpc.Importrow(row);
the memory spike goes completely, obviously because the main datatable has no rows. I thought you only run into memory issues when you have thousands of rows?

Any help would be much appreciated.

thank you