I have a a dataset with bank register data in it. I am adding a column, and I want to loop through the DS and compute the balance on the fly. I have:

oRegisterDS = csRegister.oDataProcs.ExecuteQuery("b_GetRegisters", "Register", oRegParams);

DataColumn oBalanceCol = new DataColumn("Balance", System.Type.GetType("System.Double"));
oRegisterDS.Tables["Register"].Columns.Add(oBalanceCol);

double dBalance = 0;
foreach (DataRow oRow in oRegisterDS.Tables["Register"].Rows)
{
    if (oRow["Payment"] == null)
    {
        dBalance = dBalance + Convert.ToDouble(oRow["Deposit"]);
    }
    else
    {
        dBalance = dBalance - Convert.ToDouble(oRow["Payment"]);
    }

    oRow["Balance"] = dBalance;
}

The value of the Payment column on row 0 is null, yet it skips down into the ELSE section. What's the correct way to work with NULL in a DS?

Thanks