foreach (DataRow r in ds.Tables[0].Rows)
{
    string prodCode = r["PRD-CDE"].ToString();
    statCode = r["STAT"].ToString();
    DateTime firstIssueDate = (DateTime)(r["FISS"]);
    DateTime endIssueDate = (DateTime)(r["EISS"]);
    if(endIssueDate > DateTime.Now)
    { /*do some thing...*/}
    else {/*user invalid...*/}
}

foreach (DataRow r in ds.Tables[0].Rows)
            {
                string prodCode = r["PRD-CDE"].ToString(),statCode = r["STAT"].ToString();
               // r.<DateTime?>("FISS");
                if (r["FISS"] != DBNull.Value)
                {
                    DateTime firstIssueDate = Convert.ToDateTime(r["FISS"]);
                    if (r["EISS"] != DBNull.Value)
                    {
                        DateTime endIssueDate = DateTime.Parse(r["EISS"].ToString());
                        if (endIssueDate > DateTime.Now)
                        {

row.Field<DateTime?>("column")

DateTime? firstIssueDate = r.Field<DateTime?>("fiss");
DateTime? endIssueDate = r.Field<DateTime?>("eiss");

if (firstIssueDate.HasValue && endIssueDate.HasValue)
{
    firstIssueDate.Value // blah blah
    endIssueDate.Value // blah blah
}

string prodCode = r["PRD-CDE"].ToString(),statCode = r["STAT"].ToString();
 DateTime firstIssueDate = DateTime.Parse(r["FISS"]);
 DateTime endIssueDate = DateTime.Parse(r["EISS"]);

foreach (DataRow r in ds.Tables[0].Rows)
{
    string prodCode = r["PRD-CDE"].ToString();
    string statCode = r["STAT"].ToString();
    DateTime firstIssueDate = DateTime.Parse((r["FISS"]).ToString());
    DateTime endIssueDate = DateTime.Parse((r["EISS"]).ToString());
    if(endIssueDate > DateTime.Now)
    { /*do some thing...*/}
    else {/*user invalid...*/}
}

var firstIssueDate = r["FISS"] as DateTime? ?? DateTime.MinValue;
var endIssueDate = r["EISS"] as DateTime? ?? DateTime.MinValue;