Two way binding with custom expression?

I am using a gridview in my asp.net project to view and modify some records from the database. The database has two columns: start_date and end_date. When a new record is created these columns contains null, but they can be modified later using the gridview update command.

In gridview I have two template fields (having names start_date and end_date) in which I have placed two calendar controls. Upon clicking an update link of gridview it always returns an error because of the null value binding to the calendar. I have used this helper function to solve it:

protected DateTime ReplaceNull(Object param)
{
    if (param.Equals(DBNull.Value))
    {
        return DateTime.Now;
    }
    else
    {
        return Convert.ToDateTime(param);
    }
}

and used these two custom expressions in calendar control's Sel开发者_如何学JAVAectedDate:

ReplaceNull(Eval("start_date"))
ReplaceNull(Eval("end_date"))

The problem is that two-way data binding the calendars upon selecting a date does not update the database table. Are there any workarounds? Or alternatively, a better solution would be appreciated.

---

I don't know why you let them null when insert a new record , but many ways you can solve this problem i think .

one of them : in the RowDataBound event of the Gridview

 protected void gv_RowDataBound(object sender, GridViewRowEventArgs e)
        {
            if (e.Row.RowType == DataControlRowType.DataRow)
            {
               if (e.Row.Cells[1] == null) //the index of the start_date
                 {
                    e.Row.Cells[1].Text = DateTime.Now.ToString(); // in your case you will  make the selected date of the calender(through casting to calender) with the value you need.
                 }

            }
        }

Or: you can catch the exception , you meet in your Update button through

try and catch block.