Populating textbox based off bound column in DetailsView - Problem with Nulls

I'm trying to populate a date from a bound text field in a DetailsView. I keep getting a error when a DBNull is in the column. How do I avoid the DBNull when populating the text field. Any Help on this would be great at a loss.

My code is as follows:

 <asp:TemplateField HeaderText="CBYD Clear Date">
              <EditItemTemplate>
                  <asp:TextBox ID="CBYDExpDate" runat="server" ReadOnly="true" Text='<%# IIf(Eval("CBYDDate") is DBNull.Value,"", String.For开发者_如何转开发mat("{0:MM/dd/yyyy}", Eval("CBYDDate").AddDays(30)))%>' />                                                      
          </EditItemTemplate>
    </asp:TemplateField>

The simpliest would be to define a function in codebehind that you call:

Text='<%# getDateText(Eval("CBYDDate"),"{0:MM/dd/yyyy}", 30) %>'

and in codebehind:

 Protected Function getDateText(ByVal value As Object, ByVal dateFormatString As String, ByVal addDays As Int32) As String
     If value Is Nothing OrElse value Is DBNull.Value Then
         Return String.Empty
     ElseIf TypeOf value Is Date Then
         Dim d As Date = DirectCast(value, Date)
         Return String.Format(dateFormatString, d.AddDays(addDays))
     Else
         Return value.ToString
     End If
 End Function

For additional informations have a look at MSDN