I use a FormView to edit data, there is a DateTime filed.

The original for the DateTime filed  is null and the user did not enter any date into it.
It result into a run time error: "DateTime String was not recognized as a valid DateTime" when update.

How can I set the data time filed to null in this situation?

That is an error code you are getting from the Sql server. Open the table in the Sql server and check the "allow nulls" for that column. That way when the database gets no date, it wont care.

I have same problem.  

before my code is like this:

Dim NewTagName12 As String = (CType(GridView1.Rows(e.RowIndex).Cells(12).Controls(0), TextBox)).Text 

the error message is "DateTime String was not recognized as a valid DateTime" when update.

then I change to:

Dim NewTagName12 As String = Date.ParseExact(CType(GridView1.Rows(e.RowIndex).Cells(12).Controls(0), TextBox).Text(), "MM/dd/yy", New Globalization.DateTimeFormatInfo)

the error message is : "String was not recognized as a valid DateTime." when update.

does somebody know how to change this code? 

Hi,

You may convert the field value into DateTime in you code behind. Convert the Field value if the field have any valid date otherwise dont convert.

Convert.ToDateTime("") -> throws Error.

Ganesh

thank  u for your answer. the field value is already DateTime, the problem is if a datetime field is blank. then it will give error message when update, if  datetime field  have date there, it won't show the error message.

about this code below how can I change the code so it will check if the field is blank ,then just skip this field and doesn't need to do anything. 

Dim NewTagName12 As String = Date.ParseExact(CType(GridView1.Rows(e.RowIndex).Cells(12).Controls(0), TextBox).Text(), "MM/dd/yy", New Globalization.DateTimeFormatInfo)

I checked this date field to  "allow nulls" , but it doesn't work.

I have the same problem,does someone know how to fix it?