How to pass a Null date variable to SQL Server database

Question

I am looking for the best practice, real solution, to send a Null to a SQL Server 2008 R2 database table, when a date is unknown.

I read some inputs from a formview, and a date field maybe unknown. The database allows Null values in the field but the VB to store the Null prior to a parameterized query update is not working/eludes me.

    Dim tb2 As TextBox = TryCast(FormView1.FindControl("tbPurchDate"), TextBox)
    Dim purDT As Date
    If tb2.Text = "" Then
        IsDBNull(purDT)    ' Tried this along with other possible code
    Else
        purDT = Convert.ToDateTime(tb2.Text)
    End If

Any help would be greatly appreciated.

Answer 1

If the date is unknown, send DbNull.Value as the parameter's value:

If dateIsUnknown Then
    cmd.Parameters.Add(New SqlParameter _
                       With {.ParameterName = "@purDT", _
                             .SqlDbType = SqlDbType.Date, _
                             .Value = DBNull.Value})
Else
    cmd.Parameters.Add(New SqlParameter _
                       With {.ParameterName = "@purDT", _
                             .SqlDbType = SqlDbType.Date, _
                             .Value = theDateVariable})
End If

Or if you prefer,

cmd.Parameters.Add(New SqlParameter With {
                   .ParameterName = "@purDT",
                   .SqlDbType = SqlDbType.Date,
                   .Value = If(dateIsUnknown, DBNull.Value, DirectCast(theDateVariable, Object))}
                  )

It is necessary to cast the variable to be of type Object so that the If operator has a common type to return. The Value parameter expects an Object.