[RESOLVED] how to enter null value ?

[hlsc1983]

if textbox is not enabled , a null value must be entered in a database.

i tried this but there is an error " value of type DBnull cannot be converted to string"

If pretTxt.Enabled = False Then pretTxt.Text = DBNull .value

and i am trying to do this so that i can insert something like this.

 If pretTxt.Enabled = False Then pretTxt.Text = DBNull .value

        Dim cmd As New SqlCommand("insert into Marks_master (subject_id, student_id, T_PA_marks, T_ET_marks, P_PA_marks,P_ET_marks) VALUES(@si,@stdid, @tet ,@tpa, @pet, @ppa)", con)
        cmd.Parameters.AddWithValue("si", subjectLbl.Text)
        cmd.Parameters.AddWithValue("stdid", studentidTxt.Text)
        cmd.Parameters.AddWithValue("tet", thetTxt.Text)
        cmd.Parameters.AddWithValue("tpa", thpaTxt.Text)
        cmd.Parameters.AddWithValue("pet", pretTxt.Text)
        cmd.Parameters.AddWithValue("ppa", prpaTxt.Text)

       Try
            cmd.ExecuteNonQuery()
            MessageBox.Show("Record added")
        Catch ex As Exception
            MessageBox.Show("Error while adding record")
        End Try
        con.Close()

if textbox is not enabled then null value must be inserted in the table.