Question

I’ve a table with contacts, a table with events, and a table that links contacts to those events.

So, in my view, I select an event and I can see a list of contacts that are allowed to be add to this event.

For the moment, it is possible to add a contact many times to an event.

I’d like to check if the contact is already add to the event before to display the button.

Here is my view :

 <td>
      @Code
      Dim flag1 As Integer = 0
      flag1 = (From a In Model.trans_Event
               Where a.FK_id_contact = item.idContact And a.FK_id_Event = idEvent
               Select a.idtrans).Count()
      End Code

      @If flag1 = 0 Then
      <input type="button" value="add" />
      Else
      <input type="button" value="delete" />
      End If
 </td>

This code like that doesn’t work, and I don’t really see how I can check whether the contact has already been add or not. Here is the error I get :

    Exception Details: System.ArgumentNullException: Value cannot be null.
    Parameter name: source

Answer 1

at this code ( if you havent made a copy/paste mistake )

a.FK.id_Event = idEvent

you are trying to get a -> FK -> id_event and since there is no FK object in your model it breaks down to null exception.

looking at your code that should be “a.FK_id_Event”

flag1 = (From a In Model.trans_Event
               Where a.FK_id_contact = item.idContact And a.FK_id_Event = idEvent
               Select a.idtrans).Count()
      End Code

Edit:

when filtering the collections in the model i would prefer;

Model.trans_Event.Where(a => a.FK_id_contact == item.idContact && 
                             a.FK_id_Event == idEvent)