title: EntityDataSource, QueryCreated, and DropDownLists
body: <p>I have a drop down list (ddlFilterBuildings) which contains a list of buildings to choose from.</p>
  
  <p>I have an EntityDataSource and a Sub that triggers on QueryCreated. It is as follows:</p>
  
  <pre><code>    Protected Sub EntityDataSource1_QueryCreated(sender As Object, e As QueryCreatedEventArgs)
        Dim residents = e.Query.Cast(Of Resident)()
        e.Query = From p In residents _
                  Where p.building = ddlFilterBuildings.SelectedValue
      End Sub
  </code></pre>
  
  <p>This works great. It filters by whatever building is selected in the drop down list...but now a problem. Sometimes folks want to see all of the buildings - not just one. I attempted to add a null value which I hoped would select all - but this doesn't seem to work. So, now my dropdownlist in addition to the databound options includes:</p>
  
  <pre><code>    ddlFilterBuildings.Items.Insert(0, "-- Select Building --")
      ddlFilterBuildings.Items(0).Value = vbNull
  </code></pre>
  
  <p>But this just shows no rows in the table...and if I change to a specific building it doesn't change either.</p>