title: Filtering DropDownList would not work
body: <p>I have several DropDownList in my website, and they are filtering each other.</p>
  
  <p>So,  I have a school, based on that I have classes, and inside of that classes I have students.  Each one has its own table on the DB, and it's generated from a table that has all the IDs.</p>
  
  <p>I dont know why, but I can filter the classes from the school, but the students DropdownList wouldn't be affected by the filter.</p>
  
  <p>This is my code :</p>
  
  <pre><code>&lt;li&gt;School &lt;/li&gt;
  &lt;li&gt;
      &lt;asp:DropDownList ID="SchoolBox" runat="server" AutoPostBack="True" 
          DataSourceID="DropDownlistSchool" DataTextField="SchoolName" 
          DataValueField="ID"&gt;
      &lt;/asp:DropDownList&gt; 
      &lt;asp:SqlDataSource ID="DropDownlistSchool" runat="server" 
          ConnectionString="&lt;%$ ConnectionStrings:DanielConnectionString %&gt;" 
          SelectCommand="SELECT [SchoolName], [ID] FROM [Schools]"&gt;
      &lt;/asp:SqlDataSource&gt;
  &lt;/li&gt;
  &lt;li&gt;Class&lt;/li&gt;
  &lt;li&gt;
      &lt;asp:DropDownList ID="ClassBox" runat="server" AutoPostBack="True" 
          DataSourceID="Class2" DataTextField="ClassName" DataValueField="ID"&gt;
      &lt;/asp:DropDownList&gt;
  
      &lt;asp:SqlDataSource ID="Class2" runat="server" 
          ConnectionString="&lt;%$ ConnectionStrings:DanielConnectionString %&gt;" 
          SelectCommand="SELECT * FROM [Class] WHERE ([SchoolID] = @SchoolID)"&gt;
          &lt;SelectParameters&gt;
              &lt;asp:ControlParameter ControlID="SchoolBox" Name="SchoolID" 
                  PropertyName="SelectedValue" Type="Int32" /&gt;
          &lt;/SelectParameters&gt;
      &lt;/asp:SqlDataSource&gt;
  
  &lt;/li&gt;
  &lt;li&gt;Student&lt;/li&gt;
  &lt;li&gt;
      &lt;asp:DropDownList ID="StudentBox" runat="server" AutoPostBack="True" 
          DataSourceID="Student" DataTextField="Username" DataValueField="ID"&gt;
      &lt;/asp:DropDownList&gt;
      &lt;asp:SqlDataSource ID="Student" runat="server" 
          ConnectionString="&lt;%$ ConnectionStrings:DanielConnectionString %&gt;" 
  
          SelectCommand="SELECT * FROM [Users] WHERE (([ClassID] = @ClassID) AND ([SchoolID] = @SchoolID))"&gt;
          &lt;SelectParameters&gt;
              &lt;asp:ControlParameter ControlID="ClassBox" Name="ClassID" 
                  PropertyName="SelectedValue" Type="Int32" /&gt;
              &lt;asp:ControlParameter ControlID="SchoolBox" Name="SchoolID" 
                  PropertyName="SelectedValue" Type="Int32" /&gt;
          &lt;/SelectParameters&gt;
      &lt;/asp:SqlDataSource&gt;
  </code></pre>