title: SqlDataSource with DropDownList Control Parameter doesn't read value
body: <p>I have a drop down list that bound to a SqlDataSource.</p>
  
  <p>I have another drop down list that's bound to a different SqlDataSource.</p>
  
  <p>The second SqlDataSource has the first drop down as a Control Parameter.</p>
  
  <p>I'm trying to do this...</p>
  
  <pre><code>&lt;asp:SqlDataSource ID="sqlDataSource2" runat="server"
      ConnectionString="&lt;%$ ConnectionStrings:ConnectionString %&gt;"
      SelectCommand="SELECT * FROM Test WHERE Param = @param;"
      CancelSelectOnNullParameter="true"&gt;
      &lt;SelectParameters&gt;
          &lt;asp:ControlParameter ControlID="dropDown1" Name="param"
              PropertyName="SelectedValue"
              ConvertEmptyStringToNull="true" /&gt;
      &lt;/SelectParameters&gt;
  &lt;/asp:SqlDataSource&gt;
  
  dropDown1.SelectedValue = "someValue"
  dropDown2.DataBind()
  </code></pre>
  
  <p>but I don't get any results. However, if I set the second SqlDataSource's Control Parameter to a text box, it works. For example, this works:</p>
  
  <pre><code>&lt;asp:ControlParameter ControlID="txt" Name="param"
      PropertyName="Text"
      ConvertEmptyStringToNull="true" /&gt;
  
  txt.Text = "someValue"
  dropDown2.DataBind()
  </code></pre>
  
  <p>Any ideas why this is?</p>