The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>).

about the error:

The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>).


Use <%# this.txtCompany.ClientID %> to replace <%= this.txtCompany.ClientID %> to fix the problem.
Ref: http://west-wind.com/weblog/posts/5758.aspx