How to register custom server control on ASP.NET page

Well, if this control is in another class library, or even if it's in the same one, it wouldn't be a bad idea to specify control's assembly in @Register:

<%@ Register TagPrefix="myControls" Namespace="MyApp.Controls" Assembly="MyApp" %>   <myControls:CustomControl runat="server" Text="What's up!" />

Clean and rebuild your solution too in order to verify everything is compiled rightly!

If your control will be reused on several pages, you may want to register it in web.config, as one of system.web/pages/controls subelements instead of copy-pasting the same <@Register tag in all affected pages.

web.config:

<system.web>  <pages ...>    <controls>      ...      <add tagPrefix="myCompany" namespace="MyCompany.Whatever.Controls" assembly="Whatever"/>    </controls>

thepage.aspx:

<myCompany:ControlClassName ID="TheStuff" runat="server" ... />

You should put your control either under the App_Code folder (in the case if the control not in assembly) or add a reference to assembly where this control is:

<%@ Register TagPrefix="myControls" Namespace="MyApp.Controls"      Assembly="SomeAssembly" %>

But guessing, your control not under the App_Code folder.