How to share this code?

I have the following method that I include in most of my ASPX.CS files:
  /// <summary>
  /// Standard method to display a pop-up message box to the user.
  /// </summary>
  /// <param name="message"></param>
  protected void ShowMessageBox(string message)
  {
    ScriptManager.RegisterStartupScript(Page, typeof(Page), "", "alert('" + message + "');", true);
  }
 
I tried placing this method inside of a shared DLL but it won't work, I think because the ScriptManager reference is not there.  With Javascript I can share commonly used function in one file.  Is there something equivalent for C# methods that won't function in a DLL?
Robert

...