I don't think you need to use an interface. Why not just add a Setup method that a UserControl derived class?

public partial class crlShipItems : UserControl
{
   public void Setup()
   {
      ...
   }
}

private void _AddPanel(string sKey)
{
    crlShipItems control;

    switch (sKey)
    {
        case "ship_items":
            control = new crlShipItems();

            this.Controls.Add(control);
            control.Left = 0;
            control.Top = 0;
            control.Width = this.Width;
            control.Height = this.Height;

            control.Setup();
            break;
       ...
    }
}

>I'm feeling frustrated because I cannot get this to work:
>
>I want to create a series of user controls that will be added to a form at runtime. They will obviously need to have certain similar properties and methods such as Setup() and SaveChanges();
>
>So I first tried this:

>private void _AddPanel(string sKey)
>{
>    UserControl control;
>
>    switch (sKey)
>    {
>        case "ship_items":
>            control = new crlShipItems();
>
>            this.Controls.Add(control);
>            control.Left = 0;
>            control.Top = 0;
>            control.Width = this.Width;
>            control.Height = this.Height;
>
>            control.Setup();
>
>            break;
>
>        case "sview_inv":
>            break;
>
>        case "sreports":
>            break;
>    }
>}

>But this doesn't compile because there is not Setup() on a UserControl. So I tried this:
>
>I first created an interface:

>public interface IPanel
>{
>    void Setup();
>    bool SaveChanages();
>}

>I implemented the interface in the header of the Panel control's definition:

public partial class crlShipItems : UserControl, IPanel

>Then I changed the _AddPanel method:

>private void _AddPanel(string sKey)
>{
>    IPanel control;
>
>    switch (sKey)
>    {
>        case "ship_items":
>            control = new crlShipItems();
>
>            this.Controls.Add(control);
>            control.Left = 0;
>            control.Top = 0;
>            control.Width = this.Width;
>            control.Height = this.Height;
>
>            control.Setup();
>
>            break;
>
>        case "sview_inv":
>            break;
>
>        case "sreports":
>            break;
>    }
>}

>Not I get 5 compilations errors - one for each of the Top,Left, Width and Height, and on on the Controls.Add saying "The best overloaded method match for 'System.Windows.Forms.Control.ControlCollection.Add(System.Windows.Forms.Control)' has some invalid arguments", probably because it's no longer a control.