if(this.ViewState["buttonCreated"] != null)
      {
        Button button=  new Button();
        button.ID = "button";
        button.Text = "un qualsiasi controllo";
        PlaceHolder1.Controls.Add(button);

      }

if(this.ViewState["buttonCreated"] == null)
      {

          Button button=  new Button();
          button.ID = "button";
          button.Text = "un qualsiasi controllo";
          PlaceHolder1.Controls.Add(button);
          this.ViewState["buttonCreated"] = 1;
      }

aspx:
<asp:DropDownList id="sceglisito" runat="server" AutoPostBack="True">
  <asp:ListItem Value="1">aspitalia</asp:ListItem>
  <asp:ListItem Value="2">winfxitalia</asp:ListItem>
</asp:DropDownList>
<asp:Label id="messaggio" runat="server"></asp:Label>

codebheind:
private void DropDownList1_SelectedIndexChanged(object sender, System.EventArgs e)
    {
      messaggio.Text = sceglisito.SelectedValue;
    }

aspx:
<asp:LinkButton id="premimi" runat="server">premi</asp:LinkButton>
<asp:Label id="messaggio" runat="server"></asp:Label>

codebehind
private void premimi_Click(object sender, System.EventArgs e)
{
messaggio.Text="pulsante premuto";
}

public class WebForm1 : System.Web.UI.Page
  {
    protected System.Web.UI.WebControls.Button carica;
    protected System.Web.UI.WebControls.PlaceHolder inserisci;
  
    private void Page_Load(object sender, System.EventArgs e)
    {
      string contenuto = "UserControl1.ascx";
      if(this.ViewState["contenuto"] != null)
      {
        contenuto = (string)this.ViewState["contenuto"];
      }
      Control myControl = Page.LoadControl(contenuto);
      myControl.ID = "contenuto";
      inserisci.Controls.Add(myControl);
    }
    private void carica_Click(object sender, System.EventArgs e)
    {
      Control c = inserisci.FindControl("contenuto");
      if(c != null)
      {
        inserisci.Controls.Remove(c);
      }
      this.ViewState["contenuto"] = "UserControl2.ascx";
      Control myControl = Page.LoadControl("UserControl2.ascx");
      myControl.ID = "contenuto";
      inserisci.Controls.Add(myControl);
    }
  }