Multi View Problem that I Have

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C#

ASP.NET

i have the following MultiView coding

<table>
               <tr>
                   <td>
                       <asp:Button ID="bt1" runat="server" Text="Tab 1" BorderStyle="None" CssClass="Initial" OnClick="bt1_Click" />
                       <asp:Button ID="bt2" runat="server" Text="Tab 2" BorderStyle="None" CssClass="Initial" OnClick="bt2_Click" />
                       <asp:MultiView ID="mainView" runat="server">
                           <asp:View ID="View1" runat="server">
                               <table style="width: 100%; border-width: 1px; border-color: #666; border-style: solid">
                                   <tr><td>
                                       <h3><span>View 1 </span></h3>
                                     </td></tr>
                               </table>
                           </asp:View>
                           <asp:View ID="View2" runat="server">
                               <table style="width: 100%; border-width: 1px; border-color: #666; border-style: solid">
                                   <tr><td><h3><span>View 2 </span></h3>
                                     </td></tr>
                               </table>
                           </asp:View>
                       </asp:MultiView></td></tr></table>

and c# coding is

protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                bt1.CssClass = "Clicked";
                mainView.ActiveViewIndex = 0;                
            }
        }
protected void bt1_Click(object sender, EventArgs e)
        {
            bt1.CssClass = "Clicked";
            bt1.CssClass = "Initial";
            bt3.CssClass = "Initial";
            mainView.ActiveViewIndex = 0;        }
        protected void bt2_Click(object sender, EventArgs e)
        {
            bt1.CssClass = "Initial";
            bt2.CssClass = "Clicked";
            bt3.CssClass = "Initial";
            mainView.ActiveViewIndex = 1;
        }

when i run the project it give me the error which are (Compiler Error Message: CS1061: 'ASP.pages_default_aspx' does not contain a definition for 'bt1_Click' and no extension method 'bt1_Click' accepting a first argument of type 'ASP.pages_default_aspx' could be found (are you missing a using directive or an assembly reference?)