Set MenuItem as Active using CSS class based on URL of the Current Page in ASP.Net

You can check the URL and based on that select the Menu link

<li class='<%=Request.Url.AbsolutePath.ToLower().Contains("home") ? "active" : "" %>'>Home</li>
<li class='<%=Request.Url.AbsolutePath.ToLower().Contains("contact") ? "active" : "" %>'>Contact</li>

i don't understand where to write it?

In the aspx page where you have <li>

<li class='<%=Request.Url.AbsolutePath.ToLower().Contains("Home") ? "active" : "" %>'><asp:LinkButton ID="LinkButton1" runat="server" onclick="LinkButton1_Click">Home</asp:LinkButton></li>

      <li class='<%=Request.Url.AbsolutePath.ToLower().Contains("Contact") ? "active" : "" %>'><li><asp:LinkButton ID="LinkButton2" runat="server" onclick="LinkButton2_Click">Contact</asp:LinkButton></li>  

I wrote above code. but it is not working

For this to work your page must contain words Home and Contact. Put your page names instead