In a web server, when I delete the DLL files in the bin folder, the web page gives error.
How can I show message.html if there is no file in the bin folder?
ASP.NET has a feature which checks for the existence of a file called app_offline.htm in the root of your site.
If this file is present, the HTML page will be served for every request to the application. This sounds like what you're looking for.
I'm not exactly sure what you're trying to accomplish, but you can put an app_offline.htm file in your site root, then modify any files/assemblies as you need to. Then, remove app_offline.htm when you're done.