Start the DbgClr debugger that comes with the .NET Framework SDK, open the file containing the code you want to debug, and set your breakpoints. Start the ASP.NET application. Go back to DbgClr, choose "Debug Processes" from the Tools menu, and select "aspnet_wp.exe" from the list of processes. (If "aspnet_wp.exe" doesn't appear in the list then check the "Show system processes" box.) Click the "Attach" button to attach to "aspnet_wp.exe" and begin debugging.
Be sure to enable debugging in the ASPX file before debugging it with DbgClr. You can enable ASP.NET to build debug executables by placing a:
<%@ Page Debug="true" %>
statement at the top of an ASPX file or a:
<COMPILATION debug="true" />
statement in a Web.config file.