web function call

Well it is bit difficult to send response from windows to web and again from web to window. What you can do is create a unique key while creating the exe for windows application and ask the client to use it for first time installation. So this way you can secure only authenticate user can use your application.

Amit that is not a good idea I believe. It’s also risky. Sending username and password in query string is not acceptable. Now the most wired part is you are validating the username and password in web application and based on result u want to proceed in windows application. This is no way acceptable. Say one case the server for web application is down and you want to validate some user in windows application then you need to wait for hours to accomplish the goal. What you can do to validate the user in your windows form only. To do that you should have same database connected for both windows and web application or you should have at least one common table for username and password and make sure if any username and Password create by new user or update it, it should update both the places but the best way to get rid of it to have a single database for both the application.

Ya you can call it in button click of windows application

System.Diagnostics.Process.Start(www.yahoo.com);

if you are passing some query string then validate it in web page.

Try This:

System.Diagnostics.Process.Start(www.yahoo.com);