how to make online video chat application in c#,net?

What kind of a question is that? Definitely not suitable for Q&A. You obviously didn't do any research at all, just want to use other people's work? Ever heard of Google? Or tried to search here on Codeproject? If you would have done that, you would have found many articles, that might be of help. I listed some of those articles below, since I am in good mood today. Maybe you want to have a look at them, and actually do some work on your own? But probably you only wanted the complete project handed over on a silver plate. Not going to happen.

Work through this and come back, if you have a real question or problem.

Picture Chat Using C#[^]
Building an AJAX based chat room in ASP.NET[^]
Chat Client Server[^]
Chat With US DI[^]
ASP.Net chatting using WCF Services and JSon[^]
A. R. Live Support: XML Based Customer Support Chat Solution[^]
Build a Web based Chat using ASP.NET Ajax[^]
Chat Application in ASP.NET Using AJAX (Pop-up Windows)[^]
Writing ASP Chat using JavaScript and XML (Part 2)[^]
Chat Application using Web services in C#[^]
Java Chat With Customizable GUI[^]
Google Chat Desktop Application using Jabber.Net[^]
Web User control in ASP.NET with AJAX[^]
Chat program[^]
Simple Chat Application in ASP.NET[^]
Global Text Chat Room Application using C#.NET Remoting Technology[^]
[A Simple] AI Chatbot Wizard: Create, Train and Chat[^]
Writing ASP Chat using JavaScript and XML (Part 1)[^]
Simple Voice Chat[^]
Simple chat application for ASP.NET[^]

If you have the feeling, that you saw this exact answer before, then you are right. I posted exactly the same thing yesterday, and already three other times before. Seriously, how difficult is it to do at least a tiny bit of research yourself?????