hi Friends i need to comunicate my asp.net c# Web Based Application with Autocad.How can i do
How do you do it would depend on where you want the commnication occurs: on server side or on client (user) side.
It is not technocally sound to run the resource hungry desktop app AutoCAD on a web server and let your APS.NET app on the server to communicate to it, not to metion the AutoCAD license restriction.
If the communication must be on the server side, you may look to other alternatives to running AutoCAD itself on unattented server. For example, if all you need it get data froom DWG files, you may want to use RealDwg SDK; or using accoreconsole.exe available since AutoCAD2013, or wait for Autodesk's core services as cloud services.
Without going into details of what your real need is, there are ways to incoporate AST.NET technologies with AutoCAD. See following articles I posted recently (in the AutoCAD side, it uses AutoCAD .NET API, but you get the ideas in terms of working with ASP.NET):
http://drive-cad-with-code.blogspot.ca/2014/01/using-astnet-web-api-to-automate-autocad.html
http://drive-cad-with-code.blogspot.ca/2014/03/real-time-autocad-command-monitoring.html
HTH