using C# how to make any browser to open only one site ?

You can embed BrowserControl (whatever it is; it will depend on your application type) to implement such behavior. As to the "any browser", the question does not even make sense. Here is why: 1) this is not the behavior of the existing browsers; even if you modify one more of existing products, it won't be "any"; 2) if you don't modify some browser product, it will browse to any Web site, by definition; 3) if you do modify any of the browsers in this way, it cannot be called "browser" anymore. :-)

You can only do it by blocking and redirecting the whole network traffic on some system of network. For example, you could can hide the outer world behind a proxy: http://en.wikipedia.org/wiki/Proxy_server#Web_proxy_servers[^].

Anyway, the whole requirements looks like totally dirty busyness; and this is the main part of my answer. Why needs the Web if everything is redirected to Yahoo, and who needs Yahoo if all Yahoo links are blocked?

—SA