Is working with the Session thread-safe?

Two requests to an ASP.NET application for the same same session, where the handlers do not use the IReadOnlySessionState marker interface or have the EnableSessionState="ReadOnly" turned on for your pages, will be serialized by the ASP.NET runtime to guarantee consistency of the state. So if you have two pages that are able to write to the session state, they will be accessed serially no matter what the client does on their side.

It's up to your application code to signal to ASP.NET with the afforementioned techniques whether or not a page/handler is going to write to the session state. If you do not, all requests will be serialized and the performance of your web application will suffer.

As always, the answer depends on what you mean by "safety." In ASP .NET, each request gets exclusive access to its session state. This means that you don't have to worry about synchronizing access within the scope of a single request. If Session["user"] is non-null, then it will be non-null for the entire duration of the current request. In your example, request 1 will never throw a null reference exception.

In ASP.NET, the Session module uses a pair of reader/writer locks per session, so Request 1 will have consistent reads, and Request 2 will block until Request 1 completes.