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I consider myself aware of the concepts of threading and why certain code is or isn’t “thread-safe,” but as someone who primarily works with ASP.NET, threading and thread safety is something I rarely think about. However, I seem to run across numerous comments and answers (not necessarily for ASP.NET) on Stack Overflow to the effect of “warning – that’s not thread-safe!,” and it tends to make me second guess whether I’ve written similar code that actually could cause a problem in my applications. [shock, horror, etc.] So I’m compelled to ask:
Do ASP.NET developers really need to be concerned with thread safety?
My Take: While a web application is inherently multi-threaded, each particular request comes in on a single thread, and all non-static types you create, modify, or destroy are exclusive to that single thread/request. If the request creates an instance of a DAL object which creates an instance of a business object and I want to lazy-initialize a collection within this object, it doesn’t matter if it’s not thread-safe, because it will never be touched by another thread. ...Right? (Let’s assume I’m not starting a new thread to kick off a long-running asynchronous process during the request. I’m well aware that changes everything.)
Of course, static classes, methods and variables are just the opposite. They are shared by every request, and the developer must be very careful not to have “unsafe” code that when executed by one user, can have an unintended effect on all others.
But that’s about it, and thus thread safety in ASP.NET mostly boils down to this: Be careful how you design and use statics. Other than that, you don’t need to worry about it much at all.
Am I wrong about any of this? Do you disagree? Enlighten me!
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NET class libraries are not thread safe by default. Avoid providing static methods that alter static state. In common server scenarios, static state is shared across requests, which means multiple threads can execute that code at the same time. This opens up the possibility of threading bugs.
Thread safety becomes a concern if there is at least a single entry point which can be accessed by multiple threads. If a piece of code is accessed by multiple threads and is calling other method/class/etc., then all this code tree becomes vulnerable.
Every ASP.NET web application has its own pool of threads that is used for serving requests.
Conditionally safe: Different threads can access different objects simultaneously, and access to shared data is protected from race conditions. Not thread safe: Data structures should not be accessed simultaneously by different threads.
There are certain objects in addition to static items that are shared across all requests to an application. Be careful about putting items in the application cache that are not thread-safe, for example. Also, nothing prevents you from spawning your own threads for background processing while handling a request.
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There are different levels of ASP.NET Developers. You could make a perfectly fine career as an ASP.NET Developer without knowing anything threads, mutexes, locks, semaphores and even design patterns because a high percentage of ASP.NET applications are basically CRUD applications with little to no additional business logic.
However, most great ASP.NET Developers which I have come across aren't just ASP.NET Developers, their skills run the gamut so they know all about threading and other good stuff because they don't limit themselves to ASP.NET.
So no, for the most part ASP.NET Developers do not need to know about thread safety. But what fun is there in only knowing the bare minimum?
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