[ThreadStatic] dosen't work with instance fields

ThreadStatic 属性对于字段无效

最近使用RESHARPER,发现有这个提示，查了下资料

Occa­sion­ally you might want to make the value of a sta­tic or instance field local to a thread (i.e. each thread holds an inde­pen­dent copy of the field), what you need in this case, is a thread-local stor­age.

In C#, there are mainly two ways to do this.

Thread­Sta­tic

You can mark a field with the Thread­Sta­tic attribute:

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[ThreadStatic] public static int _x; … Enumerable.Range(1, 10).Select(i => new Thread(() => Console.WriteLine(_x++))).ToList()           .ForEach(t => t.Start()); // prints 0 ten times

Whilst this is the eas­i­est way to imple­ment thread-local stor­age in C# it’s impor­tant to under­stand the lim­i­ta­tions here:

• the Thread­Sta­tic attribute doesn’t work with instance fields, it com­piles and runs but does nothing..

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[ThreadStatic] public int _x; … Enumerable.Range(1, 10).Select(i => new Thread(() => Console.WriteLine(_x++))).ToList()           .ForEach(t => t.Start()); // prints 0, 1, 2, … 9

• field always start with the default value

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[ThreadStatic] public static int _x = 1; … Enumerable.Range(1, 10).Select(i => new Thread(() => Console.WriteLine(_x++))).ToList()           .ForEach(t => t.Start()); // prints 0 ten times

ThreadLocal<T>

C#  4 has intro­duced a new class specif­i­cally for the thread-local stor­age of data – the ThreadLocal<T> class:

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private readonly ThreadLocal<int> _localX = new ThreadLocal<int>(() => 1); … Enumerable.Range(1, 10).Select(i => new Thread(() => Console.WriteLine(_localX++))).ToList()           .ForEach(t => t.Start()); // prints 1 ten times

There are some bonuses to using the ThreadLocal<T> class:

• val­ues are lazily eval­u­ated, the fac­tory func­tion eval­u­ates on the first call for each thread
• you have more con­trol over the ini­tial­iza­tion of the field and is able to ini­tial­ize the field with a non-default value

Sum­mary

As you can see, using ThreadLocal<T> has some clear advan­tages over Thread­Sta­tic, though using 4.0 only fea­tures like ThreadLocal<T> means you have to tar­get your project at the .Net 4 frame­work and is there­fore not back­ward com­pat­i­ble with pre­vi­ous ver­sions of the framework.

It’s also worth not­ing that besides ThreadLocal<T> and Thread­Sta­tic you can also use Thread.GetData and Thread.SetData to fetch and store thread spe­cific data from and to a named Local­Data­S­toreS­lot though this is usu­ally cumbersome…