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Protecting Shared Fields in Asynchronous Processing

Sometimes, in a C# application with multiple threads running simultaneously, you need to share data. So you set up a variable declared outside of any method or property (a "field") and have the different threads share data by updating and reading that variable. It should work ... but it doesn't and you can't figure out why.

The odds are that there's some bug in your asynchronous code that's creating the problem. However (and this is a long shot), it might be the compiler's fault. Some of the optimizations the compiler applies to a field assume that the field will only be accessed by a single thread at a time. This means your source code might look fine ... but the optimized code you're executing is doing something different.

There are two solutions in C#: First, put a lock around any code that accesses the field (this option is also available in Visual Basic). You might take a hit on performance as threads queue up to get to your field, but it ensures that you both keep the compiler's optimizations and have only one thread accessing the field at a time.

If you don't think you can find all the places that the field is being read or written (or are too lazy to look) you can, in C#, mark the variable as volatile:

internal volatile string Status;

The volatile keyword causes the compiler to omit the optimizations that assume single-thread access. You'll still take a performance hit because, internally, the compiler will cause reads and writes to your variable to be processed in sequence. But, now, when your application still doesn't work, you'll know it's your fault.

Visual Basic doesn't have an equivalent to the volatile keyword. However, if you can track down all the places where you read or write the field, you can use the Thread object's VolatileRead and VolatileWrite methods when working with the field.

Posted by Peter Vogel on 11/04/2015 at 2:20 PM


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