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    <p>Thread safety is not something you should add to your variables, it is something you should add to your "logic". If you add locks to all your variables, your code will still not necessarily be thread safe, but it will be slow as hell. To write a thread-safe program, Look at your code and decide where multiple threads could be using the same data/objects. Add locks or other safety measures to all those critical places.</p> <p>For instance, assuming the following bit of pseudo code:</p> <pre><code>void updateAvgBuyPrice() { float oldPrice = AvgBuyPrice; float newPrice = oldPrice + &lt;Some other logic here&gt; //Some more new price calculation here AvgBuyPrice = newPrice; } </code></pre> <p>If this code is called from multiple threads at the same time, your locking logic has no use. Imagine thread A getting AvgBuyPrice and doing some calculations. Now before it is done, thread B is also getting the AvgBuyPrice and starting calculations. Thread A in the meantime is done and will assign the new value to AvgBuyPrice. However, just moments later, it will be overwritten by thread B (which still used the old value) and the work of thread A has been lost completely.</p> <p>So how do you fix this? If we were to use locks (which would be the ugliest and slowest solution, but the easiest if you're just starting with multithreading), we need to put all the logic which changes AvgBuyPrice in locks:</p> <pre><code>void updateAvgBuyPrice() { lock(AvgBuyPriceLocker) { float oldPrice = AvgBuyPrice; float newPrice = oldPrice + &lt;Some other code here&gt; //Some more new price calculation here AvgBuyPrice = newPrice; } } </code></pre> <p>Now, if thread B wants to do the calculations while thread A is still busy, it will wait until thread A is done and then do its work using the new value. Keep in mind though, that any other code that also modifies AvgBuyPrice should also lock AvgBuyPriceLocker while it's working!</p> <p>Still, this will be slow if used often. Locks are expensive and there are a lot of other mechanism to avoid locks, just search for lock-free algorithms.</p>
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    1. POThread Safe Properties in C#
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    1. USMarcel Gosselin
    1. USMart
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    1. COTo add to Mart's comment, Joseph Albahari has written [an excellent online book](http://www.albahari.com/threading/) regarding all threading aspects. The book is available as HTML or PDF.
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