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POHow does the garbage collector work with unit tests?
 

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  1. POHow does the garbage collector work with unit tests?
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    <p>Recently, I asked (and answered) a question on StackOverflow about why a unit test would work when run by itself and then fail sporadically when run with the whole batch of unit tests. See here: <a href="https://stackoverflow.com/questions/3072986/sql-server-and-transactionscope-with-msdtc-sporadically-cant-get-connection">SQL Server and TransactionScope (with MSDTC): Sporadically can&#39;t get connection</a></p> <p>Unit tests passing when running one at a time and then failing when run together is a classic sign that something is seriously wrong with the code.</p> <p>I discovered that there is a bit of a resource-leak. Because of a subtle error causing connections to a SQL server to not be released, I was running out of connections and tests were failing. AFAIK, this works almost exactly like a memory leak; connections are allocated from a connection pool and never freed just as memory can be allocated and then not freed.</p> <p>However, this does leave me with a puzzling question? What is the difference between running my tests one at a time and running them as a suite? If the tests pass when run one at a time and then fail when run together, then there must be some sort of clean-up happening between test runs that happens only when the tests are run one at a time.</p> <p>I conjecture that this could have something to do with what the .net garbage collector does or doesn't do between the tests. In one case, connections are freed between tests; in another case, they aren't.</p> <p>How can I explain this?</p> <p><b>Update</b>: To those of you asking about the specifics of the code, it's rather simple. I declare a new <code>TransactionScope</code> object in my Setup method and dispose it in my <code>Teardown</code> method. However, The problem test was a data-driven test with 100 test cases; the code under test populated a <code>SqlDataReader</code> object from a select statement using the SqlHelper class and then didn't call the close method on the <code>SqlDataReader</code>. Because I used the SqlHelper class to get the SqlDataReader, I expected that the connections were handled for me. Not so!</p> <p><b>But to clarify</b>, I'm not asking about my specific situation. What I want to know is: generally, how are resources freed between tests? I would imagine this would be some application of the garbage collector. I wonder if the garbage collector could still be cleaning up a previous test as the next test runs (race condition?)</p> <p><b>Update: What I know about garbage collection with Unit Tests.</b> Following my own curiosity, I pulled out the unit tests that were failing because a connection was left open by the <code>SqlDataReader</code> object. I tried adding <code>System.GC.Collect()</code> to the end of each test. This successfully freed the connections, but does impose an ~50% performance penalty.</p>
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    1. USDaniel Allen Langdon
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    1. COIf you are running out of connections because you do not release enough of them between tests, I'd suspect that there is a problem with your tear down methods. When running them one at a time, the OS takes care of some tear down, but run as a suite your resource usage persists longer.
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    2. COMaybe you just need some more `Dispose` or `Close` calls somewhere?
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