Not a good idea according to some
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* ''Some people might advise you to invest time upfront to create a whole set of tests. Instead I recommend that every time you touch a piece of legacy code (either to fix a bug or to enhance the functionality), you perform a couple of safe refactoring to enable you to create a few scaffolding tests, then clean up the code (may be even test drive the new code) and then get rid of the scaffolding tests.''
* ''Even though this approach might appear to be slower, why does this approach work better?''
** ''You start seeing some immediate returns.''
** ''On any given system, there are parts of the system which are more fragile and needs more attention than others. There are parts of code which we actively touch and others we rarely touch. When we have a limit time, it does not make sense in investing effort to create tests for areas that are fairly stable or rarely changed. Big upfront test creation might not take this aspect into account. So you might not get the biggest bang for your buck.''
** ''The first few tests we usually write are fragile tests. But we won’t get this feedback nor the opportunity to improve the quality of our tests until its too late.''
** ''When we get into a test creation mode, everyone is focusing on creating more and more tests.'' 
** ''Finally when we start using the tests we’ve created, a small change in production code might breaks a whole bunch of tests. First few times developers wonder what happened, but if this generates a lot of false-negative (which usually they do), then developers start ignoring or deleting those tests. So the investment does not really pay for itself.''
** ''Also when we have a whole lot of tests prematurely written, they start getting in the way of refactoring and genuinely improving the design of the code. (Defeats the whole point of creating test upfront so we can refactor.)''
** ''People get too attached to the tests they had written (it was a big investment). They somehow want to make the test work. People fail to realize that those fragile tests are slowing them down and getting in their way.''
** ''Unless the team gets into the habit of gradually building better test coverage, they will always play the catch up game with requirements constantly changing. (Remember we are chasing a moving target.)''
** ''Its usually hard to take a fixed (usually long) duration of time off from CRs and bug fixes. People will be forced to multi-task.''
*** http://blogs.agilefaqs.com/2011/03/09/big-upfront-test-creation-in-legacy-code-is-a-bad-idea/