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copied!<p>Your business objects should throw exceptions for bad inputs, but those exceptions should never be thrown in the course of a normal program run. I know that sounds contradictory, so I shall explain.</p> <p>Each public method should validate its inputs, and throw "ArgumentException"s when they are incorrect. (And private methods should validate their inputs with "Debug.Assert()"s to ease development, but that's another story.) This rule about validating inputs to public methods (and properties, of course) is true for every layer of the application. </p> <p>The requirements of the software interface should be spelled out in the interface documentation, of course, and it is the job of the calling code to make sure the arguments are correct and the exceptions will never be thrown, which means the UI needs to validate the inputs before handing them to the business object.</p> <p>While the rules given above should almost never be broken, sometimes business object validation can be very complex, and that complexity shouldn't be foisted onto the UI. In that case it's good for the BO's interface to allow some leeway in what it accepts and then provide for an explicit Validate(out string[]) predicate to check the properties and give feedback on what needs to be changed. But notice in this case that there are still well-defined interface requirements and no exceptions need ever be thrown (assuming the calling code follows the rules).</p> <p>Following this latter system, I almost never do early validation on property setters, since that soft-of complicates the use of the properties, (but in the case given in the question, I might). (As an aside, please don't prevent me from tabbing out of a field just because it has bad data in it. I get clausterphobic when I can't tab around a form! I'll go back and fix it in a minute, I promise! OK, I feel better now, sorry.)</p>

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