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POS.O.L.I.D Essentials missing points?
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copied!<p><em>I've read so many articles about this but still I have 2 questions.</em></p> <p><strong>Question #1</strong> - Regarding <a href="http://www.remondo.net/solid-principles-csharp-dependency-inversion/" rel="nofollow noreferrer">Dependency Inversion</a> :</p> <blockquote> <p>It states that high-level classes should not depend on low-level classes. Both should depend on abstractions. Abstractions should not depend on details. Details should depend on abstractions.</p> </blockquote> <p>for example :</p> <pre><code>public class BirthdayCalculator { private readonly List<Birthday> _birthdays; public BirthdayCalculator() { _birthdays = new List<Birthday>();// <----- here is a dependency } ... </code></pre> <p>The fix : put it in a ctor.</p> <pre><code>public class BirthdayCalculator { private readonly IList<Birthday> _birthdays; public BirthdayCalculator(IList<Birthday> birthdays) { _birthdays = birthdays; } </code></pre> <ul> <li><p>If it will be in ctor - I'll have to send it each and every time I use the class. So I will have to keep it when calling the <code>BirthdayCalculator</code> class. it it ok to do it like that ? </p></li> <li><p>I can argue that , after the fix , still - <code>IList<Birthday> _birthdays</code> should not by there ( the <code>Birthday</code> in <code>IList</code>) - but it should be as <code>IList<IBirthday></code>. Am I right ? </p></li> </ul> <p><strong>Question #2</strong> - Regarding <a href="http://www.remondo.net/solid-principles-csharp-liskov-substitution/" rel="nofollow noreferrer">Liskov Substitution</a> :</p> <blockquote> <p>derived classes must be substitutable for their base classes</p> </blockquote> <p>or more accurate : </p> <blockquote> <p>Let q(x) be a property provable about objects x of type T. Then q(y) should be true for objects y of type S where S is a subtype of T.</p> </blockquote> <p><em>(already read <a href="https://stackoverflow.com/questions/1735137/liskov-substitution-principle-no-overriding-virtual-methods">this</a> )</em></p> <p>example :</p> <pre><code>public abstract class Account { public abstract void Deposit(double amount); } </code></pre> <p>I have a class : </p> <pre><code>public class CheckingAccount : Account { public override void Deposit(double amount) { ... _currentBalance += amount; } } </code></pre> <p>and the bank wants to open a Mortgage account - so : </p> <pre><code>public class MortgageAccount : Account { public override void Deposit(double amount) { _currentBalance -= amount; //<-----notice the minus } } </code></pre> <p>The problem arises when there is a function which accepts mortage as <code>Account</code> and do deposit.</p> <pre><code>public class Bank { public void ReceiveMoney(Account account, double amount) { double oldBalance = account.CurrentBalance; account.Deposit(amount); //oopssss????? } } </code></pre> <p>so here , it violates the LSP.</p> <p><strong>But</strong> I don't understand.</p> <p>every overridden method will do different code when overridden so it will <em>never</em> be <em>100%</em> replaceable !</p> <p>the definition did NOT talk about " logic should continue as in base class ( always deposit(add) positive numbers)"</p> <p>example:</p> <p>What if both <code>CheckingAccount</code> class and <code>MortgageAccount</code> class were depositing positive numbers but <code>MortgageAccount</code> also log to db ? does it still breaks LSP ? What is the boundry for breaks/not-brakes LSP ?</p> <p>the definition should define whatis that boundry. and it isn't saying anything about it.</p> <p>what am i missing ?</p>

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