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    <p>There are many ways you can implement this in code, but here is in my humble opinion, the best way to go about solving the problem before programming anything:</p> <h3>Define Parts &amp; Products (Pre-code)</h3> <p>When defining all the "parts" it will be paramount to identify hierarchy and categorization for the parts. This is true because some rules may be exclusive to a unique part <em>(ex. "brown mustard only")</em>, some categorical <em>(ex. "all mustards")</em>, some by type <em>(ex. "all condiments")</em>, etc.</p> <h3>Build Rule Sets (Pre-code)</h3> <p>Define the rule sets (prerequisites, exclusions, etc.) for each unique part, category, type, and finished product.</p> <p>It may sound silly, but a lot of care must be taken to ensure the rules are defined with an appropriate scope. For example, if the finished product is a <code>Burger</code>:</p> <ul> <li>Unique Item Rule - <em>"Mushrooms only available with Blue Cheese selected"</em> <code>prerequisite</code></li> <li>Categorical Rule - <em>"Only 1 mustard may be selected"</em> <code>exclusive</code></li> <li>Type Rule - <em>"Pickles are incompatible with Peppers"</em> <code>exclusive</code> </li> </ul> <p>After having spent so much time on unique/category/type rules for "parts", many designers will overlook rules that apply only to the finished product even when the parts have no conflict.</p> <ul> <li>Product Rule - <em>"Maximum 5 condiments"</em> <code>condition</code></li> <li>Product Rule - <em>"Burger must have a bun"</em> <code>prerequisite</code></li> </ul> <p>This graph of rule can quickly grow very complex.</p> <h3>Suggestions for Building Data Structures (Code)</h3> <ol> <li><p>Ensure your structures accommodate hierarchy and categorization. For example: "brown mustard" and "dijon mustard" are individual objects, and they are both mustards, and both condiments.</p> <p>Carefully select the right combination of inheritance modeling (base classes) and object attributes (ex. <code>Category</code> property, or <code>HasCondiments</code> flag) to make this work.</p></li> <li><p>Make a <em>private field</em> for <code>RuleSets</code> at each hierarchic object level.</p></li> <li><p>Make <em>public properties</em> for a <code>HasConflicts</code> flag, and a <code>RuleViolations</code> collection.</p></li> <li><p>When a part is added to a product, check against all levels of rules (its own, category, type, and product) -- do this via a <em>public function</em> that can be called from the product. Or for better internalization, you can make an <em>event handler</em> on the part itself.</p></li> </ol> <h3>Write your algorithms (Code)</h3> <p>This is where I suck, and good thing as it is sort of beyond the scope of your question. </p> <p>The trick with this step will be how to implement in code the rules that travel up the tree/graph -- for example, when a specific part has issue with another part outside its scope, or how does it's validation get run when another part is added? My thought:</p> <ol> <li><p>Use a <em>public function</em> methodology for each part. Pass it the product's <code>CurrentParts</code> collection.</p></li> <li><p>On the Product object, have handlers defined to handle <code>OnPartAdded</code> and <code>OnPartRemoved</code>, and have them enumerate the <code>CurrentParts</code> collection and call each part's validation function.</p></li> </ol> <h3>Example Bare-bones Prototype</h3> <pre><code>interface IProduct { void AddPart(); void OnAddPart(); } // base class for products public class Product() : IProduct { // private or no setter. write functions as you like to add/remove parts. public ICollection&lt;Part&gt; CurrentParts { get; }; // Add part function adds to collection and triggers a handler. public void AddPart(Part p) { CurrentParts.Add(p); OnAddParts(); } // handler for adding a part should trigger part validations public void OnAddPart() { // validate part-scope rules, you'll want to return some message/exception foreach(var part in CurrentParts) { part.ValidateRules(CurrentParts); } ValidateRules(); // validate Product-scope rules. } } interface IProduct { // "object" should be replaced with whatever way you implement your rules void object RuleSet; void ValidateRules(ICollection&lt;Part&gt; otherParts); } // base class for parts public class Part : IPart { public object RuleSet; // see note in interface. public ValidateRules(ICollection&lt;Part&gt; otherParts) { // insert your algorithms here for validating // the product parts against this part's rule set. } } </code></pre> <p>Nice and clean.</p>
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