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copied!<p>Inheritance is better when some condition is checking something intrinsic to your object that you check constantly (like a state or type). In this example, it's an operator for a mathematical expression tree. An operator, like addition, has an expression before and after the plus sign (<a href="http://en.wikipedia.org/wiki/Infix_notation" rel="nofollow noreferrer">infix notation</a>) so I call them left and right because it is represented as a tree. There are also unary operators like a hyphen that negates a value, but it only has one child expression.</p> <p>Using inheritance means the type (simplified to just unary vs. binary) is factored into the inheritance:</p> <pre><code>class Operator { abstract string ToString(); } class UnaryOperator : Operator { Expression Expression { get; } override string ToString() {...} } class BinaryOperator : Operator { Expression LeftExpression { get; } Expression RightExpression { get; } } class AdditionOperator : BinaryOperator { override string ToString() { return this.LeftExpression.ToString() + "+" + this.RightExpression.ToString(); } } </code></pre> <p>As opposed to:</p> <pre><code>class Operator { Expression LeftExpression { get; } Expression RightExpression { get; } OperatorType Type { get; } string ToString() { switch(this.Type) { case OperatorType.Addition: return this.LeftExpression.ToString() + "+" + this.RightExpression.ToString(); case OperatorType.Negation: return "-" + this.LeftExpression.ToString(); case ...: } } } enum OperatorType { // Unary operators Negation, // Binary operators Addition, Subtraction, Multiplication } </code></pre> <p>Here your Unary operator doesn't have a Left and a Right expression, just an expression. This means you have to introduce convention that Left is used and Right is null. ToString would have to check OperatorType; so would any other method that needs to act on what operator it is.</p> <p>The latter approach is most often seen in XML DOM implementations where XmlNode is pretty much everything any XML node could contain...ever. It makes for some very confusing situations:</p> <ul> <li>How can an attribute or text node have ChildNodes?</li> <li>How can a document have ChildNodes when it can only have one? (It need not be a collection)</li> <li>How can a document have a ParentNode?</li> <li>How can a document have a Value?</li> <li>Etc.</li> </ul>

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