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copied!<p>Both covariance and contravariance in C# 4.0 refer to the ability of using a derived class instead of base class. The in/out keywords are compiler hints to indicate whether or not the type parameters will be used for input and output.</p> <h3>Covariance</h3> <p>Covariance in C# 4.0 is aided by <code>out</code> keyword and it means that a generic type using a derived class of the <code>out</code> type parameter is OK. Hence</p> <pre><code>IEnumerable<Fruit> fruit = new List<Apple>(); </code></pre> <p>Since <code>Apple</code> is a <code>Fruit</code>, <code>List<Apple></code> can be safely used as <code>IEnumerable<Fruit></code></p> <h3>Contravariance</h3> <p>Contravariance is the <code>in</code> keyword and it denotes input types, usually in delegates. The principle is the same, it means that the delegate can accept more derived class.</p> <pre><code>public delegate void Func<in T>(T param); </code></pre> <p>This means that if we have a <code>Func<Fruit></code>, it can be converted to <code>Func<Apple></code>.</p> <pre><code>Func<Fruit> fruitFunc = (fruit)=>{}; Func<Apple> appleFunc = fruitFunc; </code></pre> <h3>Why are they called co/contravariance if they are basically the same thing?</h3> <p>Because even though the principle is the same, safe casting from derived to base, when used on the input types, we can safely cast a less derived type (<code>Func<Fruit></code>) to a more derived type (<code>Func<Apple></code>), which makes sense, since any function that takes <code>Fruit</code>, can also take <code>Apple</code>.</p>

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