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copied!<p>In .Net:</p> <p>Often you can't rely on what the type of variable a function will consume, so you need to use an object variable which extends from the lowest common denominator - in .Net this is <code>object</code>.</p> <p>However <code>object</code> is a class and stores its contents as a reference.</p> <pre><code>List<int> notBoxed = new List<int> { 1, 2, 3 }; int i = notBoxed[1]; // this is the actual value List<object> boxed = new List<object> { 1, 2, 3 }; int j = (int) boxed[1]; // this is an object that can be 'unboxed' to an int </code></pre> <p>While both these hold the same information the second list is larger and slower. Each value in the second list is actually a reference to an <code>object</code> that holds the <code>int</code>.</p> <p>This is called boxed because the <code>int</code> is wrapped by the <code>object</code>. When its cast back the <code>int</code> is unboxed - converted back to it's value.</p> <p>For value types (i.e. all <code>structs</code>) this is slow, and potentially uses a lot more space. </p> <p>For reference types (i.e. all <code>classes</code>) this is far less of a problem, as they are stored as a reference anyway.</p> <p>A further problem with a boxed value type is that it's not obvious that you're dealing with the box, rather than the value. When you compare two <code>structs</code> then you're comparing values, but when you compare two <code>classes</code> then (by default) you're comparing the reference - i.e. are these the same instance?</p> <p>This can be confusing when dealing with boxed value types:</p> <pre><code>int a = 7; int b = 7; if(a == b) // Evaluates to true, because a and b have the same value object c = (object) 7; object d = (object) 7; if(c == d) // Evaluates to false, because c and d are different instances </code></pre> <p>It's easy to work around:</p> <pre><code>if(c.Equals(d)) // Evaluates to true because it calls the underlying int's equals if(((int) c) == ((int) d)) // Evaluates to true once the values are cast </code></pre> <p>However it is another thing to be careful of when dealing with boxed values.</p>

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