C# in Depth

Value types and reference types53passed by reference or the value of the argument (the reference) is passed by value.Aside from anything else, this answers the question of what happens when null isused as a by-value argument—if objects were being passed around, that would causeissues, as there wouldn’t be an object to pass! Instead, the null reference is passed byvalue in just the same way as any other reference would be.These myths aren’t the only ones around. Boxing and unboxing come in for theirfair share of misunderstanding, which I’ll try to clear up next.2.3.4 Boxing and unboxingSometimes, you just don’t want a value type value. You want a reference. There are anynumber of reasons why this can happen, and fortunately C# and .NET provide a mechanism,called boxing, that lets you create an object from a value type value and use areference to that new object. Before we leap straight into an example, let’s start off byreviewing two important facts:■■The value of a reference type variable is always a reference.The value of a value type variable is always a value of that type.Given those two facts, the following three lines of code don’t seem to make muchsense at first glance:int i = 5;object o = i;int j = (int) o;We have two variables: i is a value type variable, and o is a reference type variable. Howdoes it make sense to assign the value of i to o? The value of o has to be a reference,and the number 5 isn’t a reference—it’s an integer value. What’s actually happening isboxing: the runtime creates an object (on the heap—it’s a normal object) that containsthe value (5). The value of o is then a reference to that new object. The third lineperforms the reverse operation—unboxing. We have to tell the compiler which type tounbox the object as, and if we use the wrong type (if it’s a boxed uint or long, forexample, or not a boxed value at all) an InvalidCastException is thrown. 10That’s it, really—boxing and unboxing in a nutshell. The only remaining problemis knowing when boxing and unboxing occur. Unboxing is usually obvious, because thecast is present in the code. Boxing can be more subtle. We’ve seen the simple version,but it can also occur if you call the ToString, Equals, or GetHashCode methods on thevalue of a type that doesn’t override them, or if you use the value as an interface expression—assigningit to a variable whose type is an interface type or passing it as a parameterwith an interface type. For example, the statement IComparable x = 5; would boxthe number 5.It’s worth being aware of boxing and unboxing because of the potential performancepenalty involved. A single box or unbox operation is very cheap, but if you10 There are corner cases where the type doesn’t have to be exactly right, mostly to do with enums. These are sofiddly that even the C# language specification hasn’t got it quite right yet!Licensed to Rhona Hadida