C# in Depth

Advanced generics89ArrayListobject[]BoxedbytesListbyte[]elementsref5elements5refrefref3ref315countref15count106refref106121312......13Figure 3.3 Visual demonstration of why List takes up a lot less space than ArrayListwhen storing value typesList and ArrayList have a buffer, and they create a larger buffer when theyneed to.)The difference in efficiency here is incredible. Let’s look at the ArrayList first,considering a 32-bit CLR. 8 Each of the boxed bytes will take up 8 bytes of object overhead,plus 4 bytes (1 byte, rounded up to a word boundary) for the data itself. On topof that, you’ve got all the references themselves, each of which takes up 4 bytes. So foreach byte of useful data, we’re paying at least 16 bytes—and then there’s the extraunused space for references in the buffer.Compare this with the List. Each byte in the list takes up a single bytewithin the elements array. There’s still “wasted” space in the buffer, waiting to be usedpotentially by new items—but at least we’re only wasting a single byte per unused elementthere.We don’t just gain space, but execution speed too. We don’t need the time taken toallocate the box, the type checking involved in unboxing the bytes in order to get atthem, or the garbage collection of the boxes when they’re no longer referenced.We don’t have to go down to the CLR level to find things happening transparentlyon our behalf, however. C# has always made life easier with syntactic shortcuts,and our next section looks at a familiar example but with a generic twist: iteratingwith foreach.8When running on a 64-bit CLR, the overheads are bigger.Licensed to Rhona Hadida