Question

As I understand, using structure value types will always give better performance than using reference types in an array or list. Is there any downside involved in using struct instead of class type in a generic list?

PS : I am aware that MSDN recommends that struct should be maximum 16 bytes, but I have been using 100+ byte structure without problems so far. Also, when I get the maximum stack memory error exceeded for using a struct, I also run out of heap space if I use a class instead.

Answer 1

As others have pointed out, there are many downsides to using large structures in a list. Some ramifications of what others have said:

Say you’re sorting a list whose members are 100+ byte structures. Every time items have to be swapped, the following occurs:

var temp = list[i];
list[i] = list[j];
list[j] = temp;

The amount of data copied is 3*sizeof(your_struct). If you’re sorting a list that’s made up of reference types, the amount of data copied is 3*sizeof(IntPtr): 12 bytes in the 32-bit runtime, or 24 bytes in the 64-bit runtime. I can tell you from experience that copying large structures is far more expensive than the indirection inherent in using reference types.

Using structures also reduces the maximum number of items you can have in a list. In .NET, the maximum size of any single data structure is 2 gigabytes (minus a little bit). A list of structures has a maximum capacity of 2^31/sizeof(your_struct). So if your structure is 100 bytes in size, you can have at most about 21.5 million of them in a list. But if you use reference types, your maximum is about 536 million in the 32-bit runtime (although you’ll run out of memory before you reach that limit), or 268 million in the 64-bit runtime. And, yes, some of us really do work with that many things in memory.