Value Types and Memory Usage

Last week a respected colleague mentioned off hand that sorting on a value type takes a lot of memory in C#. Interested, I looked into this to see why/when this is true.

Value types (using the struct keyword) are always passed by value, unlike reference types (class keyword) which are always passed by reference. This means that every time you pass them into a method, the whole value is copied; whereas with reference types, only the reference (pointer) is copied. Pointers are 4 to 8 bytes, so his original statement is only of concern if your value types are larger than that. Some such types are DateTime, Guid, and BsonObjectId.

Some people like to think of value types as being allocated on the stack (versus the heap). In C#, this is irrelevant. The CLR allocates value and reference types wherever it feels like. Usually, local variables and parameters are stored on the stack (or registers) and values that are members of a class are usually allocated on the heap. It was done this way because the folks who wrote the CLR believe they can do a good enough job of optimizing stack and heap usage, so you shouldn’t worry about it. If you’re in C#, you shouldn’t care where they’re allocated. If you’re doing something that requires you to care, you need to either break into an unsafe C# code block or C++.

As for his actual statement – yes, using Base Class Library algorithms for sorting on value types will take more memory for value types than reference types because it has to copy values. However, there are exceptions to this.

You can always write method parameters with the ref keyword so they’re passed by reference. This would fix the problem of copying, but the all of the BCL classes* are written generically by using IComparable or some other interface. When you cast a value type like an Int32 to an interface like IComparable, it has to be boxed into a reference type. When boxing, the CLR allocates a managed reference type object and then copies the Int32 value into the managed container. It copies the value again when unboxing.

In summary, sorting on a value type can take quite a bit more memory than sorting on reference types. However, it is possible to write your own sorting algorithm that always passes by reference and doesn’t use any additional memory (but who does that?).


* One might point out that generic classes like List<int> have a Sort() method. However, this casts int to IComparable while sorting.