Class VS ref Struct

Question

I am programming a game using C#, thus, I am very concerned about performance.

I would like to know what are the main differences, and if possible, performance considerations of using either a Class to pass data around, or a struct passed by reference.

I wish not to copy the data around, for performance reasons (I assume passing by ref is much faster than by value here).

I know that a class is always passed by reference and that a struct is passed by value, but I talking about passing the struct by reference here.

An example of the data I wish to pass :

    public delegate void PathCompleteDelegate(List<PathFinderNode> path);
public struct PathFinderJob{
    public PathCompleteDelegate callback;
    public Vector3 start, end;
    public PathSize unitSize;
    public BoxCollider boxCollider;
}

In the previous example, would using a class make a difference? If so, what would the difference be? Would a class be faster than a struct in this example? Why?

Thank you. Joao Carlos

Septum

Have you tried performance profiling both approaches? If the struct is scoped outside of a method it will be on the heap anyway (as in, contained within a class on the heap). I would suspect class or ref struct to be the same performance in terms of method calls. – Downstroke

I dont have much experience profiling, and I did a quick profiling check using Unity3D's built in profiler, but, I did not see a difference, however, I dont know how good the built in profiler is with this type of "small fish", or if it would see a difference at all among all the other game related profiling happening. – Septum

I wouldn't waste my time on structs while programming a game in a OO language. The problem I would face before getting performance problems is not finishing the game. Classes and objects increase the dev speed IMO. – Inventor

Answer 1

Your delegate receives a reference type - a List, so you're passing the entire list by reference anyway.

Passing a large structure by value is definitely most expensive than passing just the reference. When you have a large structure, it usually doesn't make sense to use it as a structure, just turn it into a class.

Anyway, are you sure you'll have a performance issue here? Seems like a very premature optimization.

Zincograph

I am programming the game and also learning my way around writing faster code. That exact step isnt much of a problem, because at most, there wont be more than 1000 pathfinding requests per second (I assume). But in the pathfinders node structure, this could very well scale up to 100nodes*1000pathFindingRequests, and learning about this probably help there. – Septum

Dereferencing 100 nodes 1000 times - that is, 100,000 dereferences, a second shouldn't pose any kind of trouble. – Zincograph

And if it turns out that his game is lagging, profiling would probably reveal some single line of code that's responsible. Computers these days are very fast indeed. – Reluctance

Answer 2

I know that a class is always passed by reference and that a struct is passed by value, but I talking about passing the struct by reference here.

You probably have the right idea, but this is incorrect. Everything in C# is passed by value unless you use the ref keyword.

Class instances are reference types, struct instances are value types.

When you pass a reference type by value, you pass a copy of the reference (small). When you pass a value type by value, you pass a copy of the whole data (potentially large).

Jon Skeet has a good explanation of all this here.

Sard