Which is best for data store Struct/Classes?

Question

We have seen lots of discussion in SO regarding the class vs struct in c#. Mostly ended with conclusions saying its a heap/stack memory allocation. And reco

Answer 1

I would make the choice based on the following criteria

• reference type vs value type semantics. If 2 objects are only equal if they are the same object, it indicates reference type semantics => class. If the value of its members defines equality (e.g. 2 DateTimes are equal if both represent the same point in time even if they are 2 distinct objects), value type semantics => struct
• Memory footprint of the object. If the object is huge and frequently allocated, making it a struct would consume the stack much faster, hence I'd rather have it as a class. On the contrary, I'd rather avoid the GC penalty for small value types; hence make them a struct.
• can you make the object immutable? I find structs great for 'value objects' - from the DDD book.
• Would you face some boxing-unboxing penalty based on the usage of this object? If yes, go for class.

Answer 2

I remember one advice given on MSDN that struct should not be larget than 16 or 21 bytes. Looking for the link, but can't find it yet.

The main implication was that once you have a string in your data type - make it a class without thinking. Otherwise the struct shouldn't hold much.

Answer 3

I think you have the right idea. Structs are made to mimic data-types. They are value driven not reference based. If you look at the MSDN documentation for most of the base data classes (int, double, decimal, ect.) they are all based on structs. That being said however, structs should not be overused for that very same reason. Room to store all everything in that struct is allocated as soon as it is instantiated, where as classes just allocate room for a reference to everything inside. If the data is in small enough chunks where this is not a problem than structs are the way to go. If this is an issue go with classes. If you don't know than it might just be best to stick with what you are familiar with.

Answer 4

If you have low latency requirements and A LOT of objects slow garbage collections can be a problem. In that case struct can be very helpful because the garbage collector does not need to scan through a hierarchy of value types if the value types does not contain any reference types.

You can find a benchmark here: http://00sharp.wordpress.com/2013/07/03/a-case-for-the-struct/

Answer 5

A pretty cool, not so well known advantage of Structs over Classes is that there is an automatic implementation of GetHashcode and Equals in structs.
That's pretty useful when keys are required for dictionaries

The struct implementation of GetHashcode and Equals is based on the binary content of the struct instances + reflection for the reference members (like String members and other instances of classes)

So the following code works for GethashCode/Equals :

public struct Person
{
    public DateTime Birthday { get; set; }
    public int Age{ get; set; }
    public String Firstname { get; set; }
}
class Program
{
    static void Main(string[] args)
    {
        Person p1 = new Person { Age = 44, Birthday = new DateTime(1971, 5, 24), Firstname = "Emmanuel" };
        Person p2 = new Person { Age = 44, Birthday = new DateTime(1971, 5, 24), Firstname = "Emmanuel" };
        Debug.Assert(p1.Equals(p2));
        Debug.Assert(p1.GetHashCode() == p2.GetHashCode());
    }
}

Both assertions succeed when Person is a struct Both assertions fail if Person is a class instead of a struct

Reference : https://msdn.microsoft.com/en-Us/library/2dts52z7%28v=vs.110%29.aspx

Regards, best coding

Answer 6

I can never really seem to remember, exactly how structs are different, but they are. In subtle ways. In fact, sometimes they come and bite you.

So. Unless you know what you are doing, just stick to classes.

I know this sounds a little newbie. I know I should right now go and look up the differences and display them here - but that has already been done by others. All I'm saying is that adding a different type of objects creates a semantical burden, a bit of extra complexity that you are wise to consider carefully.

If I remember correctly, one of the biggest problem is the value semantics of structs: Passing them around will result in different objects (as they get passed by value). If you then change some field in one place, beware that in all other places the field did not get changed! That is why everyone is recommending immutability for structs!

EDIT: For the case you are describing, structs won't work!