Q:

Which is best for data store Struct/Classes?

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 recommending to use structs in small data structures.
Now I have a situation to decide the simple data store among these two choices. Currenlty in our application we have thousands of classes, just acts as simple data stores (only exposed public fields) and they passed among different modules and services.
As per my understanding, I felt it's better to move ahead with struct instead classes for the performance reasons. Because these are simple data structures only act as data stores.
Before proceeding with this, I need some expert advice from the people who have experienced this struggle.

is my understanding correct?
I have seen most ORMs have classes as data stores. So I doubt there should a reason to go ahead with classes instead structs. what would that be?

A:

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.

A:

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

A:

structs should be defined immutable where in classes should not. If you think your objects are going to be small and immutable you can go ahead with making them structs or else let them be classes.

