When should you use struct and not class in C#? My conceptual model is that structs are used in times when the item is merely a collection of value types. A way to logically hold them all together into a cohesive whole.
I came across these rules here:
Do these rules work? What does a struct mean semantically?
Structs are preferable if they are relatively small and copiable because copying is way safer than having multiple references to the same instance as happens with classes. This is especially important when passing around a variable to many classes and/or in a multithreaded environment.
In classes, two variables can contain the reference of the same object and any operation on one variable can affect another variable. In this way, struct should be used only when you are sure that, It logically represents a single value, like primitive types (int, double, etc.).
The source referenced by the OP has some credibility ...but what about Microsoft - what is the stance on struct usage? I sought some extra learning from Microsoft, and here is what I found:
Consider defining a structure instead of a class if instances of the type are small and commonly short-lived or are commonly embedded in other objects.
Do not define a structure unless the type has all of the following characteristics:
- It logically represents a single value, similar to primitive types (integer, double, and so on).
- It has an instance size smaller than 16 bytes.
- It is immutable.
- It will not have to be boxed frequently.
Okay, #2 and #3 anyway. Our beloved dictionary has 2 internal structs:
[StructLayout(LayoutKind.Sequential)] // default for structs private struct Entry //<Tkey, TValue> { // View code at *Reference Source } [Serializable, StructLayout(LayoutKind.Sequential)] public struct Enumerator : IEnumerator<KeyValuePair<TKey, TValue>>, IDisposable, IDictionaryEnumerator, IEnumerator { // View code at *Reference Source }
*Reference Source
The 'JonnyCantCode.com' source got 3 out of 4 - quite forgivable since #4 probably wouldn't be an issue. If you find yourself boxing a struct, rethink your architecture.
Let's look at why Microsoft would use these structs:
Entry
and Enumerator
, represent single values.Entry
is never passed as a parameter outside of the Dictionary class. Further investigation shows that in order to satisfy implementation of IEnumerable, Dictionary uses the Enumerator
struct which it copies every time an enumerator is requested ...makes sense.Enumerator
is public because Dictionary is enumerable and must have equal accessibility to the IEnumerator interface implementation - e.g. IEnumerator getter. Update - In addition, realize that when a struct implements an interface - as Enumerator does - and is cast to that implemented type, the struct becomes a reference type and is moved to the heap. Internal to the Dictionary class, Enumerator is still a value type. However, as soon as a method calls GetEnumerator()
, a reference-type IEnumerator
is returned.
What we don't see here is any attempt or proof of requirement to keep structs immutable or maintaining an instance size of only 16 bytes or less:
readonly
- not immutableEntry
has an undetermined lifetime (from Add()
, to Remove()
, Clear()
, or garbage collection);And ... 4. Both structs store TKey and TValue, which we all know are quite capable of being reference types (added bonus info)
Hashed keys notwithstanding, dictionaries are fast in part because instancing a struct is quicker than a reference type. Here, I have a Dictionary<int, int>
that stores 300,000 random integers with sequentially incremented keys.
Capacity: 312874
MemSize: 2660827 bytes
Completed Resize: 5ms
Total time to fill: 889ms
Capacity: number of elements available before the internal array must be resized.
MemSize: determined by serializing the dictionary into a MemoryStream and getting a byte length (accurate enough for our purposes).
Completed Resize: the time it takes to resize the internal array from 150862 elements to 312874 elements. When you figure that each element is sequentially copied via Array.CopyTo()
, that ain't too shabby.
Total time to fill: admittedly skewed due to logging and an OnResize
event I added to the source; however, still impressive to fill 300k integers while resizing 15 times during the operation. Just out of curiosity, what would the total time to fill be if I already knew the capacity? 13ms
So, now, what if Entry
were a class? Would these times or metrics really differ that much?
Capacity: 312874
MemSize: 2660827 bytes
Completed Resize: 26ms
Total time to fill: 964ms
Obviously, the big difference is in resizing. Any difference if Dictionary is initialized with the Capacity? Not enough to be concerned with ... 12ms.
What happens is, because Entry
is a struct, it does not require initialization like a reference type. This is both the beauty and the bane of the value type. In order to use Entry
as a reference type, I had to insert the following code:
/* * Added to satisfy initialization of entry elements -- * this is where the extra time is spent resizing the Entry array * **/ for (int i = 0 ; i < prime ; i++) { destinationArray[i] = new Entry( ); } /* *********************************************** */
The reason I had to initialize each array element of Entry
as a reference type can be found at MSDN: Structure Design. In short:
Do not provide a default constructor for a structure.
If a structure defines a default constructor, when arrays of the structure are created, the common language runtime automatically executes the default constructor on each array element.
Some compilers, such as the C# compiler, do not allow structures to have default constructors.
It is actually quite simple and we will borrow from Asimov's Three Laws of Robotics:
...what do we take away from this: in short, be responsible with the use of value types. They are quick and efficient, but have the ability to cause many unexpected behaviors if not properly maintained (i.e. unintentional copies).
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