Why doesn't a struct in an array have to be initialized?

问题

I researched this subject but I couldn't find any duplicate. I am wondering why you can use a struct in an array without creating an instance of it.

For example, I have a class and a struct:

public class ClassAPI
{
    public Mesh mesh { get; set; }
}

public struct StructAPI
{
    public Mesh mesh { get; set; }
}

When ClassAPI is used in an array, it has to be initialized with the new keyword before being able to use its properties and methods:

ClassAPI[] cAPI = new ClassAPI[1];
cAPI[0] = new ClassAPI(); //MUST DO THIS!
cAPI[0].mesh = new Mesh();

But this is not the case with StructAPI. It looks like StructAPI doesn't have to be initialized in the array:

StructAPI[] sAPI = new StructAPI[1];
sAPI[0].mesh = new Mesh();

If you try the same thing with ClassAPI, you would get a NullReferenceException.

Why is it different with structs when using them in an array?

I understand the difference between class and struct with struct being a value type but that still doesn't make sense. To me, without the array being involved in this, it would look like I am doing this:

StructAPI sp;
sp.mesh = new Mesh();

Notice that the sp variable is not initialized and it should result in a compile-time error that says:

Error CS0165 Use of unassigned local variable 'sp'

but that's a different story when the struct is put in an array.

Is the array initializing the struct in it? I would like to know what's going on.

回答1:

As per the specification link provided by PetSerAl in the comments:

Array elements
The elements of an array come into existence when an array instance is created, and cease to exist when there are no references to that array instance.

The initial value of each of the elements of an array is the default value (Default values) of the type of the array elements.

For the purpose of definite assignment checking, an array element is considered initially assigned.

(emphasis mine).

This means that when you declare an array of T, each "cell" in the array is being initialized using default(T). For reference types default(T) returns null, but for value types default(T) returns the type default value – 0 for numbers, false for bool, and so on.

As per the Using Structs (C# Programming Guide) page:

If you instantiate a struct object using the default, parameterless constructor, all members are assigned according to their default values.

Since structs are value types, default(T) where T is a struct initializes the struct to its default value, meaning all its members will be initialized to their default values – null for reference types and whatever default value for value types.

So this line of code StructAPI[] sAPI = new StructAPI[1]; basically creates a new array of StructAPI containing a single StructAPI instance, where its mesh property is default(Mesh).

回答2:

This is likely because the fact that classes, while they do have a default constructor, structs actually don't have one.

Why class arrays need to be intialized

Classes create the object and then return the reference. The actual variable is a reference to that value. Default values are always zeros, so the default value to a reference is null, because null is represented by an address of all zeros, meaning its not pointing to anything.

Because of this, the default value for the array of a class is all null references.

Why struct arrays don't need to be

Structs, on the other hand, are all by value. They also don't have a default parameterless constructor and C# doesn't let you create one (though the CLR does). Because of the fact that there isn't a constructor, the CLR is able to very efficiently create the struct by zeroing out all of the values, without having to call the constructor.

You can view more about why this is from this StackOverflow question.

回答3:

When you initialize an array, default values are assigned to its elements:

null for reference types,
for value types, the default value varies: zero for types representing numbers, for struct it is little bit different, its default value is struct with all fields set to their default values. Again, for reference types it's null and for value types it depends (as mentioned above).

So, basically, when you initialize array you have your structs initialized (set to default value), that's why you can access their properties.

来源：https://stackoverflow.com/questions/51462431/why-doesnt-a-struct-in-an-array-have-to-be-initialized

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arrays

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