Why C# behaves differently on two int array syntaxes

Array in C# is co-variant implicitly on reference type:

object[] listString = new string[] { "string1", "string2" };

But not on value type, so if you change string to int, you will get compiled error:

object[] listInt = new int[] {0, 1}; // compile error

Now, the concern is when you declare int array like two syntaxes below which do not explicitly declare the type int, just only differentiate on new[], compiler will treat differently:

object[] list1 = { 0, 1 };       //compile successfully
object[] list2 = new[] {0, 1};    //compile error

You will get object[] list1 = { 0, 1 }; compiled successfully, but object[] list2= new[] {0, 1}; compiled error.

It seems the C# compiler treats

object[] list1 = { 0, 1 };

as

object[] list1 = new object[]{ 0, 1 };

but

object[] list2 = new[] { 0, 1 };

as

object[] list2 = new int[]{ 0, 1 };  //error because of co-variant

Why C# compiler behaves in the different way on this case?

Solution 1:

The version that compiles uses an array initializer to initialize list1. The C# language spec, §1.110 ("Array initializers") states:

An array initializer consists of a sequence of variable initializers, enclosed by “{”and “}” tokens and separated by “,” tokens. Each variable initializer is an expression or, in the case of a multi-dimensional array, a nested array initializer.

The context in which an array initializer is used determines the type of the array being initialized. In an array creation expression, the array type immediately precedes the initializer, or is inferred from the expressions in the array initializer. In a field or variable declaration, the array type is the type of the field or variable being declared.

When an array initializer is used in a field or variable declaration, such as:

int[] a = {0, 2, 4, 6, 8};

it is simply shorthand for an equivalent array creation expression:

int[] a = new int[] {0, 2, 4, 6, 8};

So it is obvious that this should compile.

The second version uses an explicit array creation expression, where you instruct the compiler specifically what type of array to create. §1.51.10.4 ("Array creation expressions") states:

An array creation expression of the third form is referred to as an implicitly typed array creation expression. It is similar to the second form, except that the element type of the array is not explicitly given, but determined as the best common type (§1.50.2.14) of the set of expressions in the array initializer.

Therefore, the second version is equivalent to

object[] list2 = new int[] { 0, 1 };

So the question now effectively becomes "why can I not assign an int[] to an object[]", just as you mention at the end of the question. And the answer is also simple, given in §1.109 ("Array covariance"):

Array covariance specifically does not extend to arrays of value-types. For example, no conversion exists that permits an int[] to be treated as an object[].

Solution 2:

The declaration

object[] listInt = new int[] {0, 1};

is invalid because covariant array conversions are not allowed for value types (and int is a value type). Alternatively, the declaration

object[] listInt = new string[] {"0", "1"};

is valid because covariant array conversions are allowed for reference types. This is because the assignment x = (object)myString only involves a simple assignment, but y = (object)myInt requires a boxing operation.

Now on to the difference between the two declarations. In the declaration object[] list2 = new[] { 0, 1 }, due to how type inference works it first looks at the Right Hand Side expression and concludes that new[] { 0, 1 } should be treated as new int[] { 0, 1 }. Then it tries to assign this int array to an object array, giving an error because of the covariant conversion of value types issue. The declaration object[] list1 = { 0, 1 }, though, uses a collection initializer, and in those circumstances the type of the collection is where the type is defined, so each element will instead be cast to the type expected by the collection.