Why are arrays only allowed in certain places?

Question

As I understand it, C# has a syntax for writing arrays as such: { 1, 2, 3 }. Why is this invalid:

 x = { 1, 2, 3 }.GetLength(0);

while this is valid?

 int[] numbers = { 1, 2, 3 };
 x = numbers.GetLength(0);

Isn't the datatype of the expression { 1, 2, 3 } the same as numbers?

Answer 1

Arrays are allowed anywhere - but you can only use that particular syntax (which is called an array initializer for creating them as part of a variable declaration - or as part of a larger expression called an array creation expression.

You can still create them though:

x = new int[] { 1, 2, 3 }.GetLength(0);

So within that, new int[] { 1, 2, 3 } is the array creation expression, and the { 1, 2, 3 } part is the array initializer.

Array creation expressions are described in section 7.6.10.4 of the C# 5 spec, and array initializers are described in section 12.6.

Answer 2

The syntax you refer to is an object collection initializer. It is useful when initializing an instance of different types. It does not, in itself, create an instance of a given type.

For instance, you can use it to declare arrays:

int[] nums = new int[] { 1, 2, 3 };

Lists:

List<int> nums = new List<int> { 1, 2, 3 };

Dictionary:

Dictionary<string, int> pairs = { { "One", 1 }, { "Two", 2 }, { "Three", 3 } };

You can still inline things to achieve your initial intention with a little more code:

new[] { 1, 2, 3 }.GetLength(0);