What is the default value of a member in an array?

Question

I instantiate an array like this:

int array[] = new int[4]; 

What are the default values for those four members? Is it null, 0 or not exists?

Answer 1

It's 0. It can't be null, as null isn't a valid int value.

From section 7.6.10.4 of the C# 5 specification:

All elements of the new array instance are initialized to their default values (§5.2).

And from section 5.2:

The default value of a variable depends on the type of the variable and is determined as follows:

• For a variable of a value-type, the default value is the same as the value computed by the value-type’s default constructor (§4.1.2).
• For a variable of a reference-type, the default value is null.

Initialization to default values is typically done by having the memory manager or garbage collector initialize memory to all-bits-zero before it is allocated for use. For this reason, it is convenient to use all-bits-zero to represent the null reference.

(As an implementation detail, there's some trickiness around the first bullet point. Although C# itself doesn't allow you to declare a parameterless constructor for value types, you can create your own parameterless constructors for value types in IL. I don't believe those constructors are called in array initialization, but they will be called in a new X() expression in C#. It's outside the realm of the C# spec though, really.)