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

问题:

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?

回答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.)

回答2:

The default value of an automatically-initialized variable of type T, such as an array element or an instance field, is the same as the value of default(T). For reference types and pointer types, it's null. For numeric types, it is the zero of that type. For bool, it's false. For struct types, it is the struct value that has all its fields initialized to their default values.

回答3:

From Arrays (C# Programming Guide):

The default value of numeric array elements are set to zero, and reference elements are set to null.

回答4:

Integers cannot be NULL. They will have the value '0'. Even if you try to assign NULL to a int from code you will not be able to do it.