Why sometimes we need to initialize structs while sometimes not?

Question

Let's say we have the following code:

struct Point {
   public Int32 m_x, m_y;
}

class Rectangle {
   Point p;
}

//Program.cs
static void Main(string[] args) {
   Point p;
   int i = p.m_x  // does't compile, unassigned field m_x
}

so we need to initialize the struct:

static void Main(string[] args) {
   Point p;
   p.m_x = 0; 
   int i = p.m_x;  // compile OK now
}

or we can all struct's default parameterless constructor to initialize its fields:

static void Main(string[] args) {
   Point p = new P();
   int i = p.m_x   //OK
}

so it looks like we need to initialize structs, but if the struct is a field in a in a class then it doesn't need to be initialized:

static void Main(string[] args) {
   Rectangle r = new Rectangle();
   int i = r.p.m_x;    //   comile OK
}

it still compile, even though I didn't initialize structs. And I checked the IL code and found Rectangle's constructor doesn't call the struct's parameterless constructor neither.

"*In C#, you must initialize a declared variable before it can be used. Because a structure-type variable can't be null (unless it's a variable of a nullable value type), you must instantiate an instance of the corresponding type*" - Fields that are of a value type, do not need to be initialized, they however will be `default` — TheGeneral, Feb 04 '21 at 01:58
The compiler may elide the actual default initializer, as the default value (using `localsinit` rather than an actual constructor) is the same anyway. Furthermore, it's not your (calling code's) business how `Rectangle` initializes, just that it does so, and therefore `new Rectangle()` is guaranteed to have an initialized field. Aside: `Rectangle` is a struct, not a class, not that it matters. — Charlieface, Feb 04 '21 at 02:12

score 1 · Answer 1 · edited Feb 06 '21 at 14:00

A local variable need to be initialised:

Instantiation of a structure type

In C#, you must initialize a declared variable before it can be used. Because a structure-type variable can't be null (unless it's a variable of a nullable value type), you must instantiate an instance of the corresponding type.

In regards to fields, let's look at the C# ECMA 334 specification

15.5.5 Field initialization

The initial value of a field, whether it be a static field or an instance field, is the default value

10.3 Default values

...

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.

9.3.3 Default constructors

All value types implicitly declare a public parameterless instance constructor called the default constructor. The default constructor returns a zero-initialized instance known as the default value for the value type:

For all simple-types, the default value is the value produced by a bit pattern of all zeros:

For sbyte, byte, short, ushort, int, uint, long, and ulong, the default value is 0.

For char, the default value is '\x0000'.

For float, the default value is 0.0f.

For double, the default value is 0.0d.

For decimal, the default value is 0.0m.

For bool, the default value is false.

For an enum-type E, the default value is 0, converted to the type E. 9 Types

For a struct-type, the default value is the value produced by setting all value type fields to their default value and all reference type fields to null.

@Charlieface do you mean Suppress emitting of localsinit flag? — TheGeneral, Feb 04 '21 at 02:25
I was referring to the actual flag. Changed my mind, looks like it doesn't actually rely on it even in Release. — Charlieface, Feb 04 '21 at 02:30

Why sometimes we need to initialize structs while sometimes not?

1 Answers1