What Is the Static Variable Initialization Order Across Classes in C#

What is the static variable initialization order across classes in C#?

It's fine for one type to depend on another type being initialized, so long as you don't end up in a cycle.

Basically this is fine:

public class Child
{
    static Child() {} // Added static constructor for extra predictability
    public static readonly int X = 10;
}

public class Parent
{
    static Parent() {} // Added static constructor for extra predictability
    public static readonly int Y = Child.X;
}

The result is well-defined. Child's static variable initializers are executed prior to the first access to any static field in the class, as per section 10.5.5.1 of the spec.

This isn't though:

public class Child
{
    public static readonly int Nasty = Parent.Y;
    public static readonly int X = 10;
}

public class Parent
{
    public static readonly int Y = Child.X;
}

In this latter case, you either end up with Child.Nasty=0, Parent.Y=10, Child.X=10 or Child.Nasty=0, Parent.Y=0, Child.X=10 depending on which class is accessed first.

Accessing Parent.Y first will start initializing Parent first, which triggers the initialization of Child. The initialization of Child will realise that Parent needs to be initialized, but the CLR knows that it's already being initialized, so carries on regardless, leading to the first set of numbers - because Child.X ends up being initialized before its value is used for Parent.Y.

Accessing Child.Nasty will start initializing Child first, which will then start to initialize Parent. The initialization of Parent will realise that Child needs to be initialized, but the CLR knows that it's already being initialized, so carries on regardless, leading to the second set of numbers.

Don't do this.

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EDIT: Okay, more detailed explanation, as promised.

When is a type initialized?

If a type has a static constructor, it will only be initialized
when it's first used (either when a static member is referenced, or
when an instance is created). If it doesn't have a static
constructor, it can be initialized earlier. In theory, it could also
be initialized later; you could theoretically call a constructor or
a static method without the static variables being initialized - but
it must be initialized before static variables are referenced.

What happens during initialization?

First, all static variables receive their default values (0, null
etc).

Then the static variables of the type are initialized in textual
order. If the initializer expression for a static variable requires
another type to be initialized, then that other type will be
completely initialized before the variable's value is assigned -
unless that second type is already being initialized (due to a
cyclic dependency). Essentially, a type is either:

• Already initialized
• Being initialized at the moment
• Not initialized

Initialization is only triggered if the type is not initialized.
This means that when there are cyclic dependencies, it is possible
to observe a static variable's value before its initial value has
been assigned. That's what my Child/Parent example shows.

After all the static variable initializers have executed, the static
constructor executes.

See section 10.12 of the C# spec for more details on all of this.

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By popular demand, here was my original answer when I thought the question was about the initialization order of static variables within a class:

Static variables are initialized in textual order, as per section 10.5.5.1 of the C# spec:

The static field variable initializers
of a class correspond to a sequence of
assignments that are executed in the
textual order in which they appear in
the class declaration.

Note that partial types make this trickier as there's no one canonical "textual order" of the class.

Static member initialization order within a single C# class

In short, don't do this.

Standard ECMA-334 C# Language Specification

15.5.6.2 Static field initialization

The static field variable initializers of a class correspond to a
sequence of assignments that are executed in the textual order in
which they appear in the class declaration (§15.5.6.1). Within a
partial class, the meaning of "textual order" is specified by
§15.5.6.1. If a static constructor (§15.12) exists in the class,
execution of the static field initializers occurs immediately prior to
executing that static constructor. Otherwise, the static field
initializers are executed at an implementation-dependent time prior to
the first use of a static field of that class

The fix is to :

• Put them in the order and use Static Constructor,
• or just Initialise them in a Static Constructor in turn giving you the ability to control the order of initialisation (given the above information).

Personally i suggest to Initialise them in a Static Constructor, it seems to make it more concrete and understandable, and less likely to be bumped in refactoring

Is the order of static class initialization in C# deterministic?

Straight from ECMA-334:

17.4.5.1: "If a static constructor (§17.11) exists in the class, execution of the static field initializers occurs immediately prior to executing that static constructor. Otherwise, the static field initializers are executed at an implementation-dependent time prior to the first use of a static field of that class."

And:

17.11: The execution of a static constructor is triggered by the first
of the following events to occur within an application domain:

• An instance of the class is created.
• Any of the static members of the class are referenced.

If a class contains the Main method (§10.1) in which execution begins, the static constructor for that class
executes before the Main method is called. If a class contains any static fields with initializers, those
initializers are executed in textual order immediately prior to executing the static constructor (§17.4.5).

So the order is:

• A.X used, so static A() called.
• A.X needs to be initialized, but it uses B.X, so static B() called.
• B.X needs to be initialized, and it is initialized to 7. B.X = 7
• All static fields of B are initialized, so static B() is called. X is printed ("7"), then it is set to A.X. A has already started being initialized, so we get the value of A.X, which is the default value ("when a class is initialized, all static fields in that class are first initialized to their default value"); B.X = 0, and is printed ("0").
• Done initializing B, and the value of A.X is set to B.X+1. A.X = 1.
• All static fields of A are initialized, so static A() is called. A.X is printed ("1").
• Back in Main, the values of A.X and B.X are printed ("1", "0").

It actually comments upon this in the standard:

17.4.5: It is possible for static fields with variable initializers to be observed in their default value state. However, this is strongly discouraged as a matter of style.

c# initialize static variable from different classes

This is because the static constructor is called automatically before the first instance is created or any static members are referenced..

This means that when an instance of otherClass invokes IDs.someID = sym; the first operation that gets executed is the static constructor, i.e. the code inside static IDs().

At this point the static variable has not yet been initialized, and you are basically executing log.info(null);.

After the static constructor completes, the variable is initialized, so you should be able to see its value inside otherMethod, after the first reference of IDs.

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Given the OP's requirement:

I want to use the value passed in someID in a switch statement

The solution could be to simply execute a static method whenever a new value is set, with the help of explicit getters and setters:

public static class IDs
{
    private static string _someID; // backing field

    public static string SomeID
    {
        get { return _someID; }

        set
        {
            _someID = value;
            DoSomethingWithSomeID();
        }
    }

    private static DoSomethingWithSomeID()
    {
        // Use SomeID here.

        switch (IDs.SomeID)
        {
            ...
        }
    }
}

public class OtherClass
{
    public void OtherMethod(string sym)
    {
        // This will set a new value to the property
        // and invoke DoSomethingWithSomeID.
        IDs.SomeID = sym;
    }
}

DoSomethingWithSomeID will be invoked every time someone sets a new value to SomeID.

Static variable order

The static fields are initialized in the same order as the declarations. When you initialize v2 with the value of v1, v1 is not initialized yet, so its value is 0.

Ambiguity in initialization order of static variables

The static field variable initializers of a class correspond to a
sequence of assignments that are executed in the textual order in
which they appear in the class declaration.

This means that within the same class, static fields are initialized in order of appearance in the source code. For example:

class A
{
   public static int X = Test.F("Init A.X");
   public static int Y = Test.F("Init A.Y");
}

When it's time for the static fields to be initialized, X is guaranteed to be initialized before Y.

"Because the rules for when static constructors execute (as defined in
Section 10.11) provide that B's static constructor (and hence B's
static field initializers) must run before A's static constructor and
field initializers."

This means that the static constructor and member initialization for each class will run in evaluation order when expressions that access these classes appear¹. The relative order of appearance of the class definitions in source code does not play any role, even if they appear in the same source file (which they most certainly are not obliged to do). For example:

static void Main() {
    Console.WriteLine("{0} {1}", B.Y, A.X);
}

Assuming that neither A nor B has already been statically initialized, order of evaluation guarantees that all the fields of B will be initialized before any field of A. The fields of each class will be initialized in the order specified by the first rule.

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¹ for the purposes of this discussion I am ignoring the existence of beforefieldinit.

When do static variables get initialized in C#?

Oh, that is complex. It depends on whether the beforefieldinit flag is set, which in turn (in C#) depends on whether there is a static constructor. And worse; in .NET 4 I believe the behaviour changed to make it more "lazy" than it used to be.

Frankly, I wouldn't code to any specific behaviour here; simply: static fields will be initialized before you try to use them, as long as you use regular code to access them.

Jon Skeet has a good write-up on this here and here

Static field initialization order with partial classes

When the fields are present in the same file, the textual order defines the execution of their initialization:

10.5.5.1 Variable initializers - Static field initialization

The static field variable initializers of a class correspond to a sequence of assignments that are executed in the textual order in which they appear in the class declaration. If a static constructor (§10.12) exists in the class, execution of the static field initializers occurs immediately prior to executing that static constructor. Otherwise, the static field initializers are executed at an implementation-dependent time prior to the first use of a static field of that class.

However, in the case of fields declared in different files of partial classes, the order is undefined:

10.2.6 Partial types - Members

The ordering of members within a type is rarely significant to C# code, but may be significant when interfacing with other languages and environments. In these cases, the ordering of members within a type declared in multiple parts is undefined.

From the C# language specification.

Initialization order of static readonly field

It is in the order that they appear in. See here.

The static field variable initializers of a class correspond to a
sequence of assignments that are executed in the textual order in
which they appear in the class declaration.

Also, when you have a static constructor:

If a static constructor (Section 10.11) exists in the class,
execution of the static field initializers occurs immediately prior to
executing that static constructor. Otherwise, the static field
initializers are executed at an implementation-dependent time prior to
the first use of a static field of that class.

Order of static variable declaration

In the second case compiler will generate the following static constructor for type:

static Program()
{
   // Note: this type is marked as 'beforefieldinit'.
   Program.b = Program.a;
   Program.a = 5;
}

So, a equals 0 when it assign to b. Then a set to 5

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