static variable allocation time in .net

When static variable is allocated i.e. when 开发者_如何学Goyou declare class or at the time of object creation?

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It is compiled into the static constructor. So the first time anyone creates an object of the class or calls a static method or property on it, the initialization occurs.

Edit: when it is important to you if initialization happens before of after your own static constructor code (and some other edge cases),check the link in the comment by divo.

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the first time the class is accessed....

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As other answers have indicated, this will happen in the type (static) constuctor. If your class does not have a type constructor explicitly defined then the compiler will generate one for you. However, determining exactly when this constructor is called is a bit more involved.

If your class does not define an explicit type constructor, e.g.

public class Foo
{
    public static int Bar = 1;
}

then the C# compiler will generate a constructor and emit the class definition with the beforefieldinit flag. This will cause the JIT compiler to guarantee that the type constructor is called sometime before a member of the type is first used but this time is non-deterministic, i.e. it is not possible to know exactly when this will happen, and it could be at a much earlier point than when a type member is first used.

If your class declares an explicit type constructor, e.g.

public class Foo
{
    public static int Bar;
    static Foo()
    {
        Bar = 1;
    }
}

then the compiler will emit IL for the class without the beforefieldinit flag. In this case the JIT compiler will call the type constructor at a deterministic time, i.e. immediately before the first type member access.

The former JIT behaviour is known as before-field-init semantics and the latter as precise sematntics. It is important to know the difference between the two since, in some scenarios, they may have a significant performance implication.

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Static variables are allocated as soon as static (type) constructor is called. This happens when you call any method that reference the type for the first time, before method execution.