C# Static variables - scope and persistence

Question

I just did a little experiment:

public abstract class MyClass
{
  private static int myInt = 0;

  public static int Foo()
  {
    return myInt;
  }

  public static int Foo(int n)
  {
    myInt = n;
    return bar();
  }

  private static int bar()
  {
    return myInt;
  }
}

and then I ran:

MessageBox.Show(MyClass.Foo().ToString());
MessageBox.Show(MyClass.Foo(3).ToString());
MessageBox.Show(MyClass.Foo().ToString());
MessageBox.Show(MyClass.Foo(10).ToString());
MessageBox.Show(MyClass.Foo().ToString());

The results I expected were 0, 3, 0, 10, 0.

To my surprise, I got 0, 3, 3, 10, 10.

How long do these changes persist for? The duration of the program execution? The duration of the function calling the static method?

Solution 1

They will persist for the duration of AppDomain. Changes done to static variable are visible across methods.

MSDN:

If a local variable is declared with the Static keyword, its lifetime is longer than the execution time of the procedure in which it is declared. If the procedure is inside a module, the static variable survives as long as your application continues running.

See following for more details:

• C#6 Language Specification - Static Variables
• C#6 Language Specification - Application Startup
• MSDN: Static Variable
• MSDN: Variable Lifetime

Solution 2

The results I expected were 0, 3, 0, 10, 0.

To my surprise, I got 0, 3, 3, 10, 10.

I'm not sure why you would expect the static variable to revert back to its original value after being changed from within the Foo(int) method. A static variable will persist its value throughout the lifetime of the process and only one will exist per class, not instance.

Solution 3

If it's a static variable, that means it exists exactly one place in memory for the duration of the program.

Solution 4

Per the C# spec, a static variable will be initialized no later than the first time a class is loaded into an AppDomain, and will exist until that AppDomain is unloaded - usually when the program terminates.

Solution 5

For the duration of the program execution.

Static class variables are like globals. They're not connected to specific objects of a class - there's only one instance of those per program. The only variables that live during function execution time are automatic (local) variables of the function.

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Comments

[Cylon Cat]

+1 for distinguishing between class variables and instance variables.

[Chris Eberle]

I think he was assuming that the myInt = 0 part would be evaluated more than once.

[YetAnotherUser]

@Chris - he? are you sure? ;)

[Ozzah]

@YetAnotherUser yes, Chris was right... he (I) did assume that. Since the static class is never instantiated, I just assumed that it would be used, an int is returned, and then the program would have forgotten it was ever used in the first place.

[Snake Sanders]

A Static variable is shared by all the instances of the class. Not just visible across methods.

[weir]

c# does not support static local variables. The quote is from an article concerning VB.