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?

score 51 · Accepted Answer · edited Jun 04 '19 at 09:31

51

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:

edited Jun 04 '19 at 09:31

Chris F Carroll

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answered May 13 '11 at 00:34

YetAnotherUser

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A Static variable is shared by all the instances of the class. Not just visible across methods. – Snake Sanders Apr 28 '13 at 11:04
5

c# does not support `static` local variables. The quote is from an article concerning VB. – weir Mar 13 '18 at 14:29
C# now does support static local methods (from C# 8). – VahidShir Mar 28 '23 at 06:45

score 29 · Answer 2 · edited Jun 20 '20 at 09:12

29

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.

edited Jun 20 '20 at 09:12

Community

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answered May 13 '11 at 00:36

Ed S.

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+1 for distinguishing between class variables and instance variables. – Cylon Cat May 13 '11 at 00:38
I think he was assuming that the `myInt = 0` part would be evaluated more than once. – Chris Eberle May 13 '11 at 00:44
1

@Chris - he? are you sure? ;) – YetAnotherUser May 13 '11 at 00:53
@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. – Ozzah May 13 '11 at 01:20

score 9 · Answer 3 · answered May 13 '11 at 00:34

9

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

answered May 13 '11 at 00:34

Chris Eberle

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score 5 · Answer 4 · answered May 13 '11 at 00:37

5

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.

answered May 13 '11 at 00:37

Ben

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score 4 · Answer 5 · answered May 13 '11 at 00:35

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.

score 2 · Answer 6 · answered Mar 13 '18 at 14:40

They persist "for the lifetime of the application domain in which your program resides" according to Microsoft Docs: Static Classes and Static Class Members (C# Programming Guide).

C# Static variables - scope and persistence

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