Static Function Concurrency ASP.NET

Question

If you have two threads invoking a static function at the same moment in time, is there a concurrency risk? And if that function uses a static member of the class, is there even a bigger problem?

• Are the two calls seperated from each other? (the function is like copied for the two threads?)
• Are they automatically queued?

For instance in next example, is there a risk?

private static int a = 5;

public static int Sum()
{
    int b = 4;
    a = 9;
    int c = a + b;
    return c;
}

And next example, is there a risk?

public static int Sum2()
{
   int a = 5;
   int b = 4;
   int c = a + b;
   return c;
}

Update: And indeed, if both functions are in the same class, what is the risk then?

thx, Lieven Cardoen

Answer 1

See here for a discussion on local variables. before your edit neither of the above methods themselves presented a concurrency risk; the local variables are all independent per call; the shared state (static int a) is visible to multiple threads, but you don't mutate it, and you only read it once.

If you did something like:

if(a > 5) {
    Console.WriteLine(a + " is greater than 5");
} // could write "1 is greater than 5"

it would (in theory) not be safe, as the value of a could be changed by another thread - you would typically either synchronize access (via lock etc), or take a snapshot:

int tmp = a;
if(tmp > 5) {
   Console.WriteLine(tmp + " is greater than 5");
}

If you are editing the value, you would almost certainly require synchronization.

Answer 2

Yes, there is a concurrency risk when you modify a static variable in static methods.

The static functions themselves have distinct sets of local variables, but any static variables are shared.

In your specific samples you're not being exposed, but that's just because you're using constants (and assigning the same values to them). Change the code sample slightly and you'll be exposed.

Edit:

If you call both Sum1() AND Sum2() from different threads you're in trouble, there's no way to guarantee the value of a and b in this statement: int c = a + b;

private static int a = 5;

public static int Sum1()
{
    int b = 4;
    a = 9;
    int c = a + b;
    return c;
}

public static int Sum2()
{
   int b = 4;
   int c = a + b;
   return c;
}

You can also achieve concurrency problems with multiple invocations of a single method like this:

public static int Sum3(int currentA)
{
   a = currentA;
   int b = 4;
   int c = a + b;
   int d = a * b; // a may have changed here
   return c + d;
}

The issue here is that the value of a may change mid-method due to other invocations changing it.