Static Variable Instances and AppDomains, what is happening?

Question

I have

public static class A
{
   public static string ConnString;
}

[Serializable]
public class Test{
   // Accesing A's field;
   public string ConnString{get{return A.ConnString;}set{A.ConnString=value;}}
}

void Main()
{
   A.ConnString = "InitialString"; // I set A.ConnString in the current domain

   var newDomain = AppDomain.CreateDomain("DomNew");
   Test TObj = newDomain.CreateInstanceAndUnwrap(typeof(Test).Assembly.FullName, typeof(Test).FullName) as Test ;

   TObj.ConnString = "NewDomainString"; // It is supposed to set A.ConnString in the newDomain aka a different instance of A.ConnString

   // Here it is supposed to print two different values
   Console.WriteLine(A.ConnString);  // "InitialString"
   Console.WriteLine(TObj.ConnString); // "NewDomainString"
}

But NO! The two WriteLines, print out the same value "NewDomainString"! WHY???

this code

TObj.ConnString = "NewDomainString"

is supposed to change the string in the newly created domain, but it seems they both refer to the same instance!

Why, what is happening here?

Answer 1

There are only two ways for a class to be accessible from another AppDomain- one is is the class is [Serializable], as your Test class is, the other is if the class inherits from MarshalByRefObject. Because your class is Serializable, a copy of it is created for every cross-AppDomain call. So the Test that the main appdomain gets when you call...

Test TObj = newDomain.CreateInstanceAndUnwrap(typeof(Test).Assembly.FullName, typeof(Test).FullName) as Test;

is actually not the Test instance that was created in the "DomNew" AppDomain- it is a copy local to the "main" AppDomain, and therefore references the static variables from the "main" AppDomain.

If you want Test to exhibit the behavior that you expect, make it inherit from MarshalByRefObject instead of being Serializable.

Answer 2

You marked your Test class as Serializable. This is wrong. You should have derived from MarshalByRef. Otherwise TObj will just be a local copy in the current AppDomain.