Does garbage collection happen at the process level or appdomain level?

Question

FullGC normaly pauses all Threads while running. Having two AppDomains, each running several threads. When GC runs, will all threads be paused, or only those of either one A

Answer 1

Hard to answer, best thing to do is just test it:

using System;
using System.Reflection;

public class Program : MarshalByRefObject {
    static void Main(string[] args) {
        var dummy1 = new object();
        var dom = AppDomain.CreateDomain("test");
        var obj = (Program)dom.CreateInstanceAndUnwrap(Assembly.GetExecutingAssembly().FullName, typeof(Program).FullName);
        obj.Test();
        Console.WriteLine("Primary appdomain, collection count = {0}, gen = {1}",
            GC.CollectionCount(0), GC.GetGeneration(dummy1));
        Console.ReadKey();

    }
    public void Test() {
        var dummy2 = new object();
        for (int test = 0; test < 3; ++test) {
            GC.Collect();
            GC.WaitForPendingFinalizers();
        } 
        Console.WriteLine("In appdomain '{0}', collection count = {1}, gen = {2}",
            AppDomain.CurrentDomain.FriendlyName, GC.CollectionCount(0),
            GC.GetGeneration(dummy2));
    }
}

Output:

In appdomain 'test', collection count = 3, gen = 2
Primary appdomain, collection count = 3, gen = 2

Good evidence that a GC affects all AppDomains on the default CLR host. This surprised me.