Loading an assembly into an AppDomain but also in the AppDomain.CurrentDomain resulting in memory leaks

Question

I have to execute some line which comes from a jscript into another appdomain than the current one. For this i have the following piece of code.

AppDomain ad = null;
try
{
    ad = AppDomain.CreateDomain("new AD" + new Random(), null, null);

    ad.CreateInstanceAndUnwrap
            (
            assembly,
            type,
            true,
            BindingFlags.DeclaredOnly | BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.CreateInstance,
            null,
            args,
            null,
            null,
            null
            );

}
catch (Exception e)
{
    throw new Exception("Automated script engine had an error. " + e.Message + "\n" + e.InnerException.StackTrace + "\n" + e.HelpLink);
}
finally
{
    if (ad != null)
    {
            Assembly[] a = ad.GetAssemblies();
            Console.WriteLine(a.Length);
            Assembly[] mainAssemblies = AppDomain.CurrentDomain.GetAssemblies();
            Console.WriteLine(mainAssemblies.Length);
            AppDomain.Unload(ad);
            GC.AddMemoryPressure(GC.GetTotalMemory(true));
            GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
            GC.Collect();
            ad = null;
        }
    }
}

But when I inspect all assemblies loaded into the current AppDomain (via AppDomain.CurrentDomain.GetAssemblies()) my asembly is also loaded.

And as i might have to run let's say 2000 tests, i understand that 2000 assemblies will be loaded into my CurrentDomain.

Is there a way to load this assembly into an AppDomain but without adding it to the CurrentDomain ?

The method calls in the finally block are just my tries over here. Don't judge me for those if they are not useful maybe.

Panos Rontogiannis Panos Rontogiannis · Accepted Answer · 2014-01-31T12:55:19

The culprit of the leak is:

Assembly[] a = ad.GetAssemblies();

You cannot pass assemblies from one AppDomain to another without loading them. So, although you have initially loaded the assembly into "ad", the call AppDomain.GetAssemblies will load them to the current AppDomain (not the "ad" variable).

The easiest way to overcome this problem is to add a method to your MarshalByRefObject derived class (the type variable in your sample) that returns AssemblyName objects which will not cause such a leak to the main AppDomain.

public AssemblyName[] GetAssemblyNames()
{
    return AppDomain.CurrentDomain
                    .GetAssemblies()
                    .Select(asm => asm.GetName()).ToArray();
}

And instead of:

Assembly[] a = ad.GetAssemblies();

you do:

AssemblyNames[] a = someRemoteObject.GetAssemblyNames();

where someRemoteObject is the return value from the call to CreateInstanceAndUnwrap.

Loading an assembly into an AppDomain but also in the AppDomain.CurrentDomain resulting in memory leaks

3 Answers