Loading/unloading same assembly in a loop does not actually unload

[jkotas]

Repro:

using System;
using System.Reflection;
using System.Runtime.Loader;

class SimpleUnloadableAssemblyLoadContext : AssemblyLoadContext
{
    public SimpleUnloadableAssemblyLoadContext()
       : base(isCollectible: true)
    {
    }

    protected override Assembly Load(AssemblyName assemblyName) => null;
}

class Program
{
    private static int ExecuteAssembly(Assembly assembly, string[] args)
    {
        MethodInfo entry = assembly.EntryPoint;

        object result = entry.GetParameters().Length > 0 ?
                    entry.Invoke(null, new object[] { args }) :
                    entry.Invoke(null, null);

        return (result != null) ? (int)result : 0;
    }

    static void Main(string[] args)
    {
        for (;;) {
            var context = new SimpleUnloadableAssemblyLoadContext();
            Assembly assembly = context.LoadFromAssemblyPath(@"D:\repro\hello\bin\Debug\netcoreapp3.0\hello.dll");
            ExecuteAssembly(assembly, Array.Empty<string>());
            context.Unload();

            GC.Collect();

            Console.WriteLine($"Number of loaded assemblies: {AppDomain.CurrentDomain.GetAssemblies().Length}");
        }
    }
}

Update path to hello.dll (it is the stock hello world from dotnet new console).

Result: Number of assemblies and process memory loaded keep growing indefinitely. They never unload.

Tested on 3.0.0-preview1-27029-03 (CoreCLR: https://github.com/dotnet/coreclr/tree/4fbeb37c62427b51410217cd49cec84e03fdca34).

[jkotas]

cc @janvorli

Found while trying to build an example for https://github.com/dotnet/corefx/issues/19773

[janvorli]

This slightly modified sample works as expected. I need to look at the generated code difference, since it seems that your example should work too.

using System;
using System.Reflection;
using System.Runtime.Loader;
using System.Runtime.CompilerServices;

class SimpleUnloadableAssemblyLoadContext : AssemblyLoadContext
{
    public SimpleUnloadableAssemblyLoadContext()
       : base(isCollectible: true)
    {
    }

    protected override Assembly Load(AssemblyName assemblyName) => null;
}

class Program
{
    private static int ExecuteAssembly(Assembly assembly, string[] args)
    {
        MethodInfo entry = assembly.EntryPoint;

        object result = entry.GetParameters().Length > 0 ?
                    entry.Invoke(null, new object[] { args }) :
                    entry.Invoke(null, null);

        return (result != null) ? (int)result : 0;
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    private static int ExecuteAndUnload(string[] args)
    {
        var context = new SimpleUnloadableAssemblyLoadContext();
        Assembly assembly = context.LoadFromAssemblyPath(@"D:\repro\hello\bin\Debug\netcoreapp3.0\hello.dll");
        int st = ExecuteAssembly(assembly, Array.Empty<string>());
        context.Unload();

        return st;
    }

    static void Main(string[] args)
    {
        for (;;) {
            ExecuteAndUnload(Array.Empty<string>());

            GC.Collect();

            Console.WriteLine($"Number of loaded assemblies: {AppDomain.CurrentDomain.GetAssemblies().Length}");
        }
    }
}

[janvorli]

Ok, so the reason why it keeps the assemblies in your version is really simple after all. It is this call:
AppDomain.CurrentDomain.GetAssemblies().Length. JIT creates a local (Assembly[]) for the result of AppDomain.CurrentDomain.GetAssemblies(). So once you call that, all the assemblies are rooted through that. In the next loop, this local is updated with the new list of assemblies that obviously includes all the previous ones etc etc.

[jkotas]

Ah, makes sense. Thanks for taking a look