Loaded assemblies from an AssemblyLoadContext not unloaded on AssemblyLoadContext.Unload() call

[apilatosba]

Description

Good morning. I want to reload a dll at runtime but I can't. I have two programs lets name them host and plugin. Host compiles to an executable and plugin compiles to a dll. Host loads a function from the plugin at runtime and i make changes to the plugin and recompile it. And then in the host i want to reload the plugin dll and want to use the updated function without needing the relaunch the host program. i have created a small example where this issue exist. For reference here is the code:

Host.cs:

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

namespace Host {
   public class PluginAssemblyLoadContext : AssemblyLoadContext {
      public PluginAssemblyLoadContext() : base(true) { }
   }

   public class HostState {
      public Assembly                  pluginAssembly;
      public PluginAssemblyLoadContext   loadContext;
      public Action                    pluginExecute;
   }

   public static class Host {
      public const string PLUGIN_PATH = "../plugin/bin/Debug/net9.0/plugin.dll";

      public static void Main() {
         HostState host = new HostState();
         host.loadContext = new PluginAssemblyLoadContext();
         ReloadDll(host);

         Console.WriteLine("Press E to execute the plugin, R to reload, or Q to quit.");

         for (;;) {
            var key = Console.ReadKey(true);
            if (false) {
            } else if (key.Key == ConsoleKey.E) {
               host.pluginExecute.Invoke();
            } else if (key.Key == ConsoleKey.R) {
               ReloadDll(host);
            } else if (key.Key == ConsoleKey.Q) {
               return;
            }
         }
      }

      public static void ReloadDll(HostState host) {
         host.pluginAssembly = null;
         host.pluginExecute = null;

         GC.Collect();
         GC.WaitForPendingFinalizers();

         host.loadContext?.Unload();

         GC.Collect();
         GC.WaitForPendingFinalizers();

         host.loadContext = new PluginAssemblyLoadContext();
         host.pluginAssembly = host.loadContext.LoadFromAssemblyPath(Path.GetFullPath(PLUGIN_PATH));

         Type type = host.pluginAssembly.GetType("Plugin");
         MethodInfo methodInfo = type.GetMethod("Execute", BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Static, []);

         if (methodInfo != null) {
            host.pluginExecute = methodInfo.CreateDelegate<Action>();
         }
      }
   }
}

Plugin.cs:

using System;

public static class Plugin {
   public static void Execute() {
      Console.WriteLine("1");
   }
}

The Scenario

I launch the host program. I press E to execute plugin. I got the output "1". I go to Plugin.cs. I change Execute() function to output "2" instead of "1". I go back to the terminal where host program is running. I press R and then I press E.

What Happens

I got the output "1" again.

What should have happened

I should have got the output "2" instead.

What I have tried

• I tried to set every reference to null which i obtained from the PluginAssemblyLoadContext as mentioned in the third remark of ms docs
• I tried to change the assembly version when i build the plugin.dll but that didn't help either.
• Called GC.Collect() and GC.WaitForPendingFinalizers() to make sure every reference gets garbage collected.

Why I want to do this

I have a game engine and i want to be able reload the game dll without needing to restart the editor of the engine. So in this case host would be the editor of the engine and plugin would be the game.

Specs

• cpu: x86-64
• os: debian sid
• dotnet sdk version: 9.0.305

-

Any idea how to reload a dll at runtime? Any help would be appreciated.

Reproduction Steps

clone the repo and then follow the scenario above.

Expected behavior

to be able to execute the new function in recompled plugin.dll

Actual behavior

still old function is being called.

Regression?

idk

Known Workarounds

i know no workarounds

Configuration

1. dotnet --version: 9.0.305
2. os: debian sid
3. cpu architecture: x86-64
4. i dont think this problem is specific to this configuration tho i have not tried other configurations.

Other information

i think the problem is the AssemblyLoadContext.Unload() function doesn't unload the assembly thats loaded from it.

[dotnet-policy-service]

Tagging subscribers to this area: @vitek-karas, @agocke, @VSadov
See info in area-owners.md if you want to be subscribed.

[huoyaoyuan]

AssemblyLoadContext.Unload doesn't do the unload work. It's even not necessary for unloading. It just prevents loading more assemblies into it.

The actual thing to do for unloading is to clear the loadContext field. An ALC can only be unloaded if it's cleared by GC.

[apilatosba]

what do you mean by clearing the loadContext field and how can i do that? Do you mean setting the loadContext to null or another object? I already set loadContext to a new PluginAssemblyLoadContext in ReloadDll(). or do you mean something else?

[vitek-karas]

See https://learn.microsoft.com/en-us/dotnet/standard/assembly/unloadability

Things which I would definitely try is - isolate the cleanup and unloading of the assembly into its own method which is disabled for inlining - this avoid any potential problems with local keeping the assembly alive on the stack.
Loop around GC.Collect - it may take several GCs to completely unload the assembly - using the test through a weak ref as shown in the article above is one way to terminate the loop.

If nothing helps the article has some tips what to look for.

[agocke]

We found and fixed a load context bug for net10. This is likely the same bug. The fix should be in nightly builds.

[Seabizkit]

Just checking he did day. Net 9,

Are you saying the bug in 10 is in 9, if so shouldnt it be back ported to 8 and 9?

[apilatosba]

We found and fixed a load context bug for net10. This is likely the same bug. The fix should be in nightly builds.

thanks. i will right here again if the issue is resolved when dotnet10 releases next month.

[Seabizkit]

@apilatosba , thanks please do, as ive had a nightmare of a time in the past, trying to get plugin stuff working...

i dont have a specific issues now, and im aware of an issue in 10, i have that myself but that was not in 9, so that is why im pointing it out.... these types of things where you not sure if its a bug or you are consuming it wrong can be quite frustrating.

but i must say, when i do have an issue, the team's working on these things are Awesome.