Attempted to access an unloaded AppDomain exception

[hieudole]

Hello @kleisauke. I'm using your library in my website for image resizing and format converting. It performs very well, but sometimes I get AppDomainUnloadedException. I don't have any build/development on the website at that time.

System.AppDomainUnloadedException: Attempted to access an unloaded AppDomain.
   at NetVips.Internal.VipsOperation.Build(Operation operation)
   at NetVips.Operation.Call(String operationName, VOption kwargs, Image matchImage, Object[] args)
   at NetVips.Image.ExtractArea(Int32 left, Int32 top, Int32 width, Int32 height)

System.AppDomainUnloadedException: Attempted to access an unloaded AppDomain.
   at NetVips.Internal.VipsOperation.Build(Operation operation)
   at NetVips.Operation.Call(String operationName, VOption kwargs, Image matchImage, Object[] args)
   at NetVips.Image.HeifsaveTarget(Target target, Nullable`1 q, Nullable`1 lossless, Nullable`1 compression, Nullable`1 effort, Nullable`1 subsampleMode, Nullable`1 strip, Double[] background, Nullable`1 pageHeight)
   at NetVips.Image.HeifsaveStream(Stream stream, Nullable`1 q, Nullable`1 lossless, Nullable`1 compression, Nullable`1 effort, Nullable`1 subsampleMode, Nullable`1 strip, Double[] background, Nullable`1 pageHeight)

I need to restart my app pool every time this issue happens because NetVips can't work anymore. Do you know any reason that might relate to this issue?

I'm using:
.Net Framework 4.8
NetVips 2.1.0
NetVips.Native 8.12.2

[kleisauke]

Hi @hieudole,

I'm not sure what's causing this. Are you seeing this exception with ASP.NET? Is this on Windows? I'm aware that ASP.NET may recycle the AppDomain, but as far as I know that ought to work without any problems.

I tried this test program to sanity check that:

Details

using System;
using System.IO;
using System.Reflection;

namespace ConsoleApp
{
    class Program
    {
        private static readonly string NetVipsAssembly =
            Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "NetVips.dll");

        static void Main()
        {
            for (var i = 0; i < 2; ++i)
            {
                // Create the domain
                Console.WriteLine($"[Program.Main] ===Pass #{i}===");
                Console.WriteLine($"[Program.Main] Creating the domain \"My Domain {i}\"");
                var domain = AppDomain.CreateDomain($"My Domain {i}");

                // Create a Proxy object in the new domain, where we want the assembly (and NetVips) to reside
                var type = typeof(Proxy);
                var proxy = (Proxy)domain.CreateInstanceAndUnwrap(
                    type.Assembly.FullName,
                    type.FullName);

                // Use the Proxy to call into the new assembly
                proxy.RunOperation(NetVipsAssembly);

                Console.WriteLine("[Program.Main] Before Domain Unload");
                AppDomain.Unload(domain);
                Console.WriteLine("[Program.Main] After Domain Unload");

                // Validate that the assembly does not exist anymore
                try
                {
                    Console.WriteLine(
                        $"[Program.Main] The Proxy object is valid ({proxy}). Unexpected domain unload behavior");
                }
                catch (Exception)
                {
                    Console.WriteLine("[Program.Main] The Proxy object is not valid anymore, domain unload complete.");
                }
            }

            Console.ReadKey();
        }

        class Proxy : MarshalByRefObject
        {
            public void RunOperation(string assemblyPath)
            {
                Console.WriteLine("[Proxy       ] In RunOperation");

                // Register domain unload event
                AppDomain.CurrentDomain.DomainUnload += OnDomainUnload;

                var assembly = Assembly.LoadFile(assemblyPath);
                var type = assembly.GetType("NetVips.Image");
                var method1 = type.GetMethod("Black");
                var method2 = type.GetMethod("WriteToFile");
                var instance = method1.Invoke(null, new object[] { 100, 100, 3 });
                method2.Invoke(instance, new object[] { "x.jpg", null });
            }

            private static void OnDomainUnload(object sender, EventArgs e)
            {
                Console.WriteLine(
                    $"[Proxy       ] In OnDomainUnload current domain = {AppDomain.CurrentDomain.FriendlyName}");
            }
        }
    }
}

Output:

[Program.Main] ===Pass #0===
[Program.Main] Creating the domain "My Domain 0"
[Proxy       ] In RunOperation
[Program.Main] Before Domain Unload
[Proxy       ] In OnDomainUnload current domain = My Domain 0
[Program.Main] After Domain Unload
[Program.Main] The Proxy object is not valid anymore, domain unload complete.
[Program.Main] ===Pass #1===
[Program.Main] Creating the domain "My Domain 1"
[Proxy       ] In RunOperation
[Program.Main] Before Domain Unload
[Proxy       ] In OnDomainUnload current domain = My Domain 1
[Program.Main] After Domain Unload
[Program.Main] The Proxy object is not valid anymore, domain unload complete.

But this works without any issues.

[hieudole]

Thanks @kleisauke for investigating this issue. I have the same output with yours.
My app is running with ASP.NET on Windows. Perhaps the app pool was recycled while Netvips was running an operation.

[kleisauke]

I'm removing the triage label, without a reproduction it'll be hard to debug further.

[kleisauke]

@hieudole Were you able to make any progress with this?

[hieudole]

@kleisauke I'm closing this issue since I'm not able to reproduce it.