Description

The AssemblyLoadContext of any type that has undergone data contract serialization cannot be unloaded / collected. The specific offender is DataContractCriticalHelper, which has a few static caches and also retains a reference to the most-recently-serialized type.

Reproduction Steps

1. Create a collectible AssemblyLoadContext
2. Load a DataContractSerializable type into the collectible ALC
3. Instantiate the collectible type
4. Serialise the instance using something like:

var ser = new DataContractSerializer(type);
using (var ms = new System.IO.MemoryStream())
{
    ser.WriteObject(ms, document);
    var bytes = ms.ToArray();
    return System.Text.Encoding.UTF8.GetString(bytes, 0, bytes.Length);
}

5. Invoke Unload() on the collectible ALC
6. Trigger a garbage collection (collect / finalise / collect)
7. Review loaded assemblies using a debugger or something like AppDomain.CurrentDomain.GetAssemblies()
8. Observe that the collectible ALC / assembly has not unloaded.
9. Repeat steps but skip step 4 and observer that the ALC / assembly has unloaded properly

Expected behavior

Unloading and collecting results in proper collection of collectible types / assemblies / contexts.

Actual behavior

Collectible types / assemblies / contexts that have been data contract serialised cannot be collected.

Regression?

No response

Known Workarounds

The following code allows proper collection to occur:

private static void ClearDataContractCaches()
{
    var t = AppDomain.CurrentDomain.GetAssemblies()
        .SelectMany(x => x.GetTypes())
        .Single(x => x.Name == "DataContractCriticalHelper");

    var staticCache = t.GetFields(BindingFlags.NonPublic | BindingFlags.Static).Select(x => x.GetValue(t)).ToArray();
    
    foreach (var dictionary in staticCache.OfType<IDictionary>())
        dictionary.Clear();

    foreach (var array in staticCache.OfType<Array>())
    {
        for (var i = 0; i < array.Length; I++)
        {
            array.SetValue(null, i);
        }
    }
    
    var handleBreaker = new DataContractSerializer(typeof(object));
    var o = new object();
    using (var ms = new MemoryStream())
    {
        handleBreaker.WriteObject(ms, o);
    }
}

Configuration

.NET 6
macOS Ventura 13.0
ARM64

Other information

No response