AssemblyLoadContext，动态加载程序集，实例化 object 并转换为共享接口

Question

I'm trying to load an assembly with the AssemblyLoadContext (exists from the version 3.0 of netcore), instantiate an object and cast this object to an interface, but I get a cast exception error.

The interface is shared between the project that loads the assembly and the implementation instantiated. The object apparently is instantiated correctly but I get the unexpected error when I do (T)instance .

Trying with watcher I'm able to cast the instance correctly to the interface, following the code I'm using and a screenshot of the watcher:

private (ExecutionAssemblyLoadContext, T) LoadTheAssemblyAndInstance<T>(string assemblyName, string typeNameToInstance)
{
    var basePath = Path.GetDirectoryName(Assembly.GetEntryAssembly().Location);
    var assemblyContext = new ExecutionAssemblyLoadContext($"{basePath}/{assemblyName}.dll");
    var assembly = assemblyContext.LoadFromAssemblyPath($"{basePath}/{assemblyName}.dll");
    var externalCodeEvent = typeNameToInstance != null ? assembly.ExportedTypes
        .Where(x => x.FullName == typeNameToInstance)
        .Single() : assembly.ExportedTypes.First();
    var instance = Activator.CreateInstance(
            externalCodeEvent,
            _defaultConstructorParameters
        );
    return (assemblyContext, (T)instance);
}

this is the full exception message:

System.InvalidCastException: 'Unable to cast object of type 'Expriva.NewWorkflow.BPMN.ExecutionCodeTest.ExecutionContractTest' to type 'Expriva.NewWorkflow.ExternalShared.Interfaces.IExecutionContract'.'

Following a screenshot that shows that T is implemented by the instance

Answer 1

AssemblyLoadContext to support dynamic code loading and unloading, it creates an isolated context for loading code and its dependencies in their own AssemblyLoadContext instance.

The problem was that in the ExecutionAssemblyLoadContext implementation, the dependencies was resolved and isolated. Using the default implementation, siggested by the documentation of AssemblyLoadContext the shared types will not be isolated. Following the correct implementation to use to share the interface.

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

    protected override Assembly Load(AssemblyName name)
    {
        return null;
    }
}

Answer 2

I also came across this problem, think of it as if the different AssemblyLoadContext have their own copies of loaded types, so although you may reference the shared assembly in both contexts as far as they are concerned the types contained are unique.

The solution is to use the Load override in the derived ALC to resolve the assembly from the shared context.

This is simple if the shared types are loaded into the default ALC, for example.

public class ModAssemblyLoadContext : AssemblyLoadContext
{
    public ModAssemblyLoadContext()
        : base("ModAssemblyLoadContext", isCollectible: true)
    {
    }

    protected override Assembly Load(AssemblyName assemblyName)
    {
        return Default.Assemblies
            .FirstOrDefault(x => x.FullName == assemblyName.FullName);
    }
}