Autofac.Core.DependencyResolutionException: 类型 '' 的表达式不能用于 Autofac 6.2 中的 return 类型 'System.Object'

Question

我这里有代码，曾经在 Autofac 2.6 中工作，但现在在 Autofac 6.2 中不再工作：

class Program
{
    static void Main(string[] args)
    {
        Holder holder = null;

        Build(c =>
        {
            holder = new Holder() { Verifiers = c.Resolve<Verifiers>() };
        });
        var res = holder.Verifiers;
        Console.WriteLine("Success");
    }

    private static void Build(Action<IContainer> container)
    {
        var builder = new ContainerBuilder();
        builder.RegisterAssemblyTypes(typeof(Verifiers).Assembly).AsSelf().AsImplementedInterfaces();

        builder.RegisterType<ConsoleOutput>().As<IOutput>();
        builder.RegisterGeneric(typeof(EntityDisplay<>.Params));
        builder.RegisterGeneric(typeof(EntityDisplayFactory<,>));
        builder.Register<Func<DisplayFactories>>(c => () => new DisplayFactories() { Instances = c.Resolve<IEntityDisplayFactory<object>>() });
        container(builder.Build());
    }

}

public interface IOutput
{
    void Write(string content);
}

public class ConsoleOutput : IOutput
{
    public ConsoleOutput()
    {
    }
    public void Write(string content)
    {
        Console.WriteLine(content);
    }
}

public interface IEntityDisplay
{
    void Display(object parameter);
}

public class EntityDisplay<T> : IEntityDisplay
{
    public struct Params
    {
        private readonly IOutput _output;
        public Params(IOutput output)
        {
            _output = output;
        }
    }

    private readonly T _entity;
    public EntityDisplay(T entity)
    {
        _entity = entity;
    }
    public void Display(object parameter)
    {
    }
}

public interface IEntityDisplayFactory<in T>
{
    IEntityDisplay Create(T entity);
}

public class InstanceEntityDisplayFactory : EntityDisplayFactory<object, object>
{
    public InstanceEntityDisplayFactory(EntityDisplay<object>.Params parameters) : base(parameters)
    {
    }
}

public class EntityDisplayFactory<T, I> : IEntityDisplayFactory<T>
{
    private readonly EntityDisplay<T>.Params _param;
    public EntityDisplayFactory(EntityDisplay<T>.Params parameters)
    {
        _param = parameters;
    }
    public IEntityDisplay Create(T entity)
    {
        return new EntityDisplay<T>(entity);
    }
}

public class DisplayFactories
{
    public IEntityDisplayFactory<object> Instances { get; set; }
}

public class Verifiers
{
    private readonly DisplayFactories _displayFunc;
    public Verifiers(Func<DisplayFactories> displayFunc)
    {
        _displayFunc = displayFunc();
    }

    public void Show()
    {
        DisplayFactories res = _displayFunc;
        res.Instances.Create(null);
    }
}

public class Holder
{
    public Verifiers Verifiers { get; set; }
}

Autofac 6.2 抛出的异常是

> Autofac.Core.DependencyResolutionException   HResult=0x80131500  
> Message=An exception was thrown while activating
> AutofacTest46.InstanceEntityDisplayFactory.   Source=Autofac  
> StackTrace:    at
> Autofac.Core.Resolving.Middleware.ActivatorErrorHandlingMiddleware.Execute(ResolveRequestContext
> context, Action`1 next)    at
> Autofac.Core.Resolving.Pipeline.ResolvePipelineBuilder.<>c__DisplayClass14_0.<BuildPipeline>b__1(ResolveRequestContext
> ctxt)    at
> Autofac.Core.Pipeline.ResolvePipeline.Invoke(ResolveRequestContext
> ctxt)    at
> Autofac.Core.Resolving.Middleware.RegistrationPipelineInvokeMiddleware.Execute(ResolveRequestContext
> context, Action`1 next)    at
> Autofac.Core.Resolving.Pipeline.ResolvePipelineBuilder.<>c__DisplayClass14_0.<BuildPipeline>b__1(ResolveRequestContext
> ctxt)    at
> Autofac.Core.Resolving.Middleware.SharingMiddleware.Execute(ResolveRequestContext
> context, Action`1 next)    at
> Autofac.Core.Resolving.Pipeline.ResolvePipelineBuilder.<>c__DisplayClass14_0.<BuildPipeline>b__1(ResolveRequestContext
> ctxt)    at
> Autofac.Core.Resolving.Middleware.ScopeSelectionMiddleware.Execute(ResolveRequestContext
> context, Action`1 next)    at
> Autofac.Core.Resolving.Pipeline.ResolvePipelineBuilder.<>c__DisplayClass14_0.<BuildPipeline>b__1(ResolveRequestContext
> ctxt)    at
> Autofac.Core.Resolving.Middleware.CircularDependencyDetectorMiddleware.Execute(ResolveRequestContext
> context, Action`1 next)    at
> Autofac.Core.Resolving.Pipeline.ResolvePipelineBuilder.<>c__DisplayClass14_0.<BuildPipeline>b__1(ResolveRequestContext
> ctxt)    at
> Autofac.Core.Pipeline.ResolvePipeline.Invoke(ResolveRequestContext
> ctxt)    at
> Autofac.Core.Resolving.ResolveOperation.GetOrCreateInstance(ISharingLifetimeScope
> currentOperationScope, ResolveRequest request)    at
> Autofac.Core.Resolving.Pipeline.DefaultResolveRequestContext.ResolveComponent(ResolveRequest
> request)    at
> Autofac.ResolutionExtensions.TryResolveService(IComponentContext
> context, Service service, IEnumerable`1 parameters, Object& instance) 
> at Autofac.ResolutionExtensions.ResolveService(IComponentContext
> context, Service service, IEnumerable`1 parameters)    at
> Autofac.ResolutionExtensions.Resolve[TService](IComponentContext
> context, IEnumerable`1 parameters)    at
> Autofac.ResolutionExtensions.Resolve[TService](IComponentContext
> context)    at
> AutofacTest46.Program.<>c__DisplayClass1_0.<Build>b__1() in D:\.net
> Samples project\May 2021\AutofacTest46\Program.cs:line 34    at
> AutofacTest46.Verifiers..ctor(Func`1 displayFunc) in D:\.net Samples
> project\May 2021\AutofacTest46\Outputs.cs:line 94    at
> Autofac.Core.Activators.Reflection.BoundConstructor.Instantiate()
> 
>   This exception was originally thrown at this call stack:
>     [External Code]
> 
> Inner Exception 1: ArgumentException: Expression of type
> 'AutofacTest46.EntityDisplay`1+Params[System.Object]' cannot be used
> for return type 'System.Object'

为什么在 Autofac 6.2 中会发生这种情况？
如何解决这个错误？

Answer 1

问题是 EntityDisplay<>.Params 是一个值类型 - struct。将其更改为 class 就可以了。

由于 DI 和反射以及诸如此类的东西目前结合在一起的方式，我们必须能够将基于反射的操作的输出转换为 object。 You can see the current wire-up code here. 不幸的是，值类型的构造函数调用程序没有 return object，所以我们无法解析它，一切都崩溃了。

我们可能会接受带有修复代码的拉取请求，但是 DI 到值类型通常不是人们做的事情。

无论如何，切换到 class 就可以了。

顺便说一句，这有点难以完成 - 在以后的问题中，它可能有助于减少重现（例如，你不需要 Holder class所有，这只是噪音；你不需要 Verifiers，等等。在问题中的异常消息上获得更清晰的格式也会有所帮助，并确保包括所有内部异常。关键是指向构造函数调用程序的内部异常。我只能通过 copy/pasting 这段代码并进行调试才能看到它，不幸的是，我通常没有时间去做。我不得不继续回答这一个。

Autofac.Core.DependencyResolutionException: 类型 '' 的表达式不能用于 Autofac 6.2 中的 return 类型 'System.Object'

Autofac.Core.DependencyResolutionException: Expression of type '' cannot be used for return type 'System.Object' in Autofac 6.2

c#

autofac

autofac-configuration