AutoFac：容器生成上的NullReferenceException

Question

I am using AutoFac 4.6.0 with XML Configuration File and Modules in an Web API / OWIN Context.

When I call Build() of ContainerBuilder, I am getting a NullReferenceException. What am I doing wrong? Is it, because "TheFailingModule" is in another assembly? If I add the property "Foo" to "TestModule", it works.

I added a break point in the Load-Methods of the modules, theyhave not been hitted.

UPDATE

I found out, that the private property MyAppContext causes the issue.

Please see the code:

Startup.cs

namespace MyNamespace.Web
{
    // [...]
    public void Configuration(IAppBuilder app)
    {
        var configurationBuilder = new ConfigurationBuilder();
        configurationBuilder.AddXmlFile("autofac.config");
        var module = new ConfigurationModule(configurationBuilder.Build());

        var builder = new ContainerBuilder();
        builder.RegisterModule(module);

        builder.RegisterApiControllers(Assembly.GetExecutingAssembly());
        var config = new HttpConfiguration();

        builder.RegisterWebApiFilterProvider(config);
        AutoFacConfig.Register(builder);

        var container = builder.Build(); // <-- NullReferenceException
        // [...]
    }
    // [...]
}

autofac.config

<?xml version="1.0" encoding="utf-8"?>

<autofac>
  <modules name="TestModule">
    <type>MyNamespace.Web.TestModule, MyNamespace.Web</type>
  </modules>

  <modules name="TheFailingModule">
    <type>MyNamespace.Failing.TheFailingModule, MyNamespace.Failing</type>
    <properties Foo="Bar"></properties>
  </modules>
</autofac>

TheFailingModule.cs

namespace MyNamespace.Failing
{
    public class TheFailingModule : Module
    {
        private MyAppContext MyAppContext { get; set; }
        public string Foo { get; set; }

        protected override void Load(ContainerBuilder builder)
        {
            // [...]

            base.Load(builder);
        }
    }
}

TestModule

namespace MyNamespace.Web
{
    public class TestModule : Module
    {
        protected override void Load(ContainerBuilder builder)
        {
            // Do some stuff

            base.Load(builder);
        }
    }
}

StackTrace

at Autofac.Configuration.Core.ConfigurationExtensions.<>c__DisplayClass3_0.<GetProperties>b__0(ParameterInfo pi, IComponentContext c)
   at Autofac.Core.ResolvedParameter.CanSupplyValue(ParameterInfo pi, IComponentContext context, Func`1& valueProvider)
   at Autofac.Core.Activators.Reflection.ReflectionActivator.InjectProperties(Object instance, IComponentContext context)
   at Autofac.Core.Activators.Reflection.ReflectionActivator.ActivateInstance(IComponentContext context, IEnumerable`1 parameters)
   at Autofac.Configuration.Core.ModuleRegistrar.RegisterConfiguredModules(ContainerBuilder builder, IConfiguration configuration)
   at Autofac.Configuration.Core.ConfigurationRegistrar.RegisterConfiguration(ContainerBuilder builder, IConfiguration configuration)
   at Autofac.Configuration.ConfigurationModule.Load(ContainerBuilder builder)
   at Autofac.Module.Configure(IComponentRegistry componentRegistry)
   at Autofac.ContainerBuilder.Build(IComponentRegistry componentRegistry, Boolean excludeDefaultModules)
   at Autofac.ContainerBuilder.Build(ContainerBuildOptions options)

Answer 1

I found the issue. I used a private property in the module (MyAppContext) whicht was not set by autofac.config. I changed it to a private field and is now working.