Unable to cast object of type ServiceCollection to type 'Autofac.ContainerBuilder' using dotnet core and autofac

Question

I am trying to use autofac in my dotnet core 3.1 project, but I am unable to run project after writing ConfigureContainer inside the startup.cs file.

    public void ConfigureContainer(ContainerBuilder builder)
    {

        var databaseConnectionString = Configuration.GetConnectionString("Database");

        builder.RegisterModule(new MediatorModule());
        builder.RegisterModule(new ApplicationModule(databaseConnectionString));
    }

The error I am getting is :

System.InvalidCastException: Unable to cast object of type
'Microsoft.Extensions.DependencyInjection.ServiceCollection' to type 'Autofac.ContainerBuilder'. at Microsoft.Extensions.Hosting.Internal.ConfigureContainerAdapter`1.ConfigureContainer(HostBuilderContext hostContext, Object containerBuilder) at Microsoft.Extensions.Hosting.HostBuilder.CreateServiceProvider() at Microsoft.Extensions.Hosting.HostBuilder.Build() at Program.Main(String[] args) in C:\src\Program.cs:line 39

Answer 1

When you configure your host you should call UseServiceProviderFactory(new AutofacServiceProviderFactory())

public static void Main(string[] args)
{
    // ASP.NET Core 3.0+:
    // The UseServiceProviderFactory call attaches the
    // Autofac provider to the generic hosting mechanism.
    var host = Host.CreateDefaultBuilder(args)
                   .UseServiceProviderFactory(new AutofacServiceProviderFactory())
                   .ConfigureWebHostDefaults(webHostBuilder => {
                       webHostBuilder
                        .UseContentRoot(Directory.GetCurrentDirectory())
                        .UseIISIntegration()
                        .UseStartup<Startup>();
                   })
                   .Build();

    host.Run();
}

Without this, .net core will create a ServiceCollection instead of a ContainerBuilder and an InvalidCastException will be thrown.