How do I conditionally register components?

There are times when you want to pick what gets registered in the container at runtime, maybe based on an environment variable or application parameter. Here are some options...

Use Configuration

Autofac provides a configuration mechanism that allows you to specify registrations via a configuration file. You can set up different configuration files to use in different environments and/or with different parameters, then during application startup pick the appropriate configuration file to read and register.

If you choose to use the Microsoft.Extensions.Configuration abstractions (Autofac 4.0+) you can even represent configuration as environment variables directly. See the docs on Microsoft.Extensions.Configuration for how to represent configuration in environment variables.

Use Modules

Autofac modules are a programmatic configuration mechanism that bundles registrations together. You can add parameters (e.g., constructor parameters or properties) to your module such that it registers different things or behaves differently based on provided values - values you read from the runtime environment.

There is an example of this in the documentation for Autofac modules.

Lambda Registrations

You can register components using lambda expressions and make a runtime choice right in the registration for how to handle things. Note this may have an effect on performance depending on the expense of the runtime check and how often it gets executed, but it's an option.

builder.Register(c =>
  {
    var environment = Environment.GetEnvironmentVariable("environment_name");
    if(environment == "DEV")
    {
      return new DevelopmentObject();
    }
    else
    {
      return new ProductionObject();
    }
  })
  .As<IMyObject>()
  .SingleInstance();