How do I access Configuration in any class in ASP.NET Core?

The right way to do it:

In .NET Core you can inject the IConfiguration as a parameter into your Class constructor, and it will be available.

public class MyClass 
{
    private IConfiguration configuration;
    public MyClass(IConfiguration configuration)
    {
        ConnectionString = new configuration.GetValue<string>("ConnectionString");
    }

Now, when you want to create an instance of your class, since your class gets injected the IConfiguration, you won't be able to just do new MyClass(), because it needs a IConfiguration parameter injected into the constructor, so, you will need to inject your class as well to the injecting chain, which means two simple steps:

1) Add your Class/es - where you want to use the IConfiguration, to the IServiceCollection at the ConfigureServices() method in Startup.cs

services.AddTransient<MyClass>();

2) Define an instance - let's say in the Controller, and inject it using the constructor:

public class MyController : ControllerBase
{
    private MyClass _myClass;
    public MyController(MyClass myClass)
    {
        _myClass = myClass;
    }

Now you should be able to enjoy your _myClass.configuration freely...

Another option:

If you are still looking for a way to have it available without having to inject the classes into the controller, then you can store it in a static class, which you will configure in the Startup.cs, something like:

public static class MyAppData
{
    public static IConfiguration Configuration;
}

And your Startup constructor should look like this:

public Startup(IConfiguration configuration)
{
    Configuration = configuration;
    MyAppData.Configuration = configuration;
}

Then use MyAppData.Configuration anywhere in your program.

Don't confront me why the first option is the right way, I can just see experienced developers always avoid garbage data along their way, and it's well understood that it's not the best practice to have loads of data available in memory all the time, neither is it good for performance and nor for development, and perhaps it's also more secure to only have with you what you need.

I know this is old but given the IOptions patterns is relatively simple to implement:

1. Class with public get/set properties that match the settings in the configuration

   public class ApplicationSettings
   {
       public string UrlBasePath { get; set; }
   }
   

2. register your settings

   public void ConfigureServices(IServiceCollection services)
   {
    ...
    services.Configure<ApplicationSettings>(Configuration.GetSection("ApplicationSettings"));
   ...
   }
   

3. inject via IOptions

   public class HomeController
   {
      public HomeController(IOptions<ApplicationSettings> appSettings)
      { ...
       appSettings.Value.UrlBasePath
       ...
       // or better practice create a readonly private reference
       }
    }
   

I'm not sure why you wouldn't just do this.

Update

Using ASP.NET Core 2.0 will automatically add the IConfiguration instance of your application in the dependency injection container. This also works in conjunction with ConfigureAppConfiguration on the WebHostBuilder.

For example:

public static void Main(string[] args)
{
    var host = WebHost.CreateDefaultBuilder(args)
        .ConfigureAppConfiguration(builder =>
        {
            builder.AddIniFile("foo.ini");
        })
        .UseStartup<Startup>()
        .Build();

    host.Run();
}

---

It's just as easy as adding the IConfiguration instance to the service collection as a singleton object in ConfigureServices:

public void ConfigureServices(IServiceCollection services)
{
   services.AddSingleton<IConfiguration>(Configuration);

   // ...
}

Where Configuration is the instance in your Startup class.

This allows you to inject IConfiguration in any controller or service:

public class HomeController
{
   public HomeController(IConfiguration configuration)
   {
      // Use IConfiguration instance
   }
}