Hosting in ASP.NET Core

By Luke Latham

ASP.NET Core apps configure and launch a host, which is responsible for app startup and lifetime management. At a minimum, the host configures a server and a request processing pipeline.

Setting up a host

Create a host using an instance of WebHostBuilder. This is typically performed in your app's entry point, the Main method. In the project templates, Main is located in Program.cs. A typical Program.cs calls CreateDefaultBuilder to start setting up a host:

public class Program
{
    public static void Main(string[] args)
    {
        BuildWebHost(args).Run();
    }

    public static IWebHost BuildWebHost(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .UseStartup<Startup>()
            .Build();
}

CreateDefaultBuilder performs the following tasks:

The content root determines where the host searches for content files, such as MVC view files. The default content root is Directory.GetCurrentDirectory. This results in using the web project's root folder as the content root when the app is started from the root folder (for example, calling dotnet run from the project folder). This is the default used in Visual Studio and the dotnet new templates.

See Configuration in ASP.NET Core for more information on app configuration.

Note

As an alternative to using the static CreateDefaultBuilder method, creating a host from WebHostBuilder is a supported approach with ASP.NET Core 2.x. See the ASP.NET Core 1.x tab for more information.

When setting up a host, you can provide Configure and ConfigureServices methods. If you specify a Startup class, it must define a Configure method. For more information, see Application Startup in ASP.NET Core. Multiple calls to ConfigureServices append to one another. Multiple calls to Configure or UseStartup on the WebHostBuilder replace previous settings.

Host configuration values

WebHostBuilder provides methods for setting most of the available configuration values for the host, which can also be set directly with UseSetting and the associated key. When setting a value with UseSetting, the value is set as a string (in quotes) regardless of the type.

Capture Startup Errors

This setting controls the capture of startup errors.

Key: captureStartupErrors
Type: bool (true or 1)
Default: Defaults to false unless the app runs with Kestrel behind IIS, where the default is true.
Set using: CaptureStartupErrors

When false, errors during startup result in the host exiting. When true, the host captures exceptions during startup and attempts to start the server.

WebHost.CreateDefaultBuilder(args)
    .CaptureStartupErrors(true)
    ...

Content Root

This setting determines where ASP.NET Core begins searching for content files, such as MVC views.

Key: contentRoot
Type: string
Default: Defaults to the folder where the app assembly resides.
Set using: UseContentRoot

The content root is also used as the base path for the Web Root setting. If the path doesn't exist, the host fails to start.

WebHost.CreateDefaultBuilder(args)
    .UseContentRoot("c:\\mywebsite")
    ...

Detailed Errors

Determines if detailed errors should be captured.

Key: detailedErrors
Type: bool (true or 1)
Default: false
Set using: UseSetting

When enabled (or when the Environment is set to Development), the app captures detailed exceptions.

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.DetailedErrorsKey, "true")
    ...

Environment

Sets the app's environment.

Key: environment
Type: string
Default: Production
Set using: UseEnvironment

You can set the Environment to any value. Framework-defined values include Development, Staging, and Production. Values aren't case sensitive. By default, the Environment is read from the ASPNETCORE_ENVIRONMENT environment variable. When using Visual Studio, environment variables may be set in the launchSettings.json file. For more information, see Working with Multiple Environments.

WebHost.CreateDefaultBuilder(args)
    .UseEnvironment("Development")
    ...

Hosting Startup Assemblies

Sets the app's hosting startup assemblies.

Key: hostingStartupAssemblies
Type: string
Default: Empty string
Set using: UseSetting

A semicolon-delimited string of hosting startup assemblies to load on startup. This feature is new in ASP.NET Core 2.0.

Although the configuration value defaults to an empty string, the hosting startup assemblies always include the app's assembly. When you provide hosting startup assemblies, they're added to the app's assembly for loading when the app builds its common services during startup.

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.HostingStartupAssembliesKey, "assembly1;assembly2")
    ...

Prefer Hosting URLs

Indicates whether the host should listen on the URLs configured with the WebHostBuilder instead of those configured with the IServer implementation.

Key: preferHostingUrls
Type: bool (true or 1)
Default: true
Set using: PreferHostingUrls

This feature is new in ASP.NET Core 2.0.

WebHost.CreateDefaultBuilder(args)
    .PreferHostingUrls(false)
    ...

Prevent Hosting Startup

Prevents the automatic loading of hosting startup assemblies, including the app's assembly.

Key: preventHostingStartup
Type: bool (true or 1)
Default: false
Set using: UseSetting

This feature is new in ASP.NET Core 2.0.

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.PreventHostingStartupKey, "true")
    ...

Server URLs

Indicates the IP addresses or host addresses with ports and protocols that the server should listen on for requests.

Key: urls
Type: string
Default: http://localhost:5000
Set using: UseUrls

Set to a semicolon-separated (;) list of URL prefixes to which the server should respond. For example, http://localhost:123. Use "*" to indicate that the server should listen for requests on any IP address or hostname using the specified port and protocol (for example, http://*:5000). The protocol (http:// or https://) must be included with each URL. Supported formats vary between servers.

WebHost.CreateDefaultBuilder(args)
    .UseUrls("http://*:5000;http://localhost:5001;https://hostname:5002")
    ...

Kestrel has its own endpoint configuration API. For more information, see Kestrel web server implementation in ASP.NET Core.

Shutdown Timeout

Specifies the amount of time to wait for the web host to shutdown.

Key: shutdownTimeoutSeconds
Type: int
Default: 5
Set using: UseShutdownTimeout

Although the key accepts an int with UseSetting (for example, .UseSetting(WebHostDefaults.ShutdownTimeoutKey, "10")), the UseShutdownTimeout extension method takes a TimeSpan. This feature is new in ASP.NET Core 2.0.

WebHost.CreateDefaultBuilder(args)
    .UseShutdownTimeout(TimeSpan.FromSeconds(10))
    ...

Startup Assembly

Determines the assembly to search for the Startup class.

Key: startupAssembly
Type: string
Default: The app's assembly
Set using: UseStartup

You can reference the assembly by name (string) or type (TStartup). If multiple UseStartup methods are called, the last one takes precedence.

WebHost.CreateDefaultBuilder(args)
    .UseStartup("StartupAssemblyName")
    ...
WebHost.CreateDefaultBuilder(args)
    .UseStartup<TStartup>()
    ...

Web Root

Sets the relative path to the app's static assets.

Key: webroot
Type: string
Default: If not specified, the default is "(Content Root)/wwwroot", if the path exists. If the path doesn't exist, then a no-op file provider is used.
Set using: UseWebRoot

WebHost.CreateDefaultBuilder(args)
    .UseWebRoot("public")
    ...

Overriding configuration

Use Configuration to configure the host. In the following example, host configuration is optionally specified in a hosting.json file. Any configuration loaded from the hosting.json file may be overridden by command-line arguments. The built configuration (in config) is used to configure the host with UseConfiguration.

hosting.json:

{
    urls: "http://*:5005"
}

Overriding the configuration provided by UseUrls with hosting.json config first, command-line argument config second:

public class Program
{
    public static void Main(string[] args)
    {
        BuildWebHost(args).Run();
    }

    public static IWebHost BuildWebHost(string[] args)
    {
        var config = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("hosting.json", optional: true)
            .AddCommandLine(args)
            .Build();

        return WebHost.CreateDefaultBuilder(args)
            .UseUrls("http://*:5000")
            .UseConfiguration(config)
            .Configure(app =>
            {
                app.Run(context => 
                    context.Response.WriteAsync("Hello, World!"));
            })
            .Build();
    }
}
Note

The UseConfiguration extension method isn't currently capable of parsing a configuration section returned by GetSection (for example, .UseConfiguration(Configuration.GetSection("section")). The GetSection method filters the configuration keys to the section requested but leaves the section name on the keys (for example, section:urls, section:environment). The UseConfiguration method expects the keys to match the WebHostBuilder keys (for example, urls, environment). The presence of the section name on the keys prevents the section's values from configuring the host. This issue will be addressed in an upcoming release. For more information and workarounds, see Passing configuration section into WebHostBuilder.UseConfiguration uses full keys.

To specify the host run on a particular URL, you could pass in the desired value from a command prompt when executing dotnet run. The command-line argument overrides the urls value from the hosting.json file, and the server listens on port 8080:

dotnet run --urls "http://*:8080"

Ordering importance

Some of the WebHostBuilder settings are first read from environment variables, if set. These environment variables use the format ASPNETCORE_{configurationKey}. To set the URLs that the server listens on by default, you set ASPNETCORE_URLS.

You can override any of these environment variable values by specifying configuration (using UseConfiguration) or by setting the value explicitly (using UseSetting or one of the explicit extension methods, such as UseUrls). The host uses whichever option sets the value last. If you want to programmatically set the default URL to one value but allow it to be overridden with configuration, you can use command-line configuration after setting the URL. See Overriding configuration.

Starting the host

Run

The Run method starts the web app and blocks the calling thread until the host is shutdown:

host.Run();

Start

You can run the host in a non-blocking manner by calling its Start method:

using (host)
{
    host.Start();
    Console.ReadLine();
}

If you pass a list of URLs to the Start method, it listens on the URLs specified:

var urls = new List<string>()
{
    "http://*:5000",
    "http://localhost:5001"
};

var host = new WebHostBuilder()
    .UseKestrel()
    .UseStartup<Startup>()
    .Start(urls.ToArray());

using (host)
{
    Console.ReadLine();
}

You can initialize and start a new host using the pre-configured defaults of CreateDefaultBuilder using a static convenience method. These methods start the server without console output and with WaitForShutdown wait for a break (Ctrl-C/SIGINT or SIGTERM):

Start(RequestDelegate app)

Start with a RequestDelegate:

using (var host = WebHost.Start(app => app.Response.WriteAsync("Hello, World!")))
{
    Console.WriteLine("Use Ctrl-C to shutdown the host...");
    host.WaitForShutdown();
}

Make a request in the browser to http://localhost:5000 to receive the response "Hello World!" WaitForShutdown blocks until a break (Ctrl-C/SIGINT or SIGTERM) is issued. The app displays the Console.WriteLine message and waits for a keypress to exit.

Start(string url, RequestDelegate app)

Start with a URL and RequestDelegate:

using (var host = WebHost.Start("http://localhost:8080", app => app.Response.WriteAsync("Hello, World!")))
{
    Console.WriteLine("Use Ctrl-C to shutdown the host...");
    host.WaitForShutdown();
}

Produces the same result as Start(RequestDelegate app), except the app responds on http://localhost:8080.

Start(Action routeBuilder)

Use an instance of IRouteBuilder (Microsoft.AspNetCore.Routing) to use routing middleware:

using (var host = WebHost.Start(router => router
    .MapGet("hello/{name}", (req, res, data) => 
        res.WriteAsync($"Hello, {data.Values["name"]}!"))
    .MapGet("buenosdias/{name}", (req, res, data) => 
        res.WriteAsync($"Buenos dias, {data.Values["name"]}!"))
    .MapGet("throw/{message?}", (req, res, data) => 
        throw new Exception((string)data.Values["message"] ?? "Uh oh!"))
    .MapGet("{greeting}/{name}", (req, res, data) => 
        res.WriteAsync($"{data.Values["greeting"]}, {data.Values["name"]}!"))
    .MapGet("", (req, res, data) => res.WriteAsync("Hello, World!"))))
{
    Console.WriteLine("Use Ctrl-C to shutdown the host...");
    host.WaitForShutdown();
}

Use the following browser requests with the example:

Request Response
http://localhost:5000/hello/Martin Hello, Martin!
http://localhost:5000/buenosdias/Catrina Buenos dias, Catrina!
http://localhost:5000/throw/ooops! Throws an exception with string "ooops!"
http://localhost:5000/throw Throws an exception with string "Uh oh!"
http://localhost:5000/Sante/Kevin Sante, Kevin!
http://localhost:5000 Hello World!

WaitForShutdown blocks until a break (Ctrl-C/SIGINT or SIGTERM) is issued. The app displays the Console.WriteLine message and waits for a keypress to exit.

Start(string url, Action routeBuilder)

Use a URL and an instance of IRouteBuilder:

using (var host = WebHost.Start("http://localhost:8080", router => router
    .MapGet("hello/{name}", (req, res, data) => 
        res.WriteAsync($"Hello, {data.Values["name"]}!"))
    .MapGet("buenosdias/{name}", (req, res, data) => 
        res.WriteAsync($"Buenos dias, {data.Values["name"]}!"))
    .MapGet("throw/{message?}", (req, res, data) => 
        throw new Exception((string)data.Values["message"] ?? "Uh oh!"))
    .MapGet("{greeting}/{name}", (req, res, data) => 
        res.WriteAsync($"{data.Values["greeting"]}, {data.Values["name"]}!"))
    .MapGet("", (req, res, data) => res.WriteAsync("Hello, World!"))))
{
    Console.WriteLine("Use Ctrl-C to shutdown the host...");
    host.WaitForShutdown();
}

Produces the same result as Start(Action routeBuilder), except the app responds at http://localhost:8080.

StartWith(Action app)

Provide a delegate to configure an IApplicationBuilder:

using (var host = WebHost.StartWith(app => 
    app.Use(next => 
    {
        return async context => 
        {
            await context.Response.WriteAsync("Hello World!");
        };
    })))
{
    Console.WriteLine("Use Ctrl-C to shutdown the host...");
    host.WaitForShutdown();
}

Make a request in the browser to http://localhost:5000 to receive the response "Hello World!" WaitForShutdown blocks until a break (Ctrl-C/SIGINT or SIGTERM) is issued. The app displays the Console.WriteLine message and waits for a keypress to exit.

StartWith(string url, Action app)

Provide a URL and a delegate to configure an IApplicationBuilder:

using (var host = WebHost.StartWith("http://localhost:8080", app => 
    app.Use(next => 
    {
        return async context => 
        {
            await context.Response.WriteAsync("Hello World!");
        };
    })))
{
    Console.WriteLine("Use Ctrl-C to shutdown the host...");
    host.WaitForShutdown();
}

Produces the same result as StartWith(Action app), except the app responds on http://localhost:8080.

IHostingEnvironment interface

The IHostingEnvironment interface provides information about the app's web hosting environment. You can use constructor injection to obtain the IHostingEnvironment in order to use its properties and extension methods:

public class CustomFileReader
{
    private readonly IHostingEnvironment _env;

    public CustomFileReader(IHostingEnvironment env)
    {
        _env = env;
    }

    public string ReadFile(string filePath)
    {
        var fileProvider = _env.WebRootFileProvider;
        // Process the file here
    }
}

You can use a convention-based approach to configure your app at startup based on the environment. Alternatively, you can inject the IHostingEnvironment into the Startup constructor for use in ConfigureServices:

public class Startup
{
    public Startup(IHostingEnvironment env)
    {
        HostingEnvironment = env;
    }

    public IHostingEnvironment HostingEnvironment { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        if (HostingEnvironment.IsDevelopment())
        {
            // Development configuration
        }
        else
        {
            // Staging/Production configuration
        }

        var contentRootPath = HostingEnvironment.ContentRootPath;
    }
}
Note

In addition to the IsDevelopment extension method, IHostingEnvironment offers IsStaging, IsProduction, and IsEnvironment(string environmentName) methods. See Working with multiple environments for details.

The IHostingEnvironment service can also be injected directly into the Configure method for setting up your processing pipeline:

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    if (env.IsDevelopment())
    {
        // In Development, use the developer exception page
        app.UseDeveloperExceptionPage();
    }
    else
    {
        // In Staging/Production, route exceptions to /error
        app.UseExceptionHandler("/error");
    }

    var contentRootPath = env.ContentRootPath;
}

You can inject IHostingEnvironment into the Invoke method when creating custom middleware:

public async Task Invoke(HttpContext context, IHostingEnvironment env)
{
    if (env.IsDevelopment())
    {
        // Configure middleware for Development
    }
    else
    {
        // Configure middleware for Staging/Production
    }

    var contentRootPath = env.ContentRootPath;
}

IApplicationLifetime interface

The IApplicationLifetime interface allows you to perform post-startup and shutdown activities. Three properties on the interface are cancellation tokens that you can register with Action methods to define startup and shutdown events. There's also a StopApplication method.

Cancellation Token Triggered when…
ApplicationStarted The host has fully started.
ApplicationStopping The host is performing a graceful shutdown. Requests may still be processing. Shutdown blocks until this event completes.
ApplicationStopped The host is completing a graceful shutdown. All requests should be completely processed. Shutdown blocks until this event completes.
Method Action
StopApplication Requests termination of the current application.
public class Startup 
{
    public void Configure(IApplicationBuilder app, IApplicationLifetime appLifetime) 
    {
        appLifetime.ApplicationStarted.Register(OnStarted);
        appLifetime.ApplicationStopping.Register(OnStopping);
        appLifetime.ApplicationStopped.Register(OnStopped);

        Console.CancelKeyPress += (sender, eventArgs) =>
        {
            appLifetime.StopApplication();
            // Don't terminate the process immediately, wait for the Main thread to exit gracefully.
            eventArgs.Cancel = true;
        };
    }

    private void OnStarted()
    {
        // Perform post-startup activities here
    }

    private void OnStopping()
    {
        // Perform on-stopping activities here
    }

    private void OnStopped()
    {
        // Perform post-stopped activities here
    }
}

Troubleshooting System.ArgumentException

Applies to ASP.NET Core 2.0 Only

If you build the host by injecting IStartup directly into the dependency injection container rather than calling UseStartup or Configure, you may encounter the following error: Unhandled Exception: System.ArgumentException: A valid non-empty application name must be provided.

This occurs because the applicationName(ApplicationKey) (the current assembly) is required to scan for HostingStartupAttributes. If you manually inject IStartup into the dependency injection container, add the following call to your WebHostBuilder with the assembly name specified:

WebHost.CreateDefaultBuilder(args)
    .UseSetting("applicationName", "<Assembly Name>")
    ...

Alternatively, add a dummy Configure to your WebHostBuilder, which sets the applicationName(ApplicationKey) automatically:

WebHost.CreateDefaultBuilder(args)
    .Configure(_ => { })
    ...

NOTE: This is only required with the ASP.NET Core 2.0 release and only when you don't call UseStartup or Configure.

For more information, see Announcements: Microsoft.Extensions.PlatformAbstractions has been removed (comment) and the StartupInjection sample.

Additional resources