---
title: Hosting in ASP.NET Core
author: guardrex
description: Learn about the web host in ASP.NET Core, which is responsible for app startup and lifetime management.
manager: wpickett
ms.author: riande
ms.date: 09/21/2017
ms.prod: asp.net-core
ms.technology: aspnet
ms.topic: article
uid: fundamentals/hosting
---
# Hosting in ASP.NET Core

By [Luke Latham](https://github.com/guardrex)

ASP.NET Core apps configure and launch a *host*. The host 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

# [ASP.NET Core 2.x](#tab/aspnetcore2x)

Create a host using an instance of [WebHostBuilder](/dotnet/api/microsoft.aspnetcore.hosting.webhostbuilder). This is typically performed in the app's entry point, the `Main` method. In the project templates, `Main` is located in *Program.cs*. A typical *Program.cs* calls [CreateDefaultBuilder](/dotnet/api/microsoft.aspnetcore.webhost.createdefaultbuilder) to start setting up a host:

[!code-csharp[](../common/samples/WebApplication1DotNetCore2.0App/Program.cs?name=snippet_Main)]

`CreateDefaultBuilder` performs the following tasks:

* Configures [Kestrel](servers/kestrel.md) as the web server. For the Kestrel default options, see [the Kestrel options section of Kestrel web server implementation in ASP.NET Core](xref:fundamentals/servers/kestrel#kestrel-options).
* Sets the content root to the path returned by [Directory.GetCurrentDirectory](/dotnet/api/system.io.directory.getcurrentdirectory).
* Loads optional configuration from:
  * *appsettings.json*.
  * *appsettings.{Environment}.json*.
  * [User secrets](xref:security/app-secrets) when the app runs in the `Development` environment.
  * Environment variables.
  * Command-line arguments.
* Configures [logging](xref:fundamentals/logging/index) for console and debug output. Logging includes [log filtering](xref:fundamentals/logging/index#log-filtering) rules specified in a Logging configuration section of an *appsettings.json* or *appsettings.{Environment}.json* file.
* When running behind IIS, enables [IIS integration](xref:host-and-deploy/iis/index). Configures the base path and port the server listens on when using the [ASP.NET Core Module](xref:fundamentals/servers/aspnet-core-module). The module creates a reverse proxy between IIS and Kestrel. Also configures the app to [capture startup errors](#capture-startup-errors). For the IIS default options, see [the IIS options section of Host ASP.NET Core on Windows with IIS](xref:host-and-deploy/iis/index#iis-options).
* Sets [ServiceProviderOptions.ValidateScopes](/dotnet/api/microsoft.extensions.dependencyinjection.serviceprovideroptions.validatescopes) to `true` if the app's environment is Development. For more information, see [Scope validation](#scope-validation).

The *content root* determines where the host searches for content files, such as MVC view files. When the app is started from the project's root folder, the project's root folder is used as the content root. This is the default used in [Visual Studio](https://www.visualstudio.com/) and the [dotnet new templates](/dotnet/core/tools/dotnet-new).

For more information on app configuration, see [Configuration in ASP.NET Core](xref:fundamentals/configuration/index).

> [!NOTE]
> As an alternative to using the static `CreateDefaultBuilder` method, creating a host from [WebHostBuilder](/dotnet/api/microsoft.aspnetcore.hosting.webhostbuilder) is a supported approach with ASP.NET Core 2.x. For more information, see the ASP.NET Core 1.x tab.

# [ASP.NET Core 1.x](#tab/aspnetcore1x)

Create a host using an instance of [WebHostBuilder](/dotnet/api/microsoft.aspnetcore.hosting.webhostbuilder). Creating a host is typically performed in the app's entry point, the `Main` method. In the project templates, `Main` is located in *Program.cs*:

[!code-csharp[](../common/samples/WebApplication1/Program.cs)]

`WebHostBuilder` requires a [server that implements IServer](servers/index.md). The built-in servers are [Kestrel](servers/kestrel.md) and [HTTP.sys](servers/httpsys.md) (prior to the release of ASP.NET Core 2.0, HTTP.sys was called [WebListener](xref:fundamentals/servers/weblistener)). In this example, the [UseKestrel extension method](/dotnet/api/microsoft.aspnetcore.hosting.webhostbuilderkestrelextensions.usekestrel?view=aspnetcore-1.1) specifies the Kestrel server.

The *content root* determines where the host searches for content files, such as MVC view files. The default content root is obtained for `UseContentRoot` by [Directory.GetCurrentDirectory](/dotnet/api/system.io.directory.getcurrentdirectory?view=netcore-1.1). When the app is started from the project's root folder, the project's root folder is used as the content root. This is the default used in [Visual Studio](https://www.visualstudio.com/) and the [dotnet new templates](/dotnet/core/tools/dotnet-new).

To use IIS as a reverse proxy, call [UseIISIntegration](/aspnet/core/api/microsoft.aspnetcore.hosting.webhostbuilderiisextensions) as part of building the host. `UseIISIntegration` doesn't configure a *server*, like [UseKestrel](/dotnet/api/microsoft.aspnetcore.hosting.webhostbuilderkestrelextensions.usekestrel?view=aspnetcore-1.1) does. `UseIISIntegration` configures the base path and port the server listens on when using the [ASP.NET Core Module](xref:fundamentals/servers/aspnet-core-module) to create a reverse proxy between Kestrel and IIS. To use IIS with ASP.NET Core, `UseKestrel` and `UseIISIntegration` must be specified. `UseIISIntegration` only activates when running behind IIS or IIS Express. For more information, see [Introduction to ASP.NET Core Module](xref:fundamentals/servers/aspnet-core-module) and [ASP.NET Core Module configuration reference](xref:host-and-deploy/aspnet-core-module).

A minimal implementation that configures a host (and an ASP.NET Core app) includes specifying a server and configuration of the app's request pipeline:

```csharp
var host = new WebHostBuilder()
    .UseKestrel()
    .Configure(app =>
    {
        app.Run(context => context.Response.WriteAsync("Hello World!"));
    })
    .Build();

host.Run();
```

---

When setting up a host, [Configure](/dotnet/api/microsoft.aspnetcore.hosting.webhostbuilderextensions.configure?view=aspnetcore-1.1) and [ConfigureServices](/dotnet/api/microsoft.aspnetcore.hosting.webhostbuilder.configureservices?view=aspnetcore-1.1) methods can be provided. If a `Startup` class is specified, it must define a `Configure` method. For more information, see [Application Startup in ASP.NET Core](startup.md). Multiple calls to `ConfigureServices` append to one another. Multiple calls to `Configure` or `UseStartup` on the `WebHostBuilder` replace previous settings.

## Host configuration values

[WebHostBuilder](/dotnet/api/microsoft.aspnetcore.hosting.webhostbuilder) relies on the following approaches to set the host configuration values:

* Host builder configuration, which includes environment variables with the format `ASPNETCORE_{configurationKey}`. For example, `ASPNETCORE_URLS`.
* Explicit methods, such as `CaptureStartupErrors`.
* [UseSetting](/dotnet/api/microsoft.aspnetcore.hosting.webhostbuilder.usesetting) and the associated key. When setting a value with `UseSetting`, the value is set as a string regardless of the type.

The host uses whichever option sets a value last. For more information, see [Overriding configuration](#overriding-configuration) in the next section.

### 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`  
**Environment variable**: `ASPNETCORE_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.

# [ASP.NET Core 2.x](#tab/aspnetcore2x)

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

# [ASP.NET Core 1.x](#tab/aspnetcore1x)

```csharp
var host = new WebHostBuilder()
    .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`  
**Environment variable**: `ASPNETCORE_CONTENTROOT`

The content root is also used as the base path for the [Web Root setting](#web-root). If the path doesn't exist, the host fails to start.

# [ASP.NET Core 2.x](#tab/aspnetcore2x)

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

# [ASP.NET Core 1.x](#tab/aspnetcore1x)

```csharp
var host = new WebHostBuilder()
    .UseContentRoot("c:\\mywebsite")
    ...
```

---

### Detailed Errors

Determines if detailed errors should be captured.

**Key**: detailedErrors  
**Type**: *bool* (`true` or `1`)  
**Default**: false  
**Set using**: `UseSetting`  
**Environment variable**: `ASPNETCORE_DETAILEDERRORS`

When enabled (or when the <a href="#environment">Environment</a> is set to `Development`), the app captures detailed exceptions.

# [ASP.NET Core 2.x](#tab/aspnetcore2x)

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

# [ASP.NET Core 1.x](#tab/aspnetcore1x)

```csharp
var host = new WebHostBuilder()
    .UseSetting(WebHostDefaults.DetailedErrorsKey, "true")
    ...
```

---

### Environment

Sets the app's environment.

**Key**: environment  
**Type**: *string*  
**Default**: Production  
**Set using**: `UseEnvironment`  
**Environment variable**: `ASPNETCORE_ENVIRONMENT`

The environment can be set 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](https://www.visualstudio.com/), environment variables may be set in the *launchSettings.json* file. For more information, see [Work with multiple environments](xref:fundamentals/environments).

# [ASP.NET Core 2.x](#tab/aspnetcore2x)

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

# [ASP.NET Core 1.x](#tab/aspnetcore1x)

```csharp
var host = new WebHostBuilder()
    .UseEnvironment("Development")
    ...
```

---

### Hosting Startup Assemblies

Sets the app's hosting startup assemblies.

**Key**: hostingStartupAssemblies  
**Type**: *string*  
**Default**: Empty string  
**Set using**: `UseSetting`  
**Environment variable**: `ASPNETCORE_HOSTINGSTARTUPASSEMBLIES`

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 hosting startup assemblies are provided, they're added to the app's assembly for loading when the app builds its common services during startup.

# [ASP.NET Core 2.x](#tab/aspnetcore2x)

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

# [ASP.NET Core 1.x](#tab/aspnetcore1x)

This feature is unavailable in ASP.NET Core 1.x.

---

### 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`  
**Environment variable**: `ASPNETCORE_PREFERHOSTINGURLS`

This feature is new in ASP.NET Core 2.0.

# [ASP.NET Core 2.x](#tab/aspnetcore2x)

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

# [ASP.NET Core 1.x](#tab/aspnetcore1x)

This feature is unavailable in ASP.NET Core 1.x.

---

### Prevent Hosting Startup

Prevents the automatic loading of hosting startup assemblies, including hosting startup assemblies configured by the app's assembly. See [Add app features using a platform-specific configuration](xref:host-and-deploy/platform-specific-configuration) for more information.

**Key**: preventHostingStartup  
**Type**: *bool* (`true` or `1`)  
**Default**: false  
**Set using**: `UseSetting`  
**Environment variable**: `ASPNETCORE_PREVENTHOSTINGSTARTUP`

This feature is new in ASP.NET Core 2.0.

# [ASP.NET Core 2.x](#tab/aspnetcore2x)

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

# [ASP.NET Core 1.x](#tab/aspnetcore1x)

This feature is unavailable in ASP.NET Core 1.x.

---

### 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`  
**Environment variable**: `ASPNETCORE_URLS`

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.

# [ASP.NET Core 2.x](#tab/aspnetcore2x)

```csharp
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](xref:fundamentals/servers/kestrel?tabs=aspnetcore2x#endpoint-configuration).

# [ASP.NET Core 1.x](#tab/aspnetcore1x)

```csharp
var host = new WebHostBuilder()
    .UseUrls("http://*:5000;http://localhost:5001;https://hostname:5002")
    ...
```

---

### Shutdown Timeout

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

**Key**: shutdownTimeoutSeconds  
**Type**: *int*  
**Default**: 5  
**Set using**: `UseShutdownTimeout`  
**Environment variable**: `ASPNETCORE_SHUTDOWNTIMEOUTSECONDS`

Although the key accepts an *int* with `UseSetting` (for example, `.UseSetting(WebHostDefaults.ShutdownTimeoutKey, "10")`), the [UseShutdownTimeout](/dotnet/api/microsoft.aspnetcore.hosting.hostingabstractionswebhostbuilderextensions.useshutdowntimeout) extension method takes a [TimeSpan](/dotnet/api/system.timespan). This feature is new in ASP.NET Core 2.0.

During the timeout period, hosting:

* Triggers [IApplicationLifetime.ApplicationStopping](/dotnet/api/microsoft.aspnetcore.hosting.iapplicationlifetime.applicationstopping).
* Attempts to stop hosted services, logging any errors for services that fail to stop.

If the timeout period expires before all of the hosted services stop, any remaining active services are stopped when the app shuts down. The services stop even if they haven't finished processing. If services require additional time to stop, increase the timeout.

# [ASP.NET Core 2.x](#tab/aspnetcore2x)

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

# [ASP.NET Core 1.x](#tab/aspnetcore1x)

This feature is unavailable in ASP.NET Core 1.x.

---

### Startup Assembly

Determines the assembly to search for the `Startup` class.

**Key**: startupAssembly  
**Type**: *string*  
**Default**: The app's assembly  
**Set using**: `UseStartup`  
**Environment variable**: `ASPNETCORE_STARTUPASSEMBLY`

The assembly by name (`string`) or type (`TStartup`) can be referenced. If multiple `UseStartup` methods are called, the last one takes precedence.

# [ASP.NET Core 2.x](#tab/aspnetcore2x)

```csharp
WebHost.CreateDefaultBuilder(args)
    .UseStartup("StartupAssemblyName")
    ...
```

```csharp
WebHost.CreateDefaultBuilder(args)
    .UseStartup<TStartup>()
    ...
```

# [ASP.NET Core 1.x](#tab/aspnetcore1x)

```csharp
var host = new WebHostBuilder()
    .UseStartup("StartupAssemblyName")
    ...
```

```csharp
var host = new WebHostBuilder()
    .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`  
**Environment variable**: `ASPNETCORE_WEBROOT`

# [ASP.NET Core 2.x](#tab/aspnetcore2x)

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

# [ASP.NET Core 1.x](#tab/aspnetcore1x)

```csharp
var host = new WebHostBuilder()
    .UseWebRoot("public")
    ...
```

---

## Overriding configuration

Use [Configuration](xref:fundamentals/configuration/index) 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`.

# [ASP.NET Core 2.x](#tab/aspnetcore2x)

*hosting.json*:

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

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

```csharp
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();
    }
}
```

# [ASP.NET Core 1.x](#tab/aspnetcore1x)

*hosting.json*:

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

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

```csharp
public class Program
{
    public static void Main(string[] args)
    {
        var config = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("hosting.json", optional: true)
            .AddCommandLine(args)
            .Build();

        var host = new WebHostBuilder()
            .UseUrls("http://*:5000")
            .UseConfiguration(config)
            .UseKestrel()
            .Configure(app =>
            {
                app.Run(context => 
                    context.Response.WriteAsync("Hello, World!"));
            })
            .Build();

        host.Run();
    }
}
```

---

> [!NOTE]
> The [UseConfiguration](/dotnet/api/microsoft.aspnetcore.hosting.hostingabstractionswebhostbuilderextensions.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](https://github.com/aspnet/Hosting/issues/839).

To specify the host run on a particular URL, the desired value can be passed in from a command prompt when executing [dotnet run](/dotnet/core/tools/dotnet-run). The command-line argument overrides the `urls` value from the *hosting.json* file, and the server listens on port 8080:

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

## Starting the host

# [ASP.NET Core 2.x](#tab/aspnetcore2x)

**Run**

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

```csharp
host.Run();
```

**Start**

Run the host in a non-blocking manner by calling its `Start` method:

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

If a list of URLs is passed to the `Start` method, it listens on the URLs specified:

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

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

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

The app 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](/dotnet/api/microsoft.aspnetcore.hosting.webhostextensions.waitforshutdown) wait for a break (Ctrl-C/SIGINT or SIGTERM):

**Start(RequestDelegate app)**

Start with a `RequestDelegate`:

```csharp
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`:

```csharp
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<IRouteBuilder> routeBuilder)**

Use an instance of `IRouteBuilder` ([Microsoft.AspNetCore.Routing](https://www.nuget.org/packages/Microsoft.AspNetCore.Routing/)) to use routing middleware:

```csharp
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<IRouteBuilder> routeBuilder)**

Use a URL and an instance of `IRouteBuilder`:

```csharp
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<IRouteBuilder> routeBuilder)**, except the app responds at `http://localhost:8080`.

**StartWith(Action<IApplicationBuilder> app)**

Provide a delegate to configure an `IApplicationBuilder`:

```csharp
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<IApplicationBuilder> app)**

Provide a URL and a delegate to configure an `IApplicationBuilder`:

```csharp
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<IApplicationBuilder> app)**, except the app responds on `http://localhost:8080`.

# [ASP.NET Core 1.x](#tab/aspnetcore1x)

**Run**

The `Run` method starts the web app and blocks the calling thread until the host is shut down:

```csharp
host.Run();
```

**Start**

Run the host in a non-blocking manner by calling its `Start` method:

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

If a list of URLs is passed to the `Start` method, it listens on the URLs specified:


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

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

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

---

## IHostingEnvironment interface

The [IHostingEnvironment interface](/aspnet/core/api/microsoft.aspnetcore.hosting.ihostingenvironment) provides information about the app's web hosting environment. Use [constructor injection](xref:fundamentals/dependency-injection) to obtain the `IHostingEnvironment` in order to use its properties and extension methods:

```csharp
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
    }
}
```

A [convention-based approach](xref:fundamentals/environments#startup-conventions) can be used to configure the app at startup based on the environment. Alternatively, inject the `IHostingEnvironment` into the `Startup` constructor for use in `ConfigureServices`:

```csharp
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 [Work with multiple environments](xref:fundamentals/environments) for details.

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

```csharp
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;
}
```

`IHostingEnvironment` can be injected into the `Invoke` method when creating custom [middleware](xref:fundamentals/middleware/index#writing-middleware):

```csharp
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

[IApplicationLifetime](/aspnet/core/api/microsoft.aspnetcore.hosting.iapplicationlifetime) allows for post-startup and shutdown activities. Three properties on the interface are cancellation tokens used to register `Action` methods that define startup and shutdown events.

| Cancellation Token    | Triggered when&#8230; |
| --------------------- | --------------------- |
| `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 processed. Shutdown blocks until this event completes. |

```csharp
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
    }
}
```

[StopApplication](/dotnet/api/microsoft.aspnetcore.hosting.iapplicationlifetime.stopapplication) requests termination of the app. The following class uses `StopApplication` to gracefully shutdown an app when the class's `Shutdown` method is called:

```csharp
public class MyClass
{
    private readonly IApplicationLifetime _appLifetime;

    public MyClass(IApplicationLifetime appLifetime)
    {
        _appLifetime = appLifetime;
    }

    public void Shutdown()
    {
        _appLifetime.StopApplication();
    }
}
```

## Scope validation

In ASP.NET Core 2.0 or later, [CreateDefaultBuilder](/dotnet/api/microsoft.aspnetcore.webhost.createdefaultbuilder) sets [ServiceProviderOptions.ValidateScopes](/dotnet/api/microsoft.extensions.dependencyinjection.serviceprovideroptions.validatescopes) to `true` if the app's environment is Development.

When `ValidateScopes` is set to `true`, the default service provider performs checks to verify that:

* Scoped services aren't directly or indirectly resolved from the root service provider.
* Scoped services aren't directly or indirectly injected into singletons.

The root service provider is created when [BuildServiceProvider](/dotnet/api/microsoft.extensions.dependencyinjection.servicecollectioncontainerbuilderextensions.buildserviceprovider) is called. The root service provider's lifetime corresponds to the app/server's lifetime when the provider starts with the app and is disposed when the app shuts down.

Scoped services are disposed by the container that created them. If a scoped service is created in the root container, the service's lifetime is effectively promoted to singleton because it's only disposed by the root container when app/server is shut down. Validating service scopes catches these situations when `BuildServiceProvider` is called.

To always validate scopes, including in the Production environment, configure the [ServiceProviderOptions](/dotnet/api/microsoft.extensions.dependencyinjection.serviceprovideroptions) with [UseDefaultServiceProvider](/dotnet/api/microsoft.aspnetcore.hosting.webhostbuilderextensions.usedefaultserviceprovider) on the host builder:

```csharp
WebHost.CreateDefaultBuilder(args)
    .UseDefaultServiceProvider((context, options) => {
        options.ValidateScopes = true;
    })
```

## Troubleshooting System.ArgumentException

**Applies to ASP.NET Core 2.0 Only**

A host may be built by injecting `IStartup` directly into the dependency injection container rather than calling `UseStartup` or `Configure`:

```csharp
services.AddSingleton<IStartup, Startup>();
```

If the host is built this way, the following error may occur:

```
Unhandled Exception: System.ArgumentException: A valid non-empty application name must be provided.
```

This occurs because the [applicationName(ApplicationKey)](/aspnet/core/api/microsoft.aspnetcore.hosting.webhostdefaults#Microsoft_AspNetCore_Hosting_WebHostDefaults_ApplicationKey) (the current assembly) is required to scan for `HostingStartupAttributes`. If the app manually injects `IStartup` into the dependency injection container, add the following call to `WebHostBuilder` with the assembly name specified:

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

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

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

**NOTE**: This is only required with the ASP.NET Core 2.0 release and only when the app doesn't call `UseStartup` or `Configure`.

For more information, see [Announcements: Microsoft.Extensions.PlatformAbstractions has been removed (comment)](https://github.com/aspnet/Announcements/issues/237#issuecomment-323786938) and the [StartupInjection sample](https://github.com/aspnet/Hosting/blob/8377d226f1e6e1a97dabdb6769a845eeccc829ed/samples/SampleStartups/StartupInjection.cs).

## Additional resources

* [Host on Windows with IIS](xref:host-and-deploy/iis/index)
* [Host on Linux with Nginx](xref:host-and-deploy/linux-nginx)
* [Host on Linux with Apache](xref:host-and-deploy/linux-apache)
* [Host in a Windows Service](xref:host-and-deploy/windows-service)