73 lines
5.5 KiB
Markdown
73 lines
5.5 KiB
Markdown
---
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title: Application Startup in ASP.NET Core
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author: ardalis
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description: Explains the Startup class in ASP.NET Core.
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keywords: ASP.NET Core, Startup, Configure method, ConfigureServices method
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ms.author: tdykstra
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manager: wpickett
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ms.date: 02/29/2017
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ms.topic: article
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ms.assetid: 6538df00-4ec2-45e4-811a-d7ce2ee608ed
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ms.technology: aspnet
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ms.prod: asp.net-core
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uid: fundamentals/startup
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---
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# Application Startup in ASP.NET Core
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By [Steve Smith](http://ardalis.com) and [Tom Dykstra](https://github.com/tdykstra/)
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The `Startup` class configures the request pipeline that handles all requests made to the application.
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## The Startup class
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ASP.NET Core apps require a `Startup` class. By convention, the `Startup` class is named "Startup". You specify the startup class name in the `Main` programs [WebHostBuilderExtensions](https://docs.microsoft.com/aspnet/core/api/microsoft.aspnetcore.hosting.webhostbuilderextensions) [`UseStartup<TStartup>`](https://docs.microsoft.com/aspnet/core/api/microsoft.aspnetcore.hosting.webhostbuilderextensions#Microsoft_AspNetCore_Hosting_WebHostBuilderExtensions_UseStartup__1_Microsoft_AspNetCore_Hosting_IWebHostBuilder_) method.
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You can define separate `Startup` classes for different environments, and the appropriate one will be selected at runtime. If you specify `startupAssembly` in the WebHost configuration or options, hosting will load that startup assembly and search for a `Startup` or `Startup[Environment]` type. See [FindStartupType](https://github.com/aspnet/Hosting/blob/rel/1.1.0/src/Microsoft.AspNetCore.Hosting/Internal/StartupLoader.cs) in `StartupLoader` and [Working with multiple environments](environments.md#startup-conventions). `UseStartup<TStartup>` is the recommended approach.
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The `Startup` class constructor can accept dependencies that are provided through [dependency injection](dependency-injection.md). You can use `IHostingEnvironment` to set up [configuration](configuration.md) sources and `ILoggerFactory` to set up [logging](logging.md) providers.
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The `Startup` class must include a `Configure` method and can optionally include a `ConfigureServices` method, both of which are called when the application starts. The class can also include [environment-specific versions of these methods](environments.md#startup-conventions).
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Learn about [handling exceptions during application startup](error-handling.md#startup-exception-handling).
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## The Configure method
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The `Configure` method is used to specify how the ASP.NET application will respond to HTTP requests. The request pipeline is configured by adding [middleware](middleware.md) components to an `IApplicationBuilder` instance that is provided by dependency injection.
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In the following example from the default web site template, several extension methods are used to configure the pipeline with support for [BrowserLink](http://vswebessentials.com/features/browserlink), error pages, static files, ASP.NET MVC, and Identity.
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[!code-csharp[Main](../common/samples/WebApplication1/Startup.cs?highlight=8,9,10,14,17,19,21&start=58&end=84)]
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Each `Use` extension method adds a [middleware](middleware.md) component to the request pipeline. For instance, the `UseMvc` extension method adds the [routing](routing.md) middleware to the request pipeline and configures [MVC](../mvc/index.md) as the default handler.
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For more information about how to use `IApplicationBuilder`, see [Middleware](middleware.md).
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Additional services, like `IHostingEnvironment` and `ILoggerFactory` may also be specified in the method signature, in which case these services will be [injected](dependency-injection.md) if they are available.
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## The ConfigureServices method
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The [ConfigureServices](https://docs.microsoft.com/en-us/aspnet/core/api/microsoft.aspnetcore.hosting.startupbase#Microsoft_AspNetCore_Hosting_StartupBase_ConfigureServices_Microsoft_Extensions_DependencyInjection_IServiceCollection_) method is optional; but if used, it's called before the `Configure` method by the runtime (some features are added before they're wired up to the request pipeline). [Configuration options](configuration.md) are set in this method.
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For features that require substantial setup there are `Add[Service]` extension methods on [IServiceCollection](https://docs.microsoft.com/en-us/aspnet/core/api/microsoft.extensions.dependencyinjection.iservicecollection). This example from the default web site template configures the app to use services for Entity Framework, Identity, and MVC:
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[!code-csharp[Main](../common/samples/WebApplication1/Startup.cs?highlight=4,7,11&start=40&end=55)]
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Adding services to the services container makes them available within your application via [dependency injection](dependency-injection.md).
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## Services Available in Startup
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ASP.NET Core dependency injection provides application services during an application's startup. You can request these services by including the appropriate interface as a parameter on your `Startup` class's constructor or one of its `Configure` or `ConfigureServices` methods.
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Looking at each method in the `Startup` class in the order in which they are called, the following services may be requested as parameters:
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* In the constructor: `IHostingEnvironment`, `ILoggerFactory`
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* In the `ConfigureServices` method: `IServiceCollection`
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* In the `Configure` method: `IApplicationBuilder`, `IHostingEnvironment`, `ILoggerFactory`, `IApplicationLifetime`
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## Additional Resources
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* [Working with Multiple Environments](environments.md)
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* [Middleware](middleware.md)
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* [Logging](logging.md)
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* [Configuration](configuration.md)
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