75 lines
4.8 KiB
Markdown
75 lines
4.8 KiB
Markdown
---
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title: Application Startup | Microsoft Docs
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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: 11/29/2016
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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: aspnet-core
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uid: fundamentals/startup
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---
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# Application Startup
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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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All ASP.NET applications require at least one `Startup` class. When an application starts, ASP.NET searches the primary assembly for a class named `Startup` (in any namespace). You can specify a different assembly to search using the *Hosting:Application* configuration key. The class doesn't have to be `public`. If there are multiple `Startup` classes, ASP.NET looks for one in the project's root namespace, otherwise it chooses one in the alphabetically first namespace.
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You can define separate `Startup` classes for different environments, and the appropriate one will be selected at runtime. Learn more in [Working with multiple environments](environments.md#startup-conventions).
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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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## 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 `Startup` class can include a `ConfigureServices` method that takes an `IServiceCollection` parameter and optionally returns an `IServiceProvider`. The `ConfigureServices` method is called before `Configure`, as some features must be added before they can be wired up to the request pipeline.
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For features that require substantial setup there are `Add[Something]` extension methods on `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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The `ConfigureServices` method is also where you should add configuration option classes. Learn more in [Configuration](configuration.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`
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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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