--- title: App startup in ASP.NET Core author: rick-anderson description: Learn how the Startup class in ASP.NET Core configures services and the app's request pipeline. monikerRange: '>= aspnetcore-3.1' ms.author: riande ms.custom: mvc ms.date: 12/05/2019 uid: fundamentals/startup --- # App startup in ASP.NET Core By [Kirk Larkin](https://twitter.com/serpent5) and [Rick Anderson](https://twitter.com/RickAndMSFT) :::moniker range=">= aspnetcore-6.0" ASP.NET Core apps created with the web templates contain the application startup code in the `Program.cs` file. The following app startup code supports: * [Razor Pages](xref:tutorials/razor-pages/razor-pages-start) * [MVC controllers with views](xref:tutorials/first-mvc-app/start-mvc) * [Web API with controllers](xref:tutorials/first-web-api) * [Minimal APIs](xref:tutorials/min-web-api) [!code-csharp[](~/fundamentals/startup/6.0_samples/WebAll/Program.cs?name=snippet)] For more information on application startup, see . :::moniker-end :::moniker range="< aspnetcore-6.0" The `Startup` class configures services and the app's request pipeline. ## The Startup class ASP.NET Core apps use a `Startup` class, which is named `Startup` by convention. The `Startup` class: * Optionally includes a method to configure the app's *services*. A service is a reusable component that provides app functionality. Services are *registered* in `ConfigureServices` and consumed across the app via [dependency injection (DI)](xref:fundamentals/dependency-injection) or . * Includes a method to create the app's request processing pipeline. `ConfigureServices` and `Configure` are called by the ASP.NET Core runtime when the app starts: [!code-csharp[](startup/3.0_samples/StartupFilterSample/Startup.cs?name=snippet)] The preceding sample is for [Razor Pages](xref:razor-pages/index); the MVC version is similar. The `Startup` class is specified when the app's [host](xref:fundamentals/index#host) is built. The `Startup` class is typically specified by calling the [`WebHostBuilderExtensions.UseStartup`/``](xref:Microsoft.AspNetCore.Hosting.WebHostBuilderExtensions.UseStartup*) method on the host builder: [!code-csharp[](startup/3.0_samples/Program3.cs?name=snippet_Program&highlight=12)] The host provides services that are available to the `Startup` class constructor. The app adds additional services via `ConfigureServices`. Both the host and app services are available in `Configure` and throughout the app. Only the following service types can be injected into the `Startup` constructor when using the [Generic Host](xref:fundamentals/host/generic-host) (): * * * [!code-csharp[](startup/3.0_samples/StartupFilterSample/StartUp2.cs?name=snippet)] Most services are not available until the `Configure` method is called. ### Multiple Startup When the app defines separate `Startup` classes for different environments (for example, `StartupDevelopment`), the appropriate `Startup` class is selected at runtime. The class whose name suffix matches the current environment is prioritized. If the app is run in the Development environment and includes both a `Startup` class and a `StartupDevelopment` class, the `StartupDevelopment` class is used. For more information, see [Use multiple environments](xref:fundamentals/environments#environment-based-startup-class-and-methods). See [The host](xref:fundamentals/index#host) for more information on the host. For information on handling errors during startup, see [Startup exception handling](xref:fundamentals/error-handling#startup-exception-handling). ## The ConfigureServices method The method is: * Optional. * Called by the host before the `Configure` method to configure the app's services. * Where [configuration options](xref:fundamentals/configuration/index) are set by convention. The host may configure some services before `Startup` methods are called. For more information, see [The host](xref:fundamentals/index#host). For features that require substantial setup, there are `Add{Service}` extension methods on . For example, **Add**DbContext, **Add**DefaultIdentity, **Add**EntityFrameworkStores, and **Add**RazorPages: [!code-csharp[](startup/3.0_samples/StartupFilterSample/StartupIdentity.cs?name=snippet)] Adding services to the service container makes them available within the app and in the `Configure` method. The services are resolved via [dependency injection](xref:fundamentals/dependency-injection) or from . ## The Configure method The method is used to specify how the app responds to HTTP requests. The request pipeline is configured by adding [middleware](xref:fundamentals/middleware/index) components to an instance. `IApplicationBuilder` is available to the `Configure` method, but it isn't registered in the service container. Hosting creates an `IApplicationBuilder` and passes it directly to `Configure`. The [ASP.NET Core templates](/dotnet/core/tools/dotnet-new) configure the pipeline with support for: * [Developer Exception Page](xref:fundamentals/error-handling#developer-exception-page) * [Exception handler](xref:fundamentals/error-handling#exception-handler-page) * [HTTP Strict Transport Security (HSTS)](xref:security/enforcing-ssl#http-strict-transport-security-protocol-hsts) * [HTTPS redirection](xref:security/enforcing-ssl) * [Static files](xref:fundamentals/static-files) * ASP.NET Core [MVC](xref:mvc/overview) and [Razor Pages](xref:razor-pages/index) [!code-csharp[](startup/3.0_samples/StartupFilterSample/Startup.cs?name=snippet)] The preceding sample is for [Razor Pages](xref:razor-pages/index); the MVC version is similar. Each `Use` extension method adds one or more middleware components to the request pipeline. For instance, configures [middleware](xref:fundamentals/middleware/index) to serve [static files](xref:fundamentals/static-files). Each middleware component in the request pipeline is responsible for invoking the next component in the pipeline or short-circuiting the chain, if appropriate. Additional services, such as `IWebHostEnvironment`, `ILoggerFactory`, or anything defined in `ConfigureServices`, can be specified in the `Configure` method signature. These services are injected if they're available. For more information on how to use `IApplicationBuilder` and the order of middleware processing, see . ## Configure services without Startup To configure services and the request processing pipeline without using a `Startup` class, call `ConfigureServices` and `Configure` convenience methods on the host builder. Multiple calls to `ConfigureServices` append to one another. If multiple `Configure` method calls exist, the last `Configure` call is used. [!code-csharp[](startup/3.0_samples/StartupFilterSample/Program1.cs?name=snippet)] ## Extend Startup with startup filters Use : * To configure middleware at the beginning or end of an app's [Configure](#the-configure-method) middleware pipeline without an explicit call to `Use{Middleware}`. `IStartupFilter` is used by ASP.NET Core to add defaults to the beginning of the pipeline without having to make the app author explicitly register the default middleware. `IStartupFilter` allows a different component to call `Use{Middleware}` on behalf of the app author. * To create a pipeline of `Configure` methods. [IStartupFilter.Configure](xref:Microsoft.AspNetCore.Hosting.IStartupFilter.Configure%2A) can set a middleware to run before or after middleware added by libraries. `IStartupFilter` implements , which receives and returns an `Action`. An defines a class to configure an app's request pipeline. For more information, see [Create a middleware pipeline with IApplicationBuilder](xref:fundamentals/middleware/index#create-a-middleware-pipeline-with-iapplicationbuilder). Each `IStartupFilter` can add one or more middlewares in the request pipeline. The filters are invoked in the order they were added to the service container. Filters may add middleware before or after passing control to the next filter, thus they append to the beginning or end of the app pipeline. The following example demonstrates how to register a middleware with `IStartupFilter`. The `RequestSetOptionsMiddleware` middleware sets an options value from a query string parameter: [!code-csharp[](startup/3.0_samples/StartupFilterSample/RequestSetOptionsMiddleware.cs?name=snippet1)] The `RequestSetOptionsMiddleware` is configured in the `RequestSetOptionsStartupFilter` class: [!code-csharp[](startup/3.0_samples/StartupFilterSample/RequestSetOptionsStartupFilter.cs?name=snippet1&highlight=7)] The `IStartupFilter` is registered in the service container in . [!code-csharp[](startup/3.0_samples/StartupFilterSample/Program.cs?name=snippet&highlight=19-20)] When a query string parameter for `option` is provided, the middleware processes the value assignment before the ASP.NET Core middleware renders the response. Middleware execution order is set by the order of `IStartupFilter` registrations: * Multiple `IStartupFilter` implementations may interact with the same objects. If ordering is important, order their `IStartupFilter` service registrations to match the order that their middlewares should run. * Libraries may add middleware with one or more `IStartupFilter` implementations that run before or after other app middleware registered with `IStartupFilter`. To invoke an `IStartupFilter` middleware before a middleware added by a library's `IStartupFilter`: * Position the service registration before the library is added to the service container. * To invoke afterward, position the service registration after the library is added. ## Add configuration at startup from an external assembly An implementation allows adding enhancements to an app at startup from an external assembly outside of the app's `Startup` class. For more information, see . ## Additional resources * [The host](xref:fundamentals/index#host) * * * * :::moniker-end