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Clarify startup class selection (#3966) 

It was not clear how the two ways of specifying the startup
class differed.
pull/3967/head
Samir Aguiar 2017-08-15 11:19:03 -03:00 committed by Scott Addie
parent d5e088545a
commit d0b3833c4f
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aspnetcore/fundamentals/startup.md
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@ -22,7 +22,9 @@ The `Startup` class configures services and the application's request pipeline.
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` program's [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. See [Hosting](xref:fundamentals/hosting) to learn more about `WebHostBuilder`, which runs before `Startup`.
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). For example, if the app is run in the *Development* environment, and includes both a `Startup` and a `StartupDevelopment` class, the `StartupDevelopment` class will be used. `UseStartup<TStartup>` is the recommended approach.
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](https://docs.microsoft.com/en-us/aspnet/core/fundamentals/hosting?tabs=aspnetcore2x#configuring-a-host) or options, hosting will load that startup assembly and search for a `Startup` or `Startup[Environment]` type. The class whose name suffix matches the current environment will be prioritized, so if the app is run in the *Development* environment, and includes both a `Startup` and a `StartupDevelopment` class, the `StartupDevelopment` class will be used. 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).
Alternatively, you can define a fixed `Startup` class that will be used regardless of the environment by calling `UseStartup<TStartup>`. This is the recommended approach.
The `Startup` class constructor can accept dependencies that are provided through [dependency injection](xref:fundamentals/dependency-injection). A common approach is to use `IHostingEnvironment` to set up [configuration](xref:fundamentals/configuration) sources.