An [IHostingStartup](/dotnet/api/microsoft.aspnetcore.hosting.ihostingstartup) implementation allows adding enhancements to an app at startup from an external assembly outside of the app's `Startup` class. For example, an external tooling library can use an `IHostingStartup` implementation to provide additional configuration providers or services to an app. `IHostingStartup`*is available in ASP.NET Core 2.0 and later.*
[View or download sample code](https://github.com/aspnet/Docs/tree/master/aspnetcore/fundamentals/configuration/platform-specific-configuration/sample/) ([how to download](xref:tutorials/index#how-to-download-a-sample))
To discover hosting startup assemblies loaded by the app or by libraries, enable logging and check the application logs. Errors that occur when loading assemblies are logged. Loaded hosting startup assemblies are logged at the Debug level, and all errors are logged.
The sample app reads the [HostingStartupAssembliesKey](/dotnet/api/microsoft.aspnetcore.hosting.webhostdefaults.hostingstartupassemblieskey) into a `string` array and displays the result in the app's Index page:
When either the host setting or the environment variable is set to `true` or `1`, hosting startup assemblies aren't automatically loaded. If both are set, the host setting controls the behavior.
Disabling hosting startup assemblies using the host setting or environment variable disables them globally and may disable several characteristics of an app. It isn't currently possible to selectively disable a hosting startup assembly added by a library unless the library offers its own configuration option. A future release will offer the ability to selectively disable hosting startup assemblies (see [GitHub issue aspnet/Hosting #1243](https://github.com/aspnet/Hosting/pull/1243)).
An `IHostingStartup` enhancement is deployed as an assembly based on a console app without an entry point. The assembly references the [Microsoft.AspNetCore.Hosting.Abstractions](https://www.nuget.org/packages/Microsoft.AspNetCore.Hosting.Abstractions/) package:
A [HostingStartup](/dotnet/api/microsoft.aspnetcore.hosting.hostingstartupattribute) attribute identifies a class as an implementation of `IHostingStartup` for loading and execution when building the [IWebHost](/dotnet/api/microsoft.aspnetcore.hosting.iwebhost). In the following example, the namespace is `StartupEnhancement`, and the class is `StartupEnhancementHostingStartup`:
A class implements `IHostingStartup`. The class's [Configure](/dotnet/api/microsoft.aspnetcore.hosting.ihostingstartup.configure) method uses an [IWebHostBuilder](/dotnet/api/microsoft.aspnetcore.hosting.iwebhostbuilder) to add enhancements to an app. `IHostingStartup.Configure` in the hosting startup assembly is called by the runtime before `Startup.Configure` in user code, which allows user code to overwrite any configruation provided by the hosting startup assembly.
The runtime location is specified in the *\*.deps.json* file. To active the enhancement, the `runtime` element must specify the location of the enhancement's runtime assembly. Prefix the `runtime` location with `lib/<TARGET_FRAMEWORK_MONIKER>/`:
In the sample app, modification of the *\*.deps.json* file is performed by a [PowerShell](/powershell/scripting/powershell-scripting) script. The PowerShell script is automatically triggered by a build target in the project file.
The `IHostingStartup` implementation's assembly file must be *bin*-deployed in the app or placed in the [runtime store](/dotnet/core/deploying/runtime-store):
For per-user use, place the assembly in the user profile's runtime store at:
The shared framework version reflects the version of the shared runtime that the target app uses. The shared runtime is shown in the *\*.runtimeconfig.json* file. In the sample app, the shared runtime is specified in the *HostingStartupSample.runtimeconfig.json* file.
Only hosting startup assemblies are scanned for the `HostingStartupAttribute`. The assembly name of the implementation is provided in this environment variable. The sample app sets this value to `StartupDiagnostics`.
The value can also be set using the [Hosting Startup Assemblies](xref:fundamentals/host/web-host#hosting-startup-assemblies) host configuration setting.
When multiple hosting startup assembles are present, their [Configure](/dotnet/api/microsoft.aspnetcore.hosting.ihostingstartup.configure) methods are executed in the order that the assemblies are listed.
The [sample app](https://github.com/aspnet/Docs/tree/master/aspnetcore/fundamentals/configuration/platform-specific-configuration/sample/) ([how to download](xref:tutorials/index#how-to-download-a-sample)) uses `IHostingStartup` to create a diagnostics tool. The tool adds two middlewares to the app at startup that provide diagnostic information:
* Connection: remote IP, remote port, local IP, local port, client certificate
* Request headers
* Environment variables
To run the sample:
1. The Startup Diagnostic project uses [PowerShell](/powershell/scripting/powershell-scripting) to modify its *StartupDiagnostics.deps.json* file. PowerShell is installed by default on Windows OS starting with Windows 7 SP1 and Windows Server 2008 R2 SP1. To obtain PowerShell on other platforms, see [Installing Windows PowerShell](/powershell/scripting/setup/installing-windows-powershell).
2. Build the Startup Diagnostic project. A build target in the project file:
* Moves the assembly and symbols files to the user profile's runtime store.
* Triggers the PowerShell script to modify the *StartupDiagnostics.deps.json* file.
* Moves the *StartupDiagnostics.deps.json* file to the user profile's `additionalDeps` folder.
