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ASP.NET Core - IHostingStartup

ASP.NET Core 2.0 introduces a new interface called IHostingStartup. Through this interface you can manipulate the process of building the web host outside your ASP.NET Core project. Just by including the DLL, you can hook into the configuration pipeline.

Implementing IHostingStartup

Let’s start by implementing this interface:

  • Create a new .NET Standard Class Library
  • Reference the Microsoft.AspNetCore.Hosting.Abstractions package. This package contains the IHostingStartup interface.
  • Create a new class and implement this interface. Just as an example we add a Console.Writeline to the Configure method:

Make the magic work

Unfortunately we are not there yet and we have to take some extra steps:

  • First add the HostingStartup attribute to your class library:

[assembly: HostingStartup(typeof(ClassLibrary1.MyCustomStartup))]

  • Now the ASP.NET Core runtime has the necessary information where to find our startup inside the DLL.
  • Next step is to setup an environment variable to let the runtime scan the dependency:
    • Right click on your ASP.NET Core project and go to Properties
    • Go to the Debug tab and add the following environment variable:
      • ASPNETCORE_HOSTINGSTARTUPASSEMBLIES=ClassLibrary1
    • The environment variable is a semicolon-delimited list of assemblies.
  • As a last step we have to activate the assembly. Probably the easiest way is by adding a reference, but using the Runtime store is also possible

If we now run our application, we see that the IHostingStartup Configure method is invoked:

More information: https://docs.microsoft.com/en-us/aspnet/core/fundamentals/host/platform-specific-configuration?view=aspnetcore-2.2

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