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ASP.NET Core - Use factory based middleware with scoped services

Yesterday I talked about a problem we encountered when trying to inject a scoped service into ASP.NET Core middleware.

The middleware used was convention based middleware, one of the ways to construct middleware in ASP.NET Core.

With convention based middleware there isn’t a base class or interface to implement. The only rules your middleware class needs to apply to are:

  • It has a public constructor with a parameter of type RequestDelegate.
  • It contains a public method named Invoke or InvokeAsync. This method must:
    • Return a Task.
    • Accept a first parameter of type HttpContext.

Convention based middleware is registered with a singleton lifetime, so you can only constructor injection for the dependencies with a singleton lifetime. For transient and scoped dependencies you need to add extra parameters to the InvokeAsync() method:

There is a log of magic going on when using convention based middleware and it shouldn’t be a surprise that people are not aware of its behavior.

If you want full control on the middleware construction and avoid any kind of convention-based “magic” you can use the factory based middleware instead.

Now we need a class that implements the IMiddleware interface:

A transient or scoped dependency can now be injected through the constructor:

The only thing remaining to do is to register our factory based middleware with a transient lifetime:

More information

ASP.NET Core–Factory based vs Convention based middleware

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