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ASP.NET Core–Factory based vs Convention based middleware

Today I had to create my own ASP.NET Core middleware. I forgot how to exactly do this but I noticed that an IMiddleware interface existed. I wasn't aware of this interface which brought me to the discovery that there in fact multiple ways to define and create middleware in ASP.NET Core.

Let’s explain each one of them and discuss the differences.

Inline middleware

The simplest solution is to use an inline request delegate:

This is a great way for simple use cases but once your middleware gets more complex it is time to move to one of the 2 other approaches.

Convention based middleware

The convention based middleware is the one I was aware of that it existed and that I had used before. To use the convention based middleware you create a separate class that follows these rules:

  • 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.

Remark: If you want to inject extra dependencies, you can use constructor injection for the dependencies with a singleton lifetime and add extra parameters to the InvokeAsync() method for your transient and scoped dependencies:

To add this middleware to the request pipeline, you need to use the UseMiddleware() method:

Factory based middleware

The last option is the factory based middleware. To use this approach you need to implement the IMiddleware interface I was mentioning in the introduction.

The advantage is that there is no longer conventional magic and that dependencies are injected through the constructor. As the factory based middleware is activated per client request (connection), scoped services can be injected into the middleware's constructor without issues.

Adding this middleware can be done in the same way as with the previous approach:

The only thing you may not forget is that you have to register the middleware class in your IoC container:

More information:

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