How Does Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore Resolve ASP.NET Core Shared Framework Types?

When you noticed that your class library project was able to resolve types like WebApplication and IAuthorizationService just by having the Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore package installed, you're actually witnessing the behavior of ASP.NET Core’s shared framework.

Implicit Framework Reference

Starting with ASP.NET Core 3.0 and continuing in later versions, Microsoft introduced the concept of a shared framework (commonly known as Microsoft.AspNetCore.App). This shared framework includes a collection of assemblies such as Microsoft.AspNetCore.dll and Microsoft.AspNetCore.Authorization.dll. Projects that target frameworks like .NET 6 or .NET 7 and are set up using the ASP.NET Core SDK automatically get an implicit reference to this shared framework without you having to list each dependency explicitly in your project file.

How Your Package Brings It In

When you add a package like Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore to your project, it brings in its own set of dependencies as defined by the package metadata. Even though the NuGet Package Manager might not show a direct dependency on Microsoft.AspNetCore.dll or Microsoft.AspNetCore.Authorization.dll, the ASP.NET Core shared framework is being referenced implicitly because of your project’s target framework and SDK. This means that even in a class library project, the compiler is aware of those APIs (such as WebApplication and IAuthorizationService) because the shared framework assemblies are available.

What Happens When You Remove the Package

If removing the package causes a failure to resolve WebApplication, it likely means that your project was indirectly getting the shared framework reference via that package (or its targets). In some cases, especially in class libraries, references to the ASP.NET Core shared framework might not be brought in unless a package or a project file explicitly triggers the behavior. By removing Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore, you might lose that trigger, so the build system no longer includes the shared framework assemblies by default for your library.

Summary

• ASP.NET Core applications (and libraries targeting ASP.NET Core) have implicit references to the shared framework (Microsoft.AspNetCore.App) where many core assemblies reside.
• The package you installed brings in build targets or transitively triggers those implicit references.
• Removing the package removes this effect, which is why types like WebApplication become unresolved.

This behavior is not about NuGet transitively referencing all assemblies directly, but rather about the SDK and target framework automatically including the assemblies provided by the ASP.NET Core shared runtime.