Reference Microsoft.AspNetCore.Routing in Library

So, I’m confused by the Migrate from ASP.NET Core 2.2 to 3.0 documentation when it comes to the deprecated libraries.

In the Remove obsolete package references section, there is a long list of packages hidden under “Click to expand the list of packages no longer being produced”. The package from that list that I’m going to focus on is Microsoft.AspNetCore.Routing. That’s the package that contains IEndpointRouteBuilder.

The article explains that the removed packages are now available through the shared framework Microsoft.AspNetCore.App. So, let’s test out if that works. I’m going to:

• Create a new class library which will reference IEndpointRouteBuilder.
• Attempt to get that class library to successfully compile.

The examples in this post can be found on github at IEndpointRouteBuilderDemo.

Here’s the sample IEndpointRouteBuilderExtensions.cs class that will be used in our test:

So, let’s try it with the suggested .csproj file settings:

And the error we get back is:

C:\Program Files\dotnet\sdk\3.1.102\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(283,5): error NETSDK1073: The FrameworkReference 'Microsoft.AspNetCore.App' was not recognized

Okay … so, what can we do? Well, there is a closed issue on github about this problem: AspNetCore Issue #16638, Cannot find the AspNetCore Nuget packages for 3.0 (specifically routing). The response to that issue is to do what was demonstrated above. So, what else can we try.

That issue accurately describes that the Microsoft.AspNetCore.Routing dll is embedded in Microsoft.AspNetCore.App, which is located on disk at C:\Program Files\dotnet\packs\Microsoft.AspNetCore.App.Ref\3.1.2\ref\netcoreapp3.1\Microsoft.AspNetCore.Routing.dll. But, how can you get it referenced propely?

One way to get it referenced is to change the .csproj file to use the SDK of Microsoft.NET.SDK.Web and change the TargetFramework to netcoreapp3.1. Like this:

But, when you do that, you get a new error:

CSC : error CS5001: Program does not contain a static 'Main' method suitable for an entry point

Which, kind of makes sense. The project has been changed over to a netcoreapp, so it kind of expects to create an executable. The great part about executables is that they also create .dlls, which is what we are looking for in the first place.

We just need to get it to compile in order to get the .dll. To do that, let’s create a DummyMain.cs class which will provide the required static ‘Main’ method:

Which provides a successful compile:

Done building project "IEndpointRouteBuilderDemo-Compiles.csproj".

Build succeeded.
    0 Warning(s)
    0 Error(s)

Of course, this isn’t the ideal result. And, it would be hard to believe that the ASP.NET Core team expected this to occur. So, it’s most likely my misunderstanding of how to reference the Shared Framework correctly in order to prevent the need for the rest of these workarounds to occur.

Hopefully the Microsoft team will be able to shed more light on this in AspNetCore Issue #19481, Reference IEndpointRouteBuilder in a class libary (ASP.NET Core 3.1).

Which they did!

So, the documentation from Migrate from ASP.NET Core 2.2 to 3.0 doesn’t use the Microsoft.NET.SDK.Web SDK reference in the .csproj file. It uses the Microsoft.NET.SDK reference instead. Along with the Shared Framework reference, this allows for the class library to be compiled without needing the DummyMain method: