I had to migrate bunch of aspnet core projects from 2.2 to 3.1 recently. I managed to automate a small portion of the process to get things rolling:

fd Dockerfile | xargs sed -i '' "s#sdk:2.2-alpine3.10#sdk:3.1.101-alpine3.10#g"
fd Dockerfile | xargs sed -i '' "s#aspnet:2.2.7-alpine3.10#aspnet:3.1.1-alpine3.10#g"
fd csproj | xargs sed -i '' "/Microsoft.AspNetCore.App/d"
fd csproj | xargs sed -i '' "/Microsoft.AspNetCore.All/d"
rg -l TargetFramework | xargs sed -i '' "s#<TargetFramework>netcoreapp2.2</TargetFramework>#<TargetFramework>netcoreapp3.1</TargetFramework>#g"

Here’s what they do in order:

• Replace sdk image in all Dockerfile with the 3.1 image.
• Replace runtime aspnet image in all Dockerfile with 3.1 image.
• Remove all instances of Microsoft.AspNetCore.App from project files.
• Remove all instances of Microsoft.AspNetCore.All from project files.
• Replace netcoreapp2.2 as target framework with netcoreapp3.1.

Aftewards I started kicking builds and fixing issues one by one. For a complete list of breaking changes from 2.2 to 3.1 see here. In my case it entailed these:

• IHostingEnvironment under Microsoft.AspNetCore.Hosting is now obsolete and is replaced by IHostEnvironment under Microsoft.Extensions.Hosting. IHostingEnvironment under Micorosft.Extensions.Hosting is also obsolete and is replaced by IWebHostEnvironment under Microsoft.AspNetCore.Hosting and drives from the new IHostEnvironment. There is a good blog post explaining the change in detail with some history here.

• Endpoint Routing is on by default. This change moves the responsibility of endpoint routing from MVC middleware to a dedicated middleware. This basically allows other middlewares to have access to routing information as it will be stored in HttpContext. For more info on the topic see here. If you want to disable this functionality, you can do simply by:

  public void ConfigureServices(IServiceCollection services)
  {
    services.AddMvc(options =>
    {
        options.EnableEndpointRouting = false;
    });
  }
  

• Json.NET is not included in aspnet core 3.1 by default anymore. If you want to keep using it you can add the package to your project via:

  dotnet add <your_project_csproj_file> package Microsoft.AspNetCore.Mvc.NewtonsoftJson

  And to configure it:

  services.AddNewtonsoftJson(opt =>
  {
    opt.SerializerSettings.ContractResolver = null;
  });
  

• EnableRewind is replaced with EnableBuffering. This is the method to call if you want multiple middlewares to read request body.

That’s all. Good luck!