net5.0 TFM defines both NET5_0 and NETCOREAPP3_1

Description

When building a project that cross compiles between net5.0 and netcoreapp3.1 there’s an ordering problem with the #if’s. If the NETCOREAPP3_1 condition is put first then the net5.0 TFM will compile using it, but if the NET5_0 condition is first then that is choosen.

Configuration

5.0.100-preview.8.20417.9 SDK

Windows 10 2004 (19041.450) x64

Regression?

Yes. This was working in prior preview SDKs when using NETCOREAPP5_0.

Other information

Program.cs:

using System;
using System.Runtime.CompilerServices;

namespace TfmConsole
{
    class Program
    {
        static void Main(string[] args)
        {
            Broken();
            Fixed();
        }

        static void Broken()
        {
            Console.WriteLine("Hello Broken World! " + Environment.Version);
#if NETCOREAPP3_1
            Console.WriteLine("NETCOREAPP3_1");
#elif NET5_0
            Console.WriteLine("NET5_0");
#elif NETCOREAPP5_0
            Console.WriteLine("NETCOREAPP5_0");
#else
#error A target framework was added to the project and needs to be added to this condition.
#endif
        }
        static void Fixed()
        {
            Console.WriteLine("Hello Fixed World! " + Environment.Version);
#if NET5_0
            Console.WriteLine("NET5_0");
#elif NETCOREAPP5_0
            Console.WriteLine("NETCOREAPP5_0");
#elif NETCOREAPP3_1
            Console.WriteLine("NETCOREAPP3_1");
#else
#error A target framework was added to the project and needs to be added to this condition.
#endif
        }
    }
}

csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFrameworks>net5.0;netcoreapp3.1</TargetFrameworks>
  </PropertyGroup>

</Project>

Output:

C:\temp\TfmConsole\TfmConsole>dotnet run --framework netcoreapp3.1
Hello Broken World! 3.1.7
NETCOREAPP3_1
Hello Fixed World! 3.1.7
NETCOREAPP3_1

C:\temp\TfmConsole\TfmConsole>dotnet run --framework net5.0
Hello Broken World! 5.0.0
NETCOREAPP3_1
Hello Fixed World! 5.0.0
NET5_0

C:\temp\TfmConsole\TfmConsole>dotnet run --framework netcoreapp5.0
Hello Broken World! 5.0.0
NETCOREAPP3_1
Hello Fixed World! 5.0.0
NET5_0

[sharwell]

It’s fine if the SDK defines both NETCOREAPP and NET5_0 (the former is allowed to be renamed to just NET but not required). However, the SDK must not define both NETCOREAPP3_1 and NET5_0. Doing so would break the entire mental model of the feature and we’ll be stuck with the ugly workaround of writing custom build tooling that fails the build if a user fails to manually remove the extraneous definition.

[marcpopMSFT]

We’ve reviewed the feedback both from external and internal customers and for net5.0, we will remove the change that defines NETCOREAPP3_1 for this release. @terrajobst will own developing a new proposal for a future SDK release. For now, customers can use #if NET to mean .net 5 and higher.

Based on our analysis of existing projects, most customers use these symbols as >= so we want to provide that option. However, we recognize that changing the previous behavior that existed for 3.1 and before will impact some customers and cause confusion.

Thanks all for the feedback.