如何针对已安装的Target Framework编译并执行测试？

问题背景 #

这个问题和Stack Overflow上的《How do I conditionally build .Net Core 3.0 only if supported by tooling?》重复，但现有方案不适用。需求是让项目能在.NET 6.0和7.0的官方Docker SDK镜像中自动适配构建，原尝试的配置完全无效：

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

  <PropertyGroup Condition="'$(VisualStudioVersion)'=='16.0'">
    <TargetFramework>net6.0</TargetFramework>
  </PropertyGroup>

  <PropertyGroup Condition="'$(VisualStudioVersion)'=='17.0'">
    <TargetFrameworks>net7.0</TargetFrameworks>
  </PropertyGroup>

环境信息 #

• .NET 6.0 SDK镜像：

% docker run -t mcr.microsoft.com/dotnet/sdk:6.0 dotnet --list-sdks
6.0.410 [/usr/share/dotnet/sdk]
% docker run -t mcr.microsoft.com/dotnet/sdk:6.0 dotnet build --version
MSBuild version 17.3.2+561848881 for .NET
17.3.2.46306

• .NET 7.0 SDK镜像：

% docker run -t mcr.microsoft.com/dotnet/sdk:7.0 dotnet --list-sdks
7.0.305 [/usr/share/dotnet/sdk]
% docker run -t mcr.microsoft.com/dotnet/sdk:7.0 dotnet build --version
MSBuild version 17.6.3+07e294721 for .NET
17.6.3.22601

核心问题：原方案失效+跨版本测试失败 #

1. 原配置为什么没用？
   原配置依赖VisualStudioVersion变量，但Docker SDK镜像里根本没装Visual Studio，这个变量要么不存在，要么值和预期不符，条件判断完全不触发。

2. .NET7镜像构建.NET6项目测试失败的原因
   .NET7 SDK支持向下编译.NET6项目，但镜像里只带了.NET7 runtime，测试时需要.NET6 runtime来运行testhost，所以构建能过，测试直接报错。复现步骤和输出如下：

复现步骤 #

% docker run -it mcr.microsoft.com/dotnet/sdk:7.0
# cd /tmp
# dotnet new sln -o unit-testing-using-dotnet-test
# cd unit-testing-using-dotnet-test
# dotnet new classlib -o PrimeService
# mv PrimeService/Class1.cs PrimeService.cs
# dotnet sln add ./PrimeService/PrimeService.csproj
# dotnet new xunit -o PrimeService.Tests
# dotnet add ./PrimeService.Tests/PrimeService.Tests.csproj reference ./PrimeService/PrimeService.csproj
# dotnet sln add ./PrimeService.Tests/PrimeService.Tests.csproj

修改项目为net6.0：

# sed -e 's/net7.0/net6.0/' -i ./PrimeService/PrimeService.csproj
# sed -e 's/net7.0/net6.0/' -i ./PrimeService.Tests/PrimeService.Tests.csproj

构建结果（成功） #

# dotnet build
MSBuild version 17.6.3+07e294721 for .NET
  Determining projects to restore...
  Restored /tmp/unit-testing-using-dotnet-test/PrimeService/PrimeService.csproj (in 696 ms).
  Restored /tmp/unit-testing-using-dotnet-test/PrimeService.Tests/PrimeService.Tests.csproj (in 752 ms).
  PrimeService -> /tmp/unit-testing-using-dotnet-test/PrimeService/bin/Debug/net6.0/PrimeService.dll
  PrimeService.Tests -> /tmp/unit-testing-using-dotnet-test/PrimeService.Tests/bin/Debug/net6.0/PrimeService.Tests.dll

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

Time Elapsed 00:00:03.71

测试结果（失败） #

# dotnet test
  Determining projects to restore...
  All projects are up-to-date for restore.
  PrimeService -> /tmp/unit-testing-using-dotnet-test/PrimeService/bin/Debug/net6.0/PrimeService.dll
  PrimeService.Tests -> /tmp/unit-testing-using-dotnet-test/PrimeService.Tests/bin/Debug/net6.0/PrimeService.Tests.dll
Test run for /tmp/unit-testing-using-dotnet-test/PrimeService.Tests/bin/Debug/net6.0/PrimeService.Tests.dll (.NETCoreApp,Version=v6.0)
Microsoft (R) Test Execution Command Line Tool Version 17.6.0 (x64)
Copyright (c) Microsoft Corporation.  All rights reserved.

Starting test execution, please wait...
A total of 1 test files matched the specified pattern.
Testhost process for source(s) '/tmp/unit-testing-using-dotnet-test/PrimeService.Tests/bin/Debug/net6.0/PrimeService.Tests.dll' exited with error: You must install or update .NET to run this application.
App: /tmp/unit-testing-using-dotnet-test/PrimeService.Tests/bin/Debug/net6.0/testhost.dll
Architecture: x64
Framework: 'Microsoft.NETCore.App', version '6.0.0' (x64)
.NET location: /usr/share/dotnet/
The following frameworks were found:
  7.0.8 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]
Learn about framework resolution:
https://aka.ms/dotnet/app-launch-failed
To install missing framework, download:
https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=6.0.0&arch=x64&rid=debian.11-x64
. Please check the diagnostic logs for more information.

Test Run Aborted.

解决方案 #

方案1：基于SDK版本动态设置单目标框架 #

用$(NETCoreSdkVersion)变量判断当前SDK主版本，自动切换目标框架，适配Docker环境：

<Project Sdk="Microsoft.NET.Sdk">
  <!-- 默认目标为net6.0，可根据需求调整 -->
  <PropertyGroup>
    <TargetFramework>net6.0</TargetFramework>
  </PropertyGroup>

  <!-- 当SDK版本是7.x及以上时，切换到net7.0 -->
  <PropertyGroup Condition="$([System.Version]::Parse('$(NETCoreSdkVersion)')).Major >= 7">
    <TargetFramework>net7.0</TargetFramework>
  </PropertyGroup>
</Project>

方案2：多目标框架+条件过滤（可选） #

如果需要保留多目标声明，但只编译当前环境支持的框架：

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <!-- 先声明所有支持的框架 -->
    <TargetFrameworks>net6.0;net7.0</TargetFrameworks>
    <!-- SDK6环境只编译net6.0 -->
    <TargetFrameworks Condition="$([System.Version]::Parse('$(NETCoreSdkVersion)')).Major == 6">net6.0</TargetFrameworks>
    <!-- SDK7环境只编译net7.0 -->
    <TargetFrameworks Condition="$([System.Version]::Parse('$(NETCoreSdkVersion)')).Major == 7">net7.0</TargetFrameworks>
  </PropertyGroup>
</Project>

补充：如果必须在.NET7镜像中测试.NET6项目 #

可以在Dockerfile中安装.NET6 runtime，解决测试依赖问题：

FROM mcr.microsoft.com/dotnet/sdk:7.0

# 安装.NET6 runtime（Debian/Ubuntu镜像适用）
RUN apt-get update && apt-get install -y dotnet-runtime-6.0

内容的提问来源于stack exchange，提问作者malat