考虑到目标之间的实现差异，如何正确地对具有多个目标框架的 .NET 项目进行单元测试? [英] How to properly unit test a .NET project with multiple target frameworks, given implementation differences among targets?

问题描述

考虑一个面向以下框架的 .NET 类库:

• .NET 框架 2.0
• .NET 框架 4.6
• .NET 标准 1.0
• .NET 标准 1.3
• .NET Portable Profile336

我们不要立即担心为什么这是目标框架的确切列表.

假设所有目标框架的外部 API 都是相同的，但是每个单独的目标框架都有一些该框架独有的代码，我如何正确编写涵盖所有代码的单元测试目标框架?

我正在运行 Visual Studio 2017 社区.

我已经在 GitHub 上制作了一个现成的最小示例，到目前为止我发现这可能是我现在能做的最好的:https://github.com/airbreather/MultiTargetDemo，但这感觉不是正确的做法.一些缺陷:

• 每个目标框架都需要一个单独的 .csproj 文件，其中包含大量复制粘贴.
• 使用名为 ReferringTargetFrameworkForProjectReferences 的未充分记录的*属性.
  • *这有点轻描淡写……目前在谷歌上搜索，除了 Microsoft 的 SDK 内容外，几乎没有任何内容.
• 似乎无法与 .NET Standard 垫片一起正常工作(此处的 .NET Standard 1.3 在运行时抛出试图查找 System.IO.FileSystem).(想通了，这是 corefx 问题 #16322，有点尴尬的解决方法).

上面链接中的库正好有一个公共类，上面有一个公共实例方法.该方法有合同义务仅返回 32 位整数值 3，但它在每个目标上以完全不同的方式计算 3.显然，实际用例围绕着实现更有趣"的方法展开，但这足以进行对话.

此外，请注意，该库中的测试类实际上自己使用了一些并非在所有目标平台上都可用的东西:System.Threading.Tasks.Task<TResult> 在 .NET Framework 2.0... 这只是测试本身可能用于其自身目的的任何东西的代理，这些东西可能不一定在正在测​​试的目标框架中可用.

实际用例是NetTopologySuite，它已经针对多个全框架平台和一些PCL；有一个 老问题 要求使其与 .NET Core 一起使用.我一直在断断续续地尝试实现这一目标大约一个月，但我终于跌跌撞撞地遇到了这个问题.

我发现了另一个问题:使用 .NET Core 和 xUnit 同时针对多个框架进行单元测试代码，即确实和这个类似.这有几个不同的原因:

• 那个问题是使用 VS2015 时代的预览 SDK 工具 (project.json).这个问题是指带有漂亮的 .csproj 文件和所有文件的 VS2017 时代的非预览工具.
• 可以通过简单地对测试项目本身使用一个测试"目标框架来解决这个问题，每个框架由被测对象"定位.似乎无法以类似的方式解决此问题，尤其是对于不寻常的 Profile336 目标.
  • 此外，鉴于完整的 .NET Framework 4.6 可以引用针对列出的五个目标框架中的任何一个的程序集，我应该不必将基本测试类的实现限制为仅在为此选择的任何目标框架的交集.
  • 我确实尝试了一种技巧，例如在 TargetFrameworks 中列出合成的东西，例如 test1;test2;test3;test4;test5 然后强制 TargetFrameworkIdentifier +TargetFrameworkVersion 到 .NETFramework + 4.6，但我没有成功(尝试获取 xunit 包时失败).不过，我也没有在这方面太努力.

解决方案

首先，测试项目也可以通过实现多目标就像您用于图书馆的财产一样.虽然 VS 测试运行器目前仅显示/运行一个框架(具体来说是第一个)，但任何 dotnet test 调用都将执行所有框架(xunit 也在开发自定义控制台运行器 - dotnet xunit - 以获得更好的控制台用户体验).

另外请记住，测试项目需要指定具有运行时的目标框架.因此，您的测试项目可以以 netcoreapp* 或 net* 为目标，但不能以 netstandard* 为目标，因为它没有关联的运行时来运行测试.我不确定可移植目标，因为曾经测试项目需要引用以获取测试运行器支持的 Microsoft.NET.Tests.Sdk 不具备 NETPortable 的条件.

因此，您需要选择需要测试/支持的所有 net 和 netcoreapp 版本 - 例如net20;net45;net46;netcoreapp1.0 将涵盖您所有的 .net 框架和 .net 核心版本.

有一种方法可以通过在 <ProjectReference> 元素上设置 Properties="TargetFramework=netstandard1.3" 来强制依赖特定的目标框架，但这对于恢复和多目标可能很难正常工作.

Consider a .NET class library that targets the following frameworks:

• .NET Framework 2.0
• .NET Framework 4.6
• .NET Standard 1.0
• .NET Standard 1.3
• .NET Portable Profile336

Let's not immediately worry about why this is the exact list of target frameworks.

Supposing that the external API across all target frameworks is identical, but each individual target framework has some code that's unique to that framework, how do I properly write the unit tests that cover the code from all target frameworks?

I'm running Visual Studio 2017 Community.

I've cooked up a ready-to-go minimal sample on GitHub here with what I've found so far to be probably the best I can do right now: https://github.com/airbreather/MultiTargetDemo, but this doesn't feel like the right way to do it. Some flaws:

• Requires a separate .csproj file for each target framework with lots of copy-paste.
• Uses a not-well-documented* property called ReferringTargetFrameworkForProjectReferences.
  • *that's a bit of an understatement... Googling it at the moment, literally nothing shows up apart from Microsoft's SDK stuff.
• Doesn't seem to work properly with the .NET Standard shims (.NET Standard 1.3 here throws at runtime trying to look for System.IO.FileSystem). (edit: figured it out, this was corefx issue #16322, which has a bit of an awkward workaround).

The library in the above-linked has precisely one public class with precisely one public instance method on it. The method is contractually obligated to just return the 32-bit integer value 3, but it computes that 3 a materially different way on each target. Obviously the actual use case revolves around implementing much more "interesting" methods, but this is sufficient for conversation.

Furthermore, note that the test class in that library actually uses some stuff on its own that's not available on all target platforms: System.Threading.Tasks.Task<TResult> is not available on .NET Framework 2.0... this serves as a proxy for just anything that the test itself might be using for its own purposes, which might not necessarily be available in the target framework that's being tested.

The actual use case is NetTopologySuite, which already targets multiple full-framework platforms and some PCLs; there's a year-old issue asking to make it work with .NET Core. I've been working on-and-off to try to make that happen for just about a month now, and I'm finally stumbling into this issue.

I found that other question: Unit testing code targeting several frameworks at once with .NET Core and xUnit, which is indeed similar to this one. This is different for a few reasons:

• That question was using the VS2015-era preview SDK tooling (project.json). This question refers to the VS2017-era non-preview tooling with the pretty .csproj files and all.
• That question could be resolved by simply multi-targeting the test project itself with one "test" target framework per framework targeted by the "subject-under-test". It does not appear that this question can be resolved in a similar way, especially with the unusual Profile336 target.
  • Besides, given that the full .NET Framework 4.6 can reference assemblies targeted at any of the listed five target frameworks, there should be no need for me to have to limit the implementations of the fundamental test classes to just what's available in the intersection of whatever grab-bag of target frameworks that get selected for this.
  • I did try a hack like listing synthetic stuff in TargetFrameworks like test1;test2;test3;test4;test5 and then forcing TargetFrameworkIdentifier + TargetFrameworkVersion to .NETFramework + 4.6, but I didn't have much success (it failed when trying to grab the xunit package). I also didn't try too hard on this, however.

解决方案

First, test projects can multi-target as well by making use of the <TargetFrameworks>property just like you used for the library. While the VS test runner currently only shows/runs one framework (the first one to be specific), any invocation of dotnet test will execute all frameworks (xunit is also developing custom console runner - dotnet xunit - for a better console UX).

Also keep in mind that test projects need to specify a target framework that has a runtime. So your test projects can either target netcoreapp* or net* but never netstandard* since it does not have a runtime associated to run the tests. I'm not sure about the portable target since the Microsoft.NET.Tests.Sdk that ever test project needs to reference to get test runner support doesn't have conditions for NETPortable.

So you'd need to pick all the versions of net and netcoreapp that you need to test on / support - e.g. net20;net45;net46;netcoreapp1.0 would cover all your .net framework and .net core versions.

There is a way to force a dependency to a specific target framework by setting a Properties="TargetFramework=netstandard1.3" on the <ProjectReference> element, but this may be hard to get working correctly for restore and multi-targeting.

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