为什么 .NET Standard 2 构建引用许多程序集而不是单个 netstandard.dll 程序集

问题描述

与 .NET Standard 1 不同，.NET Standard 2 引入了单个 netstandard.dll 参考程序集.

Unlike .NET Standard 1, .NET Standard 2 introduces a single netstandard.dll reference assembly.

但是，当使用 dotnet build 编译(使用 .NET Core SDK 2.2.203)时，我们可以看到它添加了对许多程序集(113)的引用，都在 netstandard.library/2.0.3/build/netstandard2.0/ref/*.dll.这些引用之一是 netstandard.dll.

However, when compiling (with .NET Core SDK 2.2.203) with dotnet build we can see that it adds references to many assemblies (113), all under netstandard.library/2.0.3/build/netstandard2.0/ref/*.dll. One of these references is netstandard.dll.

$ grep TargetFramework *.csproj
<TargetFramework>netstandard2.0</TargetFramework>
$ dotnet build --verbosity normal | grep -o netstandard.library/2.0.3/build/netstandard2.0/ref | wc -l
113

生成的程序集确实包含对 netstandard 的单个引用:

The resulting assembly does contain a single reference to netstandard:

$ ikdasm test.exe | grep extern
.assembly extern netstandard

问题是为什么 netstandard.library NuGet 包包含所有这些其他程序集，以及为什么在构建期间使用它们，而不是引用单个 netstandard.dll.

The question is why netstandard.library NuGet package contains all these other assemblies, and why are they used during build, instead of referencing a single netstandard.dll.

推荐答案

这是历史造成的遗憾.

您说得对，理论上只需要 netstandard.dll 即可构建 .NET Standard 库.

You are right that in theory only netstandard.dll is needed to build a .NET Standard library.

然而，有两件事使这变得复杂:

However, two things complicate this:

1. .NET Standard 1.0-1.6 库使用了更多的 NuGet 包和程序集.因此，.NET Standard 1.6 库(.dll 文件)可能包含对例如的引用System.Collections.Generic.dll 可解析.此外，构建系统需要知道 1.6 库期望在此 dll 中的类实际上与 netstandard.dll 中的类相同.
2. 允许从 .NET Standard(和 .NET Core)库使用现有的 .NET Framework 库，而无需重新编译.这意味着它们也是为 .NET Framework 使用的 .dll 文件构建的 - 从 mscorlib.dll 到单个框架 dll.

1. .NET Standard 1.0-1.6 libraries used a lot more NuGet packages and thus assemblies. So a .NET Standard 1.6 library (.dll file) may contain references to e.g. System.Collections.Generic.dll to be resolvable. Also, the build system needs to know that classes that the 1.6 library expects in this dll is actually the same as the one in netstandard.dll.
2. Existing .NET Framework libraries are allowed to be consumed from .NET Standard (and .NET Core) libraries without needing to be recompiled. This means that they also have been built for the .dll files that .NET Framework uses - from mscorlib.dll to the individual framework dlls.

对于这两种情况，添加了对具有这些名称(和版本)的程序集的引用，这些程序集仅包含类型前向声明.因此，例如 mscorlib.dll 将包含一个空的 System.Object 条目，该条目标记为在 netstandard.dll 中查找".

For both of these, references to assemblies with these names (and versions) are added which contain only type forward declarations. So for example mscorlib.dll would contain an empty System.Object entry that is marked with "go look for that in netstandard.dll".

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