Problem building SDK-style .NET Framework projects

[daveaglick]

Build started 10/24/2017 4:03:49 PM.
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Project "E:\Code\Wyam.Web\release\repo\src\core\Wyam.Common\Wyam.Common.csproj" (Clean target(s)):


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

Time Elapsed 00:00:00.87
Build started 10/24/2017 4:03:50 PM.
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Project "E:\Code\Wyam.Web\release\repo\src\core\Wyam.Common\Wyam.Common.csproj" (Compile target(s)):

Target ImplicitlyExpandNETStandardFacades:
  C:\Program Files\dotnet\sdk\2.0.2\Microsoft\Microsoft.NET.Build.Extensions\Microsoft.NET.Build.Extensions.NETFramework.targets(48,5): error MSB4062: The "GetDependsOnNETStandard" task could not be loaded from the assembly C:\Program Files\dotnet\sdk\2.0.2\Microsoft\Microsoft.NET.Build.Extensions\\tools\net46\Microsoft.NET.Build.Extensions.Tasks.dll. Could not load file or assembly 'file:///C:\Program Files\dotnet\sdk\2.0.2\Microsoft\Microsoft.NET.Build.Extensions\tools\net46\Microsoft.NET.Build.Extensions.Tasks.dll' or one of its dependencies. The system cannot find the file specified. Confirm that the <UsingTask> declaration is correct, that the assembly and all its dependencies are available, and that the task contains a public class that implements Microsoft.Build.Framework.ITask.
Done building target "ImplicitlyExpandNETStandardFacades" in project "Wyam.Common.csproj" -- FAILED.

Done building project "Wyam.Common.csproj" -- FAILED.

Build FAILED.

C:\Program Files\dotnet\sdk\2.0.2\Microsoft\Microsoft.NET.Build.Extensions\Microsoft.NET.Build.Extensions.NETFramework.targets(48,5): error MSB4062: The "GetDependsOnNETStandard" task could not be loaded from the assembly C:\Program Files\dotnet\sdk\2.0.2\Microsoft\Microsoft.NET.Build.Extensions\\tools\net46\Microsoft.NET.Build.Extensions.Tasks.dll. Could not load file or assembly 'file:///C:\Program Files\dotnet\sdk\2.0.2\Microsoft\Microsoft.NET.Build.Extensions\tools\net46\Microsoft.NET.Build.Extensions.Tasks.dll' or one of its dependencies. The system cannot find the file specified. Confirm that the <UsingTask> declaration is correct, that the assembly and all its dependencies are available, and that the task contains a public class that implements Microsoft.Build.Framework.ITask.
    0 Warning(s)
    1 Error(s)

Time Elapsed 00:00:00.16

[daveaglick]

It looks like the target checks the "$(MSBuildThisFileDirectory)\tools\net46" folder for Microsoft.NET.Build.Extensions.Tasks.dll:

https://github.com/dotnet/sdk/blob/cd25d6bbfc2aac61cc790c511900427ac64fdaa8/src/Tasks/Microsoft.NET.Build.Extensions.Tasks/msbuildExtensions/Microsoft/Microsoft.NET.Build.Extensions/Microsoft.NET.Build.Extensions.targets#L21

This folder exists at "C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\MSBuild\Sdks\Microsoft.NET.Sdk\tools\net46" when Visual Studio is installed, but not at "C:\Program Files\dotnet\sdk\2.0.2\Microsoft\Microsoft.NET.Build.Extensions\tools" (the latter only has a "netcoreapp1.0" subfolder). However, we set the MSBuild extensions folder to the dotnet SDK path if we detect an SDK project.

I guess it makes sense that .NET Framework projects should be using the VS build chain. It is called the .NET Core SDK after all. Suspect we'll need to check if the project targets .NET Framework and use the FrameworkPathHelper if so.

[daveaglick]

Interesting - it looks like the behavior is also different between an SDK project that targets net46 (which works) and one that targets net462 (which doesn't).

[dsplaisted]

MicrosoftMicrosoft.NET.Build.Extensions is used to support .NET Standard 2.0 when targeting .NET 4.6.1 and up. So it's not used if you're targeting .NET 4.6, because that doesn't support .NET Standard 2.0.

You should pretty much never use the targets in C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\MSBuild\Sdks\Microsoft.NET.Sdk. Those targets correspond to the 1.x version of the SDK, so they won't work if you're trying to target .NET Standard 2.0 or .NET Core 2.0 or higher, and also don't have any other updates that came in the 2.0 SDK or later. You should use the versions from C:\Program Files\dotnet\sdk\, even if the project targets .NET Framework.

However, I don't know if you need to explicitly point MSBuild to those targets. MSBuild has a built-in Sdk import resolver which will find those targets. I don't know if it will be automatically used when you are just using the MSBuild APIs. It is installed under C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\MSBuild\15.0\Bin\SdkResolvers, so you may want to point MSBuild at that resolver instead of pointing it directly at the SDK props and targets files (that way it will respect global.json and any other logic that is normally used to find those files).

[daveaglick]

@dsplaisted Thanks a bunch, that does help. Especially the bit about a custom resolver. After popping it open in a decompiler I can see how the VS toolchain finds the SDK.

Based on what you said about the targets in the VS Sdks directory, I'll use the VS toolchain for building .NET Framework SDK projects but set MSBuildSDKsPath to the .NET Core SDK. Hopefully that'll give the right combination of targets for these types of projects.