Csc compiler error after upgrading to VS2015 Update 3 (order of import statements)

Description

After upgrading to VS2015 Update 3 I noticed that some WebAPI projects targeting framework v4.6.1 were coming up with the following error:

Build FAILED.

       "D:\Code\my-project\src\WebApiProject.Tests.Unit\WebApiProject.Tests.Unit.csproj" (Build target) (1) ->
       "D:\Code\my-project\src\WebApiProject\WebApiProject.csproj" (default target) (5) ->
       (CoreCompile target) ->
         C:\Program Files (x86)\MSBuild\14.0\bin\Microsoft.CSharp.Core.targets(76,11): error MSB4064: The "ChecksumAlgorithm" paramete
       r is not supported by the "Csc" task. Verify the parameter exists on the task, and it is a settable public instance property. [
       D:\Code\my-project\src\WebApiProject\WebApiProject.csproj]
         C:\Program Files (x86)\MSBuild\14.0\bin\Microsoft.CSharp.Core.targets(67,5): error MSB4063: The "Csc" task could not be initi
       alized with its input parameters.  [D:\Code\my-project\src\WebApiProject\WebApiProject.csproj]

Repro steps

Paket orders the import statements like so;

  <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
  <Choose>
    <When Condition="$(TargetFrameworkIdentifier) == '.NETFramework' And ($(TargetFrameworkVersion) == 'v4.6.1' Or $(TargetFrameworkVersion) == 'v4.6.2')">
      <PropertyGroup>
        <__paket__Microsoft_Net_Compilers_props>Microsoft.Net.Compilers</__paket__Microsoft_Net_Compilers_props>
      </PropertyGroup>
    </When>
  </Choose>
  <Import Project="..\packages\Microsoft.Net.Compilers\build\$(__paket__Microsoft_Net_Compilers_props).props" Condition="Exists('..\packages\Microsoft.Net.Compilers\build\$(__paket__Microsoft_Net_Compilers_props).props')" Label="Paket" />
  <Choose>
    <When Condition="$(TargetFrameworkIdentifier) == '.NETFramework' And ($(TargetFrameworkVersion) == 'v4.6.1' Or $(TargetFrameworkVersion) == 'v4.6.2')">
      <PropertyGroup>
        <__paket__Microsoft_CodeDom_Providers_DotNetCompilerPlatform_props>Microsoft.CodeDom.Providers.DotNetCompilerPlatform</__paket__Microsoft_CodeDom_Providers_DotNetCompilerPlatform_props>
      </PropertyGroup>
    </When>
  </Choose>
  <Import Project="..\packages\Microsoft.CodeDom.Providers.DotNetCompilerPlatform\build\$(__paket__Microsoft_CodeDom_Providers_DotNetCompilerPlatform_props).props" Condition="Exists('..\packages\Microsoft.CodeDom.Providers.DotNetCompilerPlatform\build\$(__paket__Microsoft_CodeDom_Providers_DotNetCompilerPlatform_props).props')" Label="Paket" />

Ordering them instead like this fixed the issue;

  <Choose>
    <When Condition="$(TargetFrameworkIdentifier) == '.NETFramework' And ($(TargetFrameworkVersion) == 'v4.6.1' Or $(TargetFrameworkVersion) == 'v4.6.2')">
      <PropertyGroup>
        <__paket__Microsoft_Net_Compilers_props>Microsoft.Net.Compilers</__paket__Microsoft_Net_Compilers_props>
      </PropertyGroup>
    </When>
  </Choose>
  <Import Project="..\packages\Microsoft.Net.Compilers\build\$(__paket__Microsoft_Net_Compilers_props).props" Condition="Exists('..\packages\Microsoft.Net.Compilers\build\$(__paket__Microsoft_Net_Compilers_props).props')" Label="Paket" />
  <Choose>
    <When Condition="$(TargetFrameworkIdentifier) == '.NETFramework' And ($(TargetFrameworkVersion) == 'v4.6.1' Or $(TargetFrameworkVersion) == 'v4.6.2')">
      <PropertyGroup>
        <__paket__Microsoft_CodeDom_Providers_DotNetCompilerPlatform_props>Microsoft.CodeDom.Providers.DotNetCompilerPlatform</__paket__Microsoft_CodeDom_Providers_DotNetCompilerPlatform_props>
      </PropertyGroup>
    </When>
  </Choose>
  <Import Project="..\packages\Microsoft.CodeDom.Providers.DotNetCompilerPlatform\build\$(__paket__Microsoft_CodeDom_Providers_DotNetCompilerPlatform_props).props" Condition="Exists('..\packages\Microsoft.CodeDom.Providers.DotNetCompilerPlatform\build\$(__paket__Microsoft_CodeDom_Providers_DotNetCompilerPlatform_props).props')" Label="Paket" />
  <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />

Expected behavior

Microsoft.CSharp.targets is below that of the Net.Compilers and CodeDom.Providers prop imports.

Actual behavior

Microsoft.CSharp.targets is above that of the Net.Compilers and CodeDom.Providers prop imports.

Known workarounds

Move them manually and correct after each Paket update.

Related information

Paket version 3.4.0.0

[hanskhe]

I am having the same issue. After updating to update 3 of VS 2015 there is a compiler error. Moving the line <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" /> as described fixes the compiler error, but breaks the loading of roslyn files to the \bin\roslyn\-directory, causing the application to be unable to load.

As part of our .csproj we have

<PropertyGroup>
    <PostBuildEvent>if not exist "$(WebProjectOutputDir)\bin\Roslyn" md "$(WebProjectOutputDir)\bin\Roslyn"  
    start /MIN xcopy /s /y /R "$(OutDir)roslyn\*.*" "$(WebProjectOutputDir)\bin\Roslyn"</PostBuildEvent>
</PropertyGroup>

to ensure roslyn files are loaded. This no longer seems to work when the above line is moved.

[ianbattersby]

@isaacabraham Possibly, this one was also fixed by uplifting the version of imports used (I think to 1.2 and something), if the change in that uplift is that described in #1887 I’d think it is the same.