Xamarin Studio doesn't want to use v14 C# 6 compiler

Question

I'm currently trying to build a C# 6 project in Xamarin Studio (6.1.9) on a Windows 10 x64 computer. I did some research and found some people saying that adding this to the .csproj file would fix the problem:

<PropertyGroup Condition="'$(OS)' != 'Unix'">
  <CscToolPath>$(MSBuildProgramFiles32)\MSBuild\14.0\Bin</CscToolPath>
</PropertyGroup>

But no matter what I do, csc keeps telling me that '6' isn't a correct value for parameter /langversion, because Xamarin Studio always use the 4.0 csc no matter what I specified in the .csproj file.

Answer 1

While writing this question, I thought about something. I went back in the .csproj file and noticed this at the beginning of the file:

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
    ...

Notice the ToolsVersion="4.0". I tried changing that into 14.0 and it worked instantly. The strange part is that my solution contained multiple projects that were using C# 6, and they were all giving errors. But changing the ToolsVersion for only the first project fixed all the errors (the other projects still have 4.0 instead of 14.0, but XS builds them correctly.