MSBuild usage

Supporting different MSBuilds

This repo must support the ability to build with a number of different MSBuild configurations:

• MSBuild via Visual Studio: this happens when developers open Roslyn.slnx in Visual Studio and execute the build action. This uses desktop MSBuild to drive the solution.
• MSBuild via CLI: the cross platform portions of the repo are built via the CLI. This is a subset of the code contained in Roslyn.slnx.
• MSBuild xcopy: an xcopyable version of MSBuild that is used to run many of our Jenkins legs. It allows Roslyn to build and run tests on a completely fresh Windows image (no pre-reqs). The xcopy-msbuild project is responsible for building this image.
• BuildTools: this is a collection of tools produced by dotnet/buildtools which build a number of dotnet repos.

This places a small burden on our repo to keep our build props / targets files simple to avoid any odd conflicts. This is rarely an issue at this point.

Picking MSBuild

Given our repo supports multiple MSBuild versions, it must pick one to use when building, restoring, etc ... The preference list is as follows:

1. Developer command prompt: when invoked from inside a developer command prompt the associated MSBuild will be used.
2. Machine MSBuild: when invoked on a machine with MSBuild 15.0 installed, the first mentioned instance will be used.
3. XCopy MSBuild: fallback when no other option is available