It is quite confusing (sorry) but the format of the project files won't change the command line interaction -- you'll still do
dotnet build
dotnet run dotnet build
dotnet run $ dotnet build
Couldn't find 'project.json' in current directory
$ xbuild
XBuild Engine Version 14.0
Mono, Version 4.6.1.0
Copyright (C) 2005-2013 Various Mono authors
Build started 17.11.2016 18:08:11.
...
Build succeeded.
the .net core sdk (tooling, the cli command line) is 1.0.0-preview2 and use `project.json`
your xbuild is from mono installed on your system, not .net core
if you download and use the .net core sdk `1.0.0-preview3` you will see the msbuild based csproj in preview3 (run `dotnet new`). msbuild based mean `dotnet build` is calling the msbuild library with `/t:Build` argument.
To explicity run msbuild, you can do `dotnet msbuild`