ResolveAssemblyReference

One of the most important tasks in the MSBuild toolset is ResolveAssemblyReference (RAR). Its purpose is to take all the references specified in .csproj files (or elsewhere) via the item and map them to paths to assembly files on disk. The compiler only can accept a .dll path on disk as a reference, so ResolveAssemblyReference converts strings like mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 to paths like C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.6.1\mscorlib.dll which are then passed to the compiler via the /r switch.

Additionally RAR determines a closure of all .dll/exe references recursively, and for each of them determines whether it should be copied to the build output directory or not. It doesn't do the actual copying (that is handled later, after the actual compile step), but it prepares an item list of files to copy.

RAR is invoked from the ResolveAssemblyReferences target: