I would like to know where variables like`$(...)` are declared and how to define a custom one. 

I mean, when you open a C# project configuration file (`.csproj`) there are a lot of variable like: `$(SolutionDir)`, `$(MSBuildBinPath)` - where are these configured? 

If I want to create a custom one to use within a `.csproj` file to point to somewhere, how to do this?