Version 2.1.3 of the .NET Core SDK includes the file watcher. Here is how to use it from the command line...

with the --project option:

// right
dotnet watch --project path\to\project.csproj run

// wrong 
dotnet watch run --project path\to\project.csproj 

See also:

https://docs.microsoft.com/en-us/aspnet/core/tutorials/dotnet-watch?view=aspnetcore-2.1