dotnet-installer/build_projects/dotnet-cli-build/DotNetRestore.cs

using Microsoft.Build.Framework;
using Microsoft.Build.Utilities;

namespace Microsoft.DotNet.Cli.Build
{
    public class DotNetRestore : DotNetTool
    {
        protected override string Command
        {
            get { return "restore"; }
        }

        protected override string Args
        {
            get { return $"{GetVerbosity()} {GetFallbackSource()}"; }
        }

        public string FallbackSource { get; set; }

        public string Verbosity { get; set; }

        private string GetFallbackSource()
        {
            if (!string.IsNullOrEmpty(FallbackSource))
            {
                return $"--fallbacksource {FallbackSource}";
            }

            return null;
        }

        private string GetVerbosity()
        {
            if (!string.IsNullOrEmpty(Verbosity))
            {
                return $"--verbosity {Verbosity}";
            }

            return null;
        }
    }
}