static void Main(string[] args)
    {
      //1.Environment.CurrentDirectory
      Console.WriteLine(Environment.CurrentDirectory);
      //2.AppDomain.CurrentDomain.BaseDirectory
      Console.WriteLine(AppDomain.CurrentDomain.BaseDirectory);
      //3.項目的目錄
      Console.WriteLine(Path.GetFullPath("../../"));
      Console.ReadKey();
    }

private void Form1_Load(object sender, EventArgs e)
    {
      //1.Environment.CurrentDirectory
      txtBox1.Text = Environment.CurrentDirectory;
      //2.Application.StartupPath
      txtBox2.Text = Application.StartupPath;
      //3.Directory.GetCurrentDirectory()
      txtBox3.Text = Directory.GetCurrentDirectory();
      //4.AppDomain.CurrentDomain.BaseDirectory
      txtBox4.Text = AppDomain.CurrentDomain.BaseDirectory;
    }

//WebForm
 <h1><%=Server.MapPath("/") %></h1>
//Mvc
 <h1>@Server.MapPath("/")</h1>

public class ValuesController : ApiController
  {
    //api/values
    public string Get()
    {
      return System.Web.Hosting.HostingEnvironment.MapPath("/");
    }
   }