webform
var s = System.Diagnostics.Process.GetCurrentProcess().MainModule.FileName;  //C盘 IIS路径

var s1 = System.Environment.CurrentDirectory; //同上

var s2 = System.IO.Directory.GetCurrentDirectory();//同上

var s3 = System.AppDomain.CurrentDomain.BaseDirectory; //当前项目根目录

var s4 = System.AppDomain.CurrentDomain.SetupInformation.ApplicationBase; //同上

winform

System.Windows.Forms.Application.StartupPath

System.Windows.Forms.Application.ExecutablePath