Executable Path of the Application

string exeDirPath = System.IO.Path.GetDirectoryName(
          System.Reflection.Assembly.GetExecutingAssembly())

or for the full path of the executable:

string exePath = new Uri(Assembly.GetExecutingAssembly().GetName().CodeBase).LocalPath;

or just (only for Windows Forms application)
System.Windows.Forms.Application.ExecutablePath - Executable path of the application in which the assembly is housed:

string exePath = System.Windows.Forms.Application.ExecutablePath;

or

string[] args = Environment.GetCommandLineArgs();
string exePath = args[0];

To get the working directory where the executable lies (but this does not work for a service or so it seems):

string exePath = System.Windows.Forms.Application.StartupPath; 

or alternatively

string cwd = Environment.CurrentDirectory;

Here is the sample output of these for a test application. The application was started in the "D:\Documents" directory but located at "D:\Documents\Devel\Tools\IdeaTester\bin\x86\Debug\IdeaTester.exe"
First the list of methods used:

1.  System.Reflection.Assembly.GetExecutingAssembly().GetName().ToString()
2.  System.Reflection.Assembly.GetExecutingAssembly().GetName().CodeBase
3.  System.IO.Path.GetDirectoryName(System.Reflection.Assembly.GetExecutingAssem
bly().GetName().CodeBase)
4.  System.IO.Path.GetDirectoryName(new Uri(Assembly.GetExecutingAssembly().GetN
ame().CodeBase).LocalPath)
5.  System.IO.Path.GetDirectoryName(new Uri(Assembly.GetExecutingAssembly().GetN
ame().CodeBase).AbsolutePath)
6.  System.IO.Path.GetDirectoryName(System.Windows.Forms.Application.ExecutableP
ath)
7.  System.IO.Path.GetDirectoryName(args[0])
8.  System.Windows.Forms.Application.StartupPath
9.  Environment.CurrentDirectory
10. System.Windows.Forms.Application.ExecutablePath

Here is the output of each method:

1.  IdeaTester, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
2.  file:///D:/Documents/Devel/Tools/IdeaTester/bin/x86/Debug/IdeaTester.exe
3.  file:\D:\Documents\Devel\Tools\IdeaTester\bin\x86\Debug
4.  D:\Documents\Devel\Tools\IdeaTester\bin\x86\Debug
5.  D:\Documents\Devel\Tools\IdeaTester\bin\x86\Debug
6.  D:\Documents\Devel\Tools\IdeaTester\bin\x86\Debug
7.  D:\Documents\Devel\Tools\IdeaTester\bin\x86\Debug
8.  D:\Documents\Devel\Tools\IdeaTester\bin\x86\Debug
9.  D:\Documents
10. D:\Documents\Devel\Tools\IdeaTester\bin\x86\Debug\IdeaTester.exe