System.Reflection.Assembly.GetExecutingAssembly().Location1
Combine that with System.IO.Path.GetDirectoryName if all you want is the directory.
1As per Mr.Mindor’s comment:
System.Reflection.Assembly.GetExecutingAssembly().Locationreturns where the executing assembly is currently located, which may or may not be where the assembly is located when not executing. In the case of shadow copying assemblies, you will get a path in a temp directory.System.Reflection.Assembly.GetExecutingAssembly().CodeBasewill return the ‘permanent’ path of the assembly.