Right click
your project ->
Add Reference
-> select
Framework
tab ->
select System.Core.Dll
3.5 version... -> click
ok
if u r not able to locate the dll , you can download from this
system.core.dll[
^]
AppendAllLines is in framework 4.0, which is not available in v 3.5..
you can use WriteAllLines instead of it incase of v 3.5