Nothing extra happens with creating a file in WCF so you can do that like this
string path = AppDomain.CurrentDomain.BaseDirectory;
String dir = Path.GetDirectoryName(path);
dir += "\\Emp_data";
string filename = dir+"\\Json_data.json";
if (!Directory.Exists(dir))
Directory.CreateDirectory(dir); // inside the if statement
FileStream fs = File.Open(filename,FileMode.OpenOrCreate, FileAccess.ReadWrite);
StreamReader reader = new StreamReader(fs);