ReadAllLines会删除所有换行符，但WriteAllLines会将它们全部收回！

而是试试这个：

ReadAllLines does remove all newlines, but WriteAllLines puts them all back!
Instead try this:

string inFile = Class1.filename;
string outFile = Class1.filename;
var contents = File.ReadAllLines(inFile);
Array.Sort(contents);      
File.WriteAllText(outFile, string.Concat(contents));