That code doesn't do any replacement, with comma or with full stop.
And to be honest, the choice of characters is a bit poor - comma and full stop will both naturally occur in lines of text, particularly if it's a message designed to be presented to a user - you should consider using less likely characters such as '|' and '¬' perhaps.
But ... assuming that you will be adding the line terminator in your
foreach
loop, the simplest way to do this is to change it to a
for
loop:
for (int i = 0; i < app.Count(); i++)
{
var item = app[i];
...
result += i == (app.Count() -1) ? "." : ",";
result += Environment.NewLine + Environment.NewLine;
}