abcdefgh
abcdefg
xbcdefe
...
aaabcdefgh

string[] str = new string[]{ "gfdsgaaaa", "fds", "fdsgfsdfds", "fdsagfdserre", "rfvffgfdssss" };
var query = str.Where(w => w.Contains("fds"));
var result = from w in query where w.Length == query.Max(m => m.Length)
             select w;
foreach (var s in result)
    Console.WriteLine(s);

------解决方案--------------------

C# code

            string[] str = new string[] { "gfdsgaaaa", "fds", "fdsgfsdfds", "fdsagfdserre", "rfvffgfdssss" };
            var query = str.Where(w => w.Contains("fds"));
            var result = query.Where(w => w.Length == query.Max(m => m.Length));
            foreach (var s in result)
                Console.WriteLine(s);
            /** result print:
             * fdsagfdserre 
             * rfvffgfdssss
             * */