int wordFrequency = 10;
string words = "hello how r u u u u  u  u u  u  u u u  u u u u  u u u ? hello there u u u u ! great to c u there. hello .hello hello hello hello hello .hello hello hello hello hello hello ";            
 
var result = (from word in words.Split(new string[] { " " }, StringSplitOptions.RemoveEmptyEntries)
                          group word by word into g
                          select new { Word = g.Key, Occurance = g.Count() }).ToList().FindAll(i => i.Occurance >= wordFrequency);

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