var box = new List<Class1>();
  var bag = from p in box
                                 group p by new { p.aa, p.bb } into Group
                                 select new
                                 {
                                     aa = Group.Key,
                                     bb = Group.Key,
                                     cc = Group.Aggregate((a, b) => new { aa = a.a, bb = a.b, cc = (a.cc + ";" + b.cc) }).cc
                                 };

 .Select (g=>new {A=g.Key.A,B=g.Key.B,C=string.Join(";",g.Select(z=> z.C)) });

 class Program
    {
        public class Class1
        {
           public string A { get; set; }
           public string B { get; set; }
           public string C { get; set; }
        }

        static void Main(string[] args)
        {
            
            var list = new List<Class1>(new[] 
                { 
                    new Class1() { A= "7", B = "8", C= "1"},
                    new Class1() { A= "7", B = "8", C= "2"},
                    new Class1() { A= "7", B = "8", C= "3"},
                    new Class1() { A= "7", B = "9", C= "1"},
                    new Class1() { A= "8", B = "9", C= "1"}
                });

            var result = list.GroupBy(c1 => new { A = c1.A, B = c1.B })
                .Select(g => new { aa = g.Key.A, bb = g.Key.B, cc = string.Join(";", g.Select(c => c.C)) });

            foreach (var g in result)
            {
                Console.WriteLine("{0} {1} {2}", g.aa, g.bb, g.cc);
            }
        }
    }