Dynamic group by linq

public string[][] GetAttributes(int attributelenght, string[] attributes)
        {
            string[][] result = new string[attributelenght][];

            string parm = "";
          
            foreach (var a in attributes)
            {
                for (int i = 0; i <= attributelenght;)
                {
                   
                       var varible = "c." + a; //c.Colour 

                        var feild = (from c in _auth.ClothingItems
                                     group c by varible into g // change each loop to match attibutes
                                     select g).ToList();

                        foreach (var f in feild)
                        {
                            parm = parm + "," + f.Key; // black , blue, yellow, pink, red

                        }
                }


            }


            return result;
        }

public string[][] GetAttributes(int attributelenght, string[] attributes)
        {
            string[][] result = new string[attributelenght][];

            string parm = "";
          
            foreach (var a in attributes)
            {
                for (int i = 0; i <= attributelenght;)
                {
                   
                       var varible = "c." + a; //c.Colour 

                        var feild = (from c in _auth.ClothingItems
                                     let var = varible
                                     group c by var into g // change each loop to match attibutes
                                     select g).ToList();

                        foreach (var f in feild)
                        {
                            parm = parm + "," + f.Key; // black , blue, yellow, pink, red

                        }
                }


            }


            return result;
        }