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     private bool isCipher1( int characterPosition)
     {
         int[] cipher1Chars = new int[] { 0, 2, 4, 6, 8, 10, 12, 14 };
         foreach ( int cipher1Char in cipher1Chars)
         {
             if (characterPosition == cipher1Char) return true;
         }

         return false;
     }

    private string CodiceControllo(string codice_fiscale)
    {
	    int somma = 0;
	    int resto = 0;

        int [] cipher1 = new int [] {1,0,5,7,9,13,15,17,19,21,2,4,18,20,11,3,6,8,12,14,16,
                                10,22,25,24,23,0,5,7,9,13,15,17,19,21};

        char [] chars_codice_fiscale = codice_fiscale.ToUpper().ToCharArray();

	    for (int characterPosition = 0; characterPosition < 15; characterPosition++)
        {
       
            if (isCipher1(characterPosition))
            {
                if (Char.IsNumber(chars_codice_fiscale[characterPosition]))
                {
                    somma += cipher1 [ Int32.Parse(chars_codice_fiscale[characterPosition]) + 26] ;
                    continue;
                }
                
                if (Char.IsLetter(chars_codice_fiscale[characterPosition]))
                {
                    somma += cipher1 [Convert.ToInt32(chars_codice_fiscale[characterPosition]) - 65];
                    continue;
                }

                throw new ArgumentException("Invalid character in argument " + codice_fiscale);
            }
            else
            {
                if (Char.IsNumber(chars_codice_fiscale[characterPosition]))
                {
                    somma += Int32.Parse(chars_codice_fiscale[characterPosition].ToString()) ;
                    continue;
                }
                
                if (Char.IsLetter(chars_codice_fiscale[characterPosition]))
                {
                    somma += Convert.ToInt32(chars_codice_fiscale[characterPosition]) - 65;
                    continue;
                }

                throw new ArgumentException("Invalid character in argument " + codice_fiscale);
        
            }
	
		    Math.DivRem(somma, 26, out resto); 
			return char.ConvertFromUtf32(resto+65);
	}

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// just for the switch

Dictionary<char,int> dict = new Dictionary<char,int>(26);
dict.Add('A',1);
dict.Add('B',0);
dict.Add('C',5);
dict.Add('D',7);
dict.Add('E',9);
dict.Add('F',13);
dict.Add('G',15);
dict.Add('H',17);
dict.Add('I',19);
dict.Add('J',21);
dict.Add('K',2);
dict.Add('L',4);
dict.Add('M',18);
dict.Add('N',20);
dict.Add('O',11);
dict.Add('P',3);
dict.Add('Q',6);
dict.Add('R',8);
dict.Add('S',12);
dict.Add('T',14);
dict.Add('U',16);
dict.Add('V',10);
dict.Add('W',22);
dict.Add('X',25);
dict.Add('Y',24);
dict.Add('Z',23);
dict.Add('0',1);
dict.Add('1',0);
dict.Add('2',5);
dict.Add('3',7);
dict.Add('4',9);
dict.Add('5',13);
dict.Add('6',15);
dict.Add('7',17);
dict.Add('8',19);
dict.Add('9',21);
// then instead of the big switch, use this:
somma += dict[codice_fiscale[i]];

using dictionary initializers

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Dictionary<char,int> dict = new Dictionary<char,int> {
  {'A', 1},
  {'B', 0},
  {'C', 5},
  //...
  {'8', 19},
  {'9', 21}
};

[cs]

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        private int TableIndex(char c)
        {
            if (Char.IsLetter(c))
            {
                return (int)Char.ToUpper(c) - (int)'A';
            }
            else if (Char.IsNumber(c))
            {
                return 26 + int.Parse(c.ToString());
            }
            throw new ArgumentException("Invalid character");
        }

        private string CodiceControllo2(string codice_fiscale)
        {
            int[] table_even = new int[] {1,0,5,7,9,13,15,17,19,21,2,4,18,20,11,3,6,8,12,14,16,10,22,25,24,23,1,0,5,7,9,13,15,17,19,21};
            int[] table_odd  = new int[] {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,0,1,2,3,4,5,6,7,8,9};

            int somma = 0;

            for (int i = 0; i < 15; i++)
            {
                char c = codice_fiscale[i];

                if (i % 2 == 0)
                {
                    somma += table_even[TableIndex(c)];
                }
                else
                {
                    somma += table_odd[TableIndex(c)];
                }
            }

            int resto = somma % 26;
            return char.ConvertFromUtf32(resto + 65);
        }