[excavator]

private int[] PopulateRandomNumbers(int arraylength, int minvalue, int maxvalue)
        {
            DateTime c = DateTime.Now;
            Random r = new Random(c.Millisecond);
            int[] RandomNumbers = new int[arraylength];
            int RandomNumber;

            for (int i = 0; i < arraylength; i++)
            {
                RandomNumber = r.Next(minvalue, maxvalue);
                while (isExistIn(RandomNumber, RandomNumbers) == true)
                {
                    RandomNumber = r.Next(minvalue, maxvalue);
                }
                RandomNumbers[i] = RandomNumber;                
            }
            return RandomNumbers;
        }

        private Boolean isExistIn(int number, int[] Numbers)
        {
            Boolean result = false;
            for (int i = 0; i < Numbers.Length; i++)
            {
                if (Numbers[i] == number)
                {
                    result = true;                    
                }
            }
            return result;
        }

to use the code

int[] RN = PopulateRandomNumbers(40, 1, 42);

there may be simpler steps but i hope this can help u a bit.