get number generate from two number

i want mathematical equation or function to get number between tow number .. for example the two number is 5 and 10 .. when i send 5 make process and return number ex:7 this result will not be repeated .. and when send 8 return ex:10

void main()
{
    int from=5,to=10;
    for(int a=from;a<=to;a++)
    {
        int result=gen(from,to,a);
    }
}

int gen(int from, int to,int value)
{
    //make process with 3 args and return value random and will not be repeated
    return result;
}

I use many methods but all of them are stored in memory and I don't want it . like this

void not_good()
{
    var ran = Enumerable.Range(from, to).OrderBy(x => Guid.NewGuid());
    foreach (int i in ran)
    {
    }
    /////////////////
    List<int> listNumbers = new List<int>();
    int number;
    for (int i = from; i < to; i++)
    {
      do {
         number = rand.Next(from, to);
      } while (listNumbers.Contains(number));
      listNumbers.Add(number);
    }
}