static void Main(string[] args)
    {

        Console.WriteLine(Sum(5));
        Console.Read();
    }


    static int Sum(int value)
    {
        if (value > 0)
        {
          return value + Sum(value - 1);
        }
        else
        {
            return 1;
        }
    }

else
{
    return 0;
}

Sum --> 5
  Sum --> 4
    Sum --> 3
      Sum --> 2
        Sum --> 1
          Sum --> 0
          1 <---
        2 <---
      4 <---
    7 <---
  11 <---
16 <---

static int Sum(int result, int value)
{
    if(value == 0)
        return result;

    Sum(result + value, value - 1);
}

static int Sum(int value)
{
    if (value > 0)
    {
        return value + Sum(value - 1);
    }
    else
    {
        return 0; //Change this.
    }
}

static unsigned int Sum(unsigned int value) {
    if (value == 0)
        return 0;
    return value + Sum(value - 1);
}