Stack overflow with a loop c#

Question

I am currently having problems with this loop. It becomes an infinite loop and i get a stack overflow error. This is for a interest rate trade swap application. i is the length of the trade and l is the increasing index.

private void button1_Click(object sender, EventArgs e)
        {
            int outp = 0;
            int i = int.Parse(tradeLength.Text);
            string month = "January";
            for (int l = 1; l <= i; l++)
            {
                Console.WriteLine("I iterated " + l + " Amount of times");
                if (l == 1)
                {
                    month = "January";
                }
                if (l == 2)
                {
                    month = "February";
                }
                if (l == 3)
                {
                    month = "March";
                }
                if (l == 4)
                {
                    month = "Aprll";
                }
                if (l == 5)
                {
                    month = "May";
                }
                if (l == 6)
                {
                    month = "June";
                }
                if (l == 7)
                {
                    month = "July";
                }
                if (l == 8)
                {
                    month = "August";
                }
                if (l == 9)
                {
                    month = "September";
                }
                if (l == 10)
                {
                    month = "October";
                }
                if (l == 11)
                {
                    month = "November";
                }
                if (l == 12)
                {
                    month = "December";
                }
                else
                {
                    month = "Null";
                    l = 1;
                }

Answer 1

The cause is the final else:

    if (l == 12) {
      month = "December";
    }
    else { // <- if l != 12 (e.g. l == 1) restart the loop 
      month = "Null";
      l = 1;
    }

you want else if:

    if (l == 1)
    {
        month = "January";
    }
    else if (l == 2) 
    {
        ... 
    }
    ...
    else if (l == 12) 
    {
        ... 
    }
    else {
      month = "Null";
      l = 1;
    }

Edit: Another problem (see FKEinternet's comment) is a user input: if i is greater than 12 l never reaches it. You have to either validate the user input:

    int i = int.Parse(tradeLength.Text);

    if (i > 12)
      i = 12;    // or ask for other value

or use modular arithmetics:

    for (int index = 1; index <= i; index++) {
      int l = index % 12 + 1;

      if (l == 1)
      {
         month = "January";
      }
      else if (l == 2)
        ...
      else if (l == 12)
        ...
      else  
      {
        month = "Null";
        l = 1;
      }
    }