error with string index going below 0

hope explanation in comments enough give basic idea of intended purpose , error.

      public string[] tbchatold = new string[19]; // array store userinput      int = 0; // counter ofr event below.            private void tbchat_enter(object sender, keypresseventargs e)      {        //purpose of snip-it 1: display text tryped in textbox tbchat richtextbox windowchat.        //now wrote code following stores userinput empty index in array tbchatold[]        //using counter int declayed above. im hoping when = end of index starts storing        //at index 0 again. in giving user 20 stored strings thay can go to.        //i "think" part of code above explanation.        string userinput;        if(e.keychar == (char)keys.enter)        {                    userinput = tbchat.text;          windowchat.text += userinput + "\r\n";          if (i != 19)          {            tbchatold[i] = userinput;            i++;          }          else if (i == 19)          {            = 0;            tbchatold[i] = userinput;          }          tbchat.text = "";          tbchat.focus();        }      }      int idnext = 0;      private void tbchat_altq(object sender, keyeventargs e)      {        //my purpose here give user ability cycle threw previous input, intention once         //reached index of 0, loop around either last stored string or 19 depending if index        //had string stored in them.          //ok here im having problem. im no means decent programmer, , far cry beginner :)        //so may written wrong.        //the error im getting during debug "commented arrow" below keeps going -2, of course causing a        //outofrangeexception,          if (e.keycode == keys.up)        {          int idnext = --i;          if (idnext != 0)          {            if (tbchat.text == tbchatold[idnext]) // <-------------error-------------, far, more on down.            {              --idnext;              tbchat.text = tbchatold[idnext];              }            else if (tbchat.text != tbchatold[idnext])            {              tbchat.text = tbchatold[idnext];              --idnext;            }          }          else if (idnext == 0)          {            if (tbchat.text == tbchatold[idnext])            {              idnext = --i;              tbchat.text = tbchatold[idnext];            }            else if (tbchat.text != tbchatold[idnext])            {              tbchat.text = tbchatold[idnext];                idnext = --i;            }          }                  }        }  

when user input reaches 19, becomes zero. , in other function decrementing seems incorrect. instead of assigning idnext --i, assign total length.

  private void tbchat_altq(object sender, keyeventargs e)  {      int idnext = tbchatold.length - 1;      ...  }      

may helps.

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please mark post answer if solved problem. happy programming!

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