c# - how to fix NullReferenceException WPF Text box -


i have case works after data has been loaded to combobox , textbox, when app first gets loaded , combobox = tobacco use? , trys set textbox empty "nullreferenceexception - object reference not set instance of object". not sure how fix , able keep clearing textbox when "tobacco use? " selected. "tobacco use?" default on combobox.

combobox cbtobacco.text

textbox = cbtobaccocode.text

private void cbtobacco_selectionchanged(object sender, selectionchangedeventargs e)  {    if (cbtobacco.text != null)    {      switch (convert.tostring(cbtobacco.text))       {         case "tobacco use?": strtobaccocode = ""; break;         case "1 - current every day smoker": strtobaccocode = "449868002"; break;         case "2 - current day smoker": strtobaccocode = "428041000124106"; break;         case "3 - former smoker": strtobaccocode = "8517006"; break;         case "4 - never smoker": strtobaccocode = "266919005"; break;         case "5 - smoker, current status unknown": strtobaccocode = "77176002"; break;         case "6 - unknown if ever smoked": strtobaccocode = "266927001"; break;         case "7 - heavy tobacco smoker": strtobaccocode = "428071000124103"; break;         case "8 - light tobacco smoker": strtobaccocode = "428061000124105"; break;       }   cbtobaccocode.text = strtobaccocode;   } }

are sure application entered on first case, , seted value of strtobaccocode? put breakpoint , check.

use "default" option make code little safer:

private void cbtobacco_selectionchanged(object sender, selectionchangedeventargs e) {     if (cbtobacco.text != null)     {         string strtobaccocode;         switch (convert.tostring(cbtobacco.text))         {             case "1 - current every day smoker": strtobaccocode = "449868002"; break;             case "2 - current day smoker": strtobaccocode = "428041000124106";     break;             case "3 - former smoker": strtobaccocode = "8517006"; break;             case "4 - never smoker": strtobaccocode = "266919005"; break;             case "5 - smoker, current status unknown": strtobaccocode = "77176002"; break;             case "6 - unknown if ever smoked": strtobaccocode = "266927001"; break;             case "7 - heavy tobacco smoker": strtobaccocode = "428071000124103"; break;             case "8 - light tobacco smoker": strtobaccocode = "428061000124105"; break;             default: strtobaccocode = ""; break;         }         cbtobaccocode.text = strtobaccocode;     } }

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