c# - Calculation is wrong -

what should happen

my exexcrise prgramm calculator able calculations in 'queue'.

example: user enters first number in txtzahl , clicks 1 of button. needed calculation should saved in list numbers , txtzahl should cleared. user can enter new number , press button etc. when user clicks on btnequ foreach should take every calculation list , calculation. if done result should displayed in txtzahl.

problem

the calculations not correct. example 0.00 result 4-3.

i know idea class not best way, keep it, see teacher thinks it. thank helping!

code:

form1.cs

double ergebniss = 0; boolean firstrun = true; list<rechnung> numbers = new list<rechnung>(); rechnung.rechenart lastart;  private void btnminus_click(object sender, eventargs e) {     if (isvalid())     {         if (firstrun)         {             ergebniss = convert.todouble(txtzahl.text);         }          numbers.add(new rechnung(convert.todouble(txtzahl.text), rechnung.rechenart.subtraktion));         lastart = rechnung.rechenart.subtraktion;         clearandfocus();     } }  private void btnequ_click(object sender, eventargs e) {     foreach (rechnung r in numbers)     {         switch (r.getart())         {             case rechnung.rechenart.subtraktion:                 {                     ergebniss -= r.getnumber();                     break;                 }         }     }     txtzahl.text = ergebniss.tostring("f2"); } 

}

if (firstrun)         {             ergebniss = convert.todouble(txtzahl.text);             firstrun = false;             return;         } 

first forgot firstrun = false; after that

then advice display clean string

txtzahl.text = ergebniss.tostring(); 

you doesn't use lastart variable don't know if that's necessary.