c# - Converting between negative Hexadecimal and negative Decimal gives wrong results -

i attempting manually convert numbers between decimal , hexadecimal. have working positive numbers , converting negative decimal 'negative' hexadecimal can't convert 'negative' hexadecimal negative decimal.

here code attempting work with:

private string hextodecimal(char[] toconvert)     {         if (negativevalue)         {             negativevalue = false;              long var = convert.toint64(hextodecimal(resultlabel.text.tochararray()));              long valuetohex = var - (long)math.pow(16, 15);              return resultlabel.text = valuetohex.tostring();         }         else         {             double total = 0;             //convert hex decimal              hexordecimallabel.text = "decimal";              //todo: create int array indivial int             char[] chararray = toconvert;             long[] numberarray = hexswitchfunction(chararray);              //todo: reverse array             array.reverse(numberarray);              //loop array, times value 16^i++, adding total. method   used convert hex decimal             double power = 0;             foreach (int in numberarray)             {                 total += (i * (math.pow(16, power)));                 power++;             }              //set result label total              ishex = false;             allowhexbuttons();              return resultlabel.text = total.tostring();         }     } 

for instance, can turn - 10 fffffffffffffff6, when attempt turn decimal, 1.15292150460685e+18, can't equations with.

does know of way around this?

this because double uses different representation negative numbers. changing type of total , power double long fix problem.