c# - Integer summing blues, short += short problem -

program in c#:

short a, b; = 10; b = 10; = + b; // error : cannot implicitly convert type 'int' 'short'.  // can write code using arithmetic assignment operator given below  += b; // running successfully, why?  console.write(a); 

there 2 questions here. first "why short plus short result in int?"

well, suppose short plus short short , see happens:

short[] prices = { 10000, 15000, 11000 }; short average = (prices[0] + prices[1] + prices[2]) / 3; 

and average is, of course, -9845 if calculation done in shorts. sum larger largest possible short, wraps around negative, , divide negative number.

in world integer arithmetic wraps around more sensible calculations in int, type have enough range typical calculations not overflow.

the second question is:

• short plus short int
• assigning int short illegal
• a +=b same = + b
• therefore short += short should illegal
• so why legal?

the question has incorrect premise; third line above wrong. c# specification states in section 7.17.2

otherwise, if selected operator predefined operator, if return type of selected operator explicitly convertible type of x, , if y implicitly convertible type of x or operator shift operator, operation evaluated x = (t)(x op y), t type of x, except x evaluated once.

the compiler inserts cast on behalf. correct reasoning is:

• short plus short int
• assigning int short illegal
• s1 += s2 same s1 = (short)(s1 + s2)
• therefore should legal

if did not insert cast impossible use compound assignment on many types.