in C# (int) decimal and Convert.Int32(decimal) give me two different results

Question

I understand that there are rounding errors but can anyone explain why I get such different results using these different methods:

decimal amount = 9.990M;
var cost = Convert.ToInt32(amount*1000);
var cost1 = (int) amount*1000;

I get:

cost = 9990
cost1 = 9000

Answer 1

Try (int)(amount*1000). In the Convert, the brackets enforce the precedence, but cast (int) takes precedence over the multiply - so you current have: ((int)amount)*1000, which rounds (during the cast) to 9.

In particular, see "7.2.1 Operator precedence and associativity" in the MS spec, which defines cast ahead of multiplication:

• 7.5: Primary: x.y f(x) a[x] x++ x-- new typeof default checked unchecked delegate
• 7.6: Unary: + - ! ~ ++x --x (T)x
• 7.7: Multiplicative: * / %
• etc