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

The second one should be

var cost1 = (int)(amount * 1000);

You have to multiply with 1000 and then convert the result. In your example you are converting first and then multiplying.

See Operator precedence and associativity