Why does Decimal.Divide(int, int) work, but not (int / int)?

int is an integer type; dividing two ints performs an integer division, i.e. the fractional part is truncated since it can’t be stored in the result type (also int!). Decimal, by contrast, has got a fractional part. By invoking Decimal.Divide, your int arguments get implicitly converted to Decimals.

You can enforce non-integer division on int arguments by explicitly casting at least one of the arguments to a floating-point type, e.g.:

int a = 42;
int b = 23;
double result = (double)a / b;