Sum of digits in C#

You could do it arithmetically, without using a string:

sum = 0;
while (n != 0) {
    sum += n % 10;
    n /= 10;
}

I use

int result = 17463.ToString().Sum(c => c - '0');

It uses only 1 line of code.

For integer numbers, Greg Hewgill has most of the answer, but forgets to account for the n < 0. The sum of the digits of -1234 should still be 10, not -10.

n = Math.Abs(n);
sum = 0;
while (n != 0) {
    sum += n % 10;
    n /= 10;
}

It the number is a floating point number, a different approach should be taken, and chaowman's solution will completely fail when it hits the decimal point.