# Dutch social security number check

For a Dutch customer I currently work for, I dealed with an incorrect social security number check (in Dutch: elfproef voor BSN’s/burgerservicenummers). After checking out the Internet for the correct definition of Dutch social security numbers (e.g. Burgerservicenummer – Wikipedia), I created the next check in C#:

```string cleanBsnNr = bsnNr.Trim().Replace(".", "");

// A BSN consists of 9 characters ...
if(cleanBsnNr.Length != 9) return false;

// ... all being numeric and not resulting in a 0 when converted to a number ...
long l;
if(!long.TryParse(cleanBsnNr, out l)) return false;
else if(l == 0) return false;

// ... the number must be validatable to the so-called 11-proof ...
long total = 0;
for(int i = 1; i <= 9; i++)
{
// 11-proof voor BSN's: (9*A + 8*B + 7*C + 6*D + 5*E + 4*F + 3*G + 2*H + (-1*I)) % 11 == 0
int number = Convert.ToInt32(cleanBsnNr[i - 1].ToString());
int multiplier = 10 - i;
if(i == 9) multiplier = -1 * multiplier;

total += number * multiplier;
}

// ... not result in a 0 when dividing by 11 ...
if(total == 0) return false;

// ... and not have a modulo when dividing by 11.