Check the credit card is valid or not with ISO 2894 http://en.wikipedia.org/wiki/Luhn_algorithm

validate.c

#include <string.h>

// Sum Luhn
int GetSumLuhn(
  const char * number
)
{
	//
	int digits = 0;
	int i = strlen( number );
	char digitv = 0;

	//
	while( i-- )
	{
		//
		digitv = number - '0';
		digitv <<= !( i % 2 );//
		digitv -= 9 * ( digitv > 9 );
		//
		digits += digitv;
	}

	//
	return digits;
}

// CheckSum Luhn
bool CheckSumLuhn(
	const char * number
)
{
	return ( GetSumLuhn( number ) % 10 == 0 );
}

// Get Luhn Key
char GetKeyLuhn(
	const char * number
)
{
	return ( 10 - GetSumLuhn( number ) % 10 ) % 10;
}

//
bool CheckKeyLuhn(
	const char * number,
	const char key
)
{
	return ( GetKeyLuhn( number ) == ( key - '0' ) );
}

ValidateCard.m

-(BOOL) validateCardNumber:(NSString *)str
{
    BOOL result = NO;
    str = [self replaceString:str withString:@"" withPattern:@"[^\\d]"];
    if(str!=nil && str.length>0)
    {
        int num=0;
        int weight = (str.length % 2 == 0 )?2:1;
        int total=0;
        int tmp=0;
        for(int i =0;i<[str length];i++)
        {
            char c = [str characterAtIndex:i] ;
            num = (int)c -'0';
            tmp = num *weight;
            tmp = (tmp>9)?tmp-9:tmp;
            total += tmp;
            weight = (weight==2)?1:2;
        }
        result = (total%10)== 0;
    }
    return result;
}

-(NSString *) replaceString:(NSString *)srcStr withString:(NSString *)replaceStr withPattern:(NSString *)pattern
{
    NSMutableString *destStr = [NSMutableString stringWithString:srcStr];
    NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:pattern options:0 error:nil];
    [regex replaceMatchesInString:destStr options:0 range:NSMakeRange(0, [destStr length]) withTemplate:replaceStr];
    return destStr;
}

ValidateCardNumber.cs

 bool ValidateCard(string str)
        {
            bool validateResult = false;
            //过滤特殊字符，也可在输入的时候过滤
            str = Regex.Replace(str,"[^\\d]","");
            if (str.Length == 0)
                return false;
            int num;
            int weight = str.Length % 2 == 0 ? 2 : 1;
            int total = 0;
            int tmp;
            foreach (char ch in str)
            {
                num = (int)ch-'0';
                tmp =num * weight;
                tmp = (tmp > 9) ? tmp - 9 : tmp;
                total += tmp;
                weight = weight == 2 ? 1 : 2;
            }
            validateResult = (total%10) == 0;
            return validateResult;
        }
        
//Implemtation with linq
// http://www.codeproject.com/Tips/515367/Validate-credit-card-number-with-Mod-10-algorithm
public static bool Mod10Check(string creditCardNumber)
{
    //// check whether input string is null or empty
    if (string.IsNullOrEmpty(creditCardNumber))
    {
        return false;
    }

    //// 1. Starting with the check digit double the value of every other digit 
    //// 2.	If doubling of a number results in a two digits number, add up
    ///   the digits to get a single digit number. This will results in eight single digit numbers                    
    //// 3. Get the sum of the digits
    int sumOfDigits = creditCardNumber.Where((e) => e >= '0' && e <= '9')
                    .Reverse()
                    .Select((e, i) => ((int)e - 48) * (i % 2 == 0 ? 1 : 2))
                    .Sum((e) => e / 10 + e % 10);


    //// If the final sum is divisible by 10, then the credit card number
    //   is valid. If it is not divisible by 10, the number is invalid.            
    return sumOfDigits % 10 == 0;            
}