Calculating an NMEA Checksum (C#)

NMEA_checksum

// Calculates the checksum for a sentence

// Calculates the checksum for a sentence
static string getChecksum(string sentence) {
        //Start with first Item
	int checksum= Convert.ToByte(sentence[sentence.IndexOf('$')+1]);
	// Loop through all chars to get a checksum
	for (int i=sentence.IndexOf('$')+2 ; i<sentence.IndexOf('*') ; i++){
		// No. XOR the checksum with this character's value
		checksum^=Convert.ToByte(sentence[i]);				
	}
	// Return the checksum formatted as a two-character hexadecimal
	return checksum.ToString("X2");
}

public bool IsValid(string sentence) 
{
    // Compare the characters after the asterisk to the calculation
    return sentence.Substring(sentence.IndexOf("*") + 1) = GetChecksum(sentence)
}

A gps checksum validator in C here:

#include <stdio.h>
#include <string.h>

int nmea0183_checksum(char *nmea_data)
{
    int crc = 0;
    int i;

    // the first $ sign and the last two bytes of original CRC + the * sign
    for (i = 1; i < strlen(nmea_data) - 3; i ++) {
        crc ^= nmea_data[i];
    }

    return crc;
}