Filtering return on serial port - visual-studio

Question

I have a CO2 sensor on my Arduino Mega and sometimes randomly when I'm reading the CO2 measurement, the sensor will return a "?". The question mark causes my program to crash and return "input string was not in a correct format".
I haven't tried anything because I don't know what approach would be the best for this. The CO2 sensor returns the measurement in the form of "Z 00000" but when this question mark appears it shows that all that returned was a "\n". Currently, I have the program just reading the 5 digits after the Z.
if (returnString != "")
{
val = Convert.ToDouble(returnString.Substring(returnString.LastIndexOf('Z')+ 1));
}
What I expect to return is the digits after Z which works but every so often I will get a random line return which crashes everything.

Answer 1

According to the C# documentation the ToDouble method throws FormatException whenever the input string is invalid. You should catch the exception to avoid further issues.
try {
val = Convert.ToDouble(returnString.Substring(returnString.LastIndexOf('Z')+ 1));
}
catch(FormatException e) {
//If you want to do anything in case of an error
//Otherwise you can leave it blank
}
Also I'd recommend using some sort of statemachine for parsing the data in your case, that could discard all invalid characters. Something like this:
bool z_received = false;
int digits = 0;
int value = 0;
//Called whenever you receive a byte from the serial port
void onCharacter(char input) {
if(input == 'Z') {
z_received = true;
}
else if(z_received && input <= '9' && input >= '0') {
value *= 10;
value += (input - '0');
digits++;
if(digits == 5) {
onData(value);
value = 0;
z_received = false;
digits = 0;
}
}
else {
value = 0;
z_received = false;
digits = 0;
}
}
void onData(int data) {
//do something with the data
}
This is just a mock-up, should work in your case if you can direct the COM port's byte stream into the onCharacter function.