Here you have a list of all the standard numeric formats. I think "N" is the one you want.
long l = 1234;
string s = l.ToString("N0"); //gives "1,234"
The "0" after the format specifier is the number of desired decimal places (usually 2 by default).
Note that this version is culture-sensitive, i.e., in my country, we use dots (".") as thousand separators, so the actual returned value will be "1.234" instead of the "1,234". If this is desired behaviour, just leave it as is, but if you need to use commas always, then you should specify a culture as a parameter to the ToString method, like
l.ToString("N0", CultureInfo.InvariantCulture); //always return "1,234"
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