validate-national-code.cs

// based on js version -> https://gist.github.com/dehghani-mehdi/df7f216d8031abad8c911b8117b7000e
public bool IsValidNationalCode(string value)
{
  // extract only numbers form the value
	value = new string(value?.Where(char.IsDigit).ToArray());
	  
	if (value.Length != 10 || Regex.IsMatch(value, @"(\d)(\1){9}")) return false;

	var sum = 0;
	var chars = value.ToCharArray();
	var lastDigit = 0;
	var remainder = 0;
	
	for (var i = 0; i < 9; i++) sum += int.Parse(chars[i].ToString()) * (10 - i);

	remainder = sum % 11;
	lastDigit = remainder < 2 ? remainder : 11 - remainder;

	return int.Parse(chars[9].ToString()) == lastDigit;
}

// usage
IsValidNationalCode("0797564411"); // -> true
IsValidNationalCode("0797564401"); // -> false