For one, the "string" you're trying to iterate over is an int. You probably meant something along the lines of
string numbers = "1234512345"
After that, there are several ways to do this, my favorite personally is iterating over each character of the string, using a TryParse on it (this eliminates any issues if the string happens to be alphanumeric) and totaling the result. See below:
static void Main(string[] args) {
string numbers = "1234512345";
int total = 0;
int num; // out result
for (int i = 0; i < numbers.Length; i++) {
int.TryParse(numbers[i].ToString(), out num);
total += num; // will equal 30
}
Console.WriteLine(total);
total = 0;
string alphanumeric = "1@23451!23cf47c";
for (int i = 0; i < alphanumeric.Length; i++) {
int.TryParse(alphanumeric[i].ToString(), out num);
total += num; // will equal 32, non-numeric characters are ignored
}
Console.WriteLine(total);
Console.ReadLine();
}
Like others have posted though, there are several ways to go about this, it's about personal preference most of all.