Fastest way to check if string contains only digits

bool IsDigitsOnly(string str)
{
    foreach (char c in str)
    {
        if (c < '0' || c > '9')
            return false;
    }

    return true;
}

Will probably be the fastest way to do it.

Here's some benchmarks based on 1000000 parses of the same string:

Updated for release stats:

IsDigitsOnly: 384588
TryParse:     639583
Regex:        1329571

Here's the code, looks like IsDigitsOnly is faster:

class Program
{
    private static Regex regex = new Regex("^[0-9]+$", RegexOptions.Compiled);

    static void Main(string[] args)
    {
        Stopwatch watch = new Stopwatch();
        string test = int.MaxValue.ToString();
        int value;

        watch.Start();
        for(int i=0; i< 1000000; i++)
        {
            int.TryParse(test, out value);
        }
        watch.Stop();
        Console.WriteLine("TryParse: "+watch.ElapsedTicks);

        watch.Reset();
        watch.Start();
        for (int i = 0; i < 1000000; i++)
        {
            IsDigitsOnly(test);
        }
        watch.Stop();
        Console.WriteLine("IsDigitsOnly: " + watch.ElapsedTicks);

        watch.Reset();
        watch.Start();
        for (int i = 0; i < 1000000; i++)
        {
            regex.IsMatch(test);
        }
        watch.Stop();
        Console.WriteLine("Regex: " + watch.ElapsedTicks);

        Console.ReadLine();
    }

    static bool IsDigitsOnly(string str)
    {
        foreach (char c in str)
        {
            if (c < '0' || c > '9')
                return false;
        }

        return true;
    }
}

Of course it's worth noting that TryParse does allow leading/trailing whitespace as well as culture specific symbols. It's also limited on length of string.

You could simply do this using LINQ

return str.All(char.IsDigit);

1. .All returns true for empty strings and exception for null strings.
2. char.IsDigit is true for all Unicode characters.