Fastest way to check if string contains only digits

Question

I know a few ways how to check this. regex,int.parse,tryparse,looping.

can anyone tell me what is the fastest way to check?

the nee

Answer 1

Function with empty validation:

public static bool IsDigitsOnly(string str)
  {             
        return !string.IsNullOrEmpty(str) && str.All(char.IsDigit);
  }