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Also: It contains some logic for globalization, which checks for Latin characters.
Info: The program defines the Main entry point and an input string. The string contains lowercase letters, punctuation, and digits.
And: In the output, IsDigit returns True only for the digits. It accurately detects digit chars.
True, FalseC# program that tests for digits
using System;
class Program
{
    static void Main()
    {
        string test = "Abc,123";
        foreach (char value in test)
        {
            bool digit = char.IsDigit(value);
            Console.Write(value);
            Console.Write(' ');
            Console.WriteLine(digit);
        }
    }
    /// <summary>
    /// Returns whether the char is a digit char.
    /// Taken from inside the char.IsDigit method.
    /// </summary>
    public static bool IsCharDigit(char c)
    {
        return ((c >= '0') && (c <= '9'));
    }
}
Output
A False
b False
c False
, False
1 True
2 True
3 True
Therefore: Using a custom IsCharDigit method is safer for simple tasks. The logic in IsCharDigit is taken directly from the main part of char.IsDigit.
Tip: You can extract the character-testing logic for more predictable behavior and slightly better performance.