Function CountDV - Practice Exercises C# Sharp

using System;
public class CDV
{
    public static void CountDV(string answer, ref int amountOfDigits, ref int amountOfVowels)
    {
        amountOfDigits = 0;
        amountOfVowels = 0;

        for (int i = 0; i < answer.Length; i++)
        {
            switch (answer.Substring(i, 1).ToLower())
            {
                case "a":
                case "e":
                case "i":
                case "o":
                case "u":
                    amountOfVowels++;
                    break;
                case "0":
                case "1":
                case "2":
                case "3":
                case "4":
                case "5":
                case "6":
                case "7":
                case "8":
                case "9":
                    amountOfDigits++;
                    break;
            }
        }
    }

    public static void Main()
    {
        int amountOfDigits = 0;
        int amountOfVowels = 0;

        CountDV("This", ref amountOfDigits, ref amountOfVowels);

        Console.WriteLine(amountOfDigits);
        Console.WriteLine(amountOfVowels);
    }
}