Роман

using System;
using System.IO;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;

открыл онлайн компилятор, действительно ошибка

mymail@mail.com

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

</Project>

using System.Collections.ObjectModel;
using System.Globalization;

namespace VowelsCount;

public class Program
{
    static void Main(string[] args)
    {
        var letters = new List<Letter> {
            new ('а', LetterType.Vowel),
            new ('б', LetterType.Consonant),
            new ('в', LetterType.Consonant),
            new ('г', LetterType.Consonant),
            new ('д', LetterType.Consonant),
            new ('е', LetterType.Vowel),
            new ('ё', LetterType.Vowel),
            new ('ж', LetterType.Consonant),
            new ('з', LetterType.Consonant),
            new ('и', LetterType.Vowel),
            new ('й', LetterType.Consonant),
            new ('к', LetterType.Consonant),
            new ('л', LetterType.Consonant),
            new ('м', LetterType.Consonant),
            new ('н', LetterType.Consonant),
            new ('о', LetterType.Vowel),
            new ('п', LetterType.Consonant),
            new ('р', LetterType.Consonant),
            new ('с', LetterType.Consonant),
            new ('т', LetterType.Consonant),
            new ('у', LetterType.Vowel),
            new ('ф', LetterType.Consonant),
            new ('х', LetterType.Consonant),
            new ('ц', LetterType.Consonant),
            new ('ч', LetterType.Consonant),
            new ('ш', LetterType.Consonant),
            new ('щ', LetterType.Consonant),
            new ('ъ', LetterType.Consonant),
            new ('ы', LetterType.Vowel),
            new ('ь', LetterType.Consonant),
            new ('э', LetterType.Vowel),
            new ('ю', LetterType.Vowel),
            new ('я', LetterType.Vowel)
        };

        var alphabet = new Alphabet(letters);
        IReadOnlySet<char> vowels = alphabet.GetVowelSet();
        IReadOnlySet<char> consonants = alphabet.GetConsonantSet();

        const string text = "Да мы хотим помочь!!!";

        int vowelCounter = 0;
        int consonantCounter = 0;

        var vowelResult = new char[text.Length];
        var consonantResult = new char[text.Length];
        var otherSymbolsResult = new char[text.Length];

        var ruRu = new CultureInfo("ru-RU");

        for (int i = 0; i < text.Length; i++)
        {
            vowelResult[i] = '_';
            consonantResult[i] = '_';
            otherSymbolsResult[i] = '_';

            char lowerCaseChar = char.ToLower(text[i], ruRu);
            if (consonants.Contains(lowerCaseChar))
            {
                ++consonantCounter;
                consonantResult[i] = text[i];
            }
            else if (vowels.Contains(lowerCaseChar))
            {
                ++vowelCounter;
                vowelResult[i] = text[i];
            }
            else
            {
                otherSymbolsResult[i] = text[i];
            }
        }

        Console.WriteLine("Исходная строка для подсчёта:");
        Console.WriteLine(text);
        Console.WriteLine("=========");
        Console.WriteLine(vowelResult);
        Console.WriteLine(consonantResult);
        Console.WriteLine(otherSymbolsResult);
        Console.WriteLine("=========");
        Console.WriteLine("Кол-во символов в исходной строке: " + text.Length);
        Console.WriteLine("Кол-во гласных: " + vowelCounter);
        Console.WriteLine("Кол-во согласных: " + consonantCounter);
        Console.WriteLine("Кол-во прочих символов: " +
            (text.Length - (vowelCounter + consonantCounter)));
    }
}

/// <summary>Тип буквы - гласная или согласная.</summary>
internal enum LetterType
{
    /// <summary>Гласная.</summary>
    Vowel,

    /// <summary>Согласная.</summary>
    Consonant
}

internal record struct Letter(char letter, LetterType type)
{
    public LetterType Type { get; init; } = type;

    public char Value { get; init; } = letter;
}

internal class Alphabet
{
    public IReadOnlyList<Letter> Letters { get; }

    public Alphabet(IEnumerable<Letter> letters)
    {
        Letters = letters switch
        {
            IList<Letter> list => new ReadOnlyCollection<Letter>(list),
            _ => new ReadOnlyCollection<Letter>(letters.ToList()),
        };
    }

    public IReadOnlyList<char> GetVowelList()
    {
        return Letters
            .Where(letter => letter.Type == LetterType.Vowel)
            .Select(letter => letter.Value)
            .ToList()
            .AsReadOnly();
    }

    public IReadOnlySet<char> GetVowelSet()
    {
        return Letters
            .Where(letter => letter.Type == LetterType.Vowel)
            .Select(letter => letter.Value)
            .ToHashSet();
    }

    public IReadOnlyList<char> GetConsonantList()
    {
        return Letters
            .Where(letter => letter.Type == LetterType.Consonant)
            .Select(letter => letter.Value)
            .ToList()
            .AsReadOnly();
    }

    public IReadOnlySet<char> GetConsonantSet()
    {
        return Letters
            .Where(letter => letter.Type == LetterType.Consonant)
            .Select(letter => letter.Value)
            .ToHashSet();
    }
}