Прикрутить API Yandex словаря в мой готовый переводчик

1) Создадим два пустых класса YandexAnswer и YandexServices.

2) Создадим тестовый проект и напишем пару тестов

[TestClass()]
public class YandexServicesTests
{
    [TestMethod()]
    public async Task GetTranslateAnswerAsyncTest()
    {
        var ru = "семь раз отмерь один раз отрежь";
        var eng = "measure twice cut once\r\n";
        YandexServices sut = new YandexServices();
        YandexServices.TranslationDirection direction
            = YandexServices.TranslationDirection.RuEng;

        YandexAnswer answer = await sut.GetTranslateAnswerAsync(ru, direction);

        Assert.AreEqual("OK", answer.Code);
        Assert.AreEqual(eng, answer.Text);
    }

    [TestMethod()]
    public async Task GetDictionaryAnswerAsyncTest()
    {
        var ru = "мысль";
        var eng = "thought";
        YandexServices sut = new YandexServices();
        YandexServices.TranslationDirection direction
            = YandexServices.TranslationDirection.RuEng;

        YandexAnswer answer = await sut.GetDictionaryAnswerAsync(ru, direction);

        Assert.AreEqual("OK", answer.Code);
        Assert.IsTrue(answer.Text.Contains(eng));
    }
}

3) Поработаем немного над тем, чтоб эти тесты стали зелеными и получим в итоге такие классы

public class YandexAnswer
{
    private Dictionary _dictionaryAnswer;
    private Translator _translatorAnswer;

    public string Code { get; set; }
    public string Lang { get; set; }
    public string Text { get; set; }
    public Dictionary DictionaryAnswer
    {
        get => _dictionaryAnswer;
        set
        {
            _dictionaryAnswer = value;
            if (_dictionaryAnswer != null)
            {
                SetTextFromDictionaryAnswer();
            }
        }
    }

    public Translator TranslatorAnswer
    {
        get => _translatorAnswer;
        set
        {
            _translatorAnswer = value;
            if (_translatorAnswer != null)
            {
                SetTextFromTranslatorAnswer();
            }
        }
    }

    private void SetTextFromTranslatorAnswer()
    {
        var sb = new StringBuilder();
        _translatorAnswer.text?.ForEach(t => sb.AppendLine(t));
        Text = sb.ToString();
    }

    private void SetTextFromDictionaryAnswer()
    {
        if (_dictionaryAnswer.def == null)
        {
            return;
        }

        var sb = new StringBuilder();
        foreach (var def in _dictionaryAnswer.def)
        {
            int counter = 0;
            foreach (var tr in def.tr)
            {
                sb.AppendLine($"{++counter}) {tr.text}");
            }
        }
        Text = sb.ToString();
    }
}

public class Translator
{
    public int code { get; set; }
    public string lang { get; set; }
    public List<string> text { get; set; }
}

public class Dictionary
{
    public Head head { get; set; }
    public Def[] def { get; set; }
}

public class Head
{
}

public class Def
{
    public string text { get; set; }
    public string pos { get; set; }
    public string gen { get; set; }
    public string anm { get; set; }
    public Tr[] tr { get; set; }
}

public class Tr
{
    public string text { get; set; }
    public string pos { get; set; }
    public Syn[] syn { get; set; }
    public Mean[] mean { get; set; }
    public Ex[] ex { get; set; }
}

public class Syn
{
    public string text { get; set; }
    public string pos { get; set; }
}

public class Mean
{
    public string text { get; set; }
}

public class Ex
{
    public string text { get; set; }
    public Tr1[] tr { get; set; }
}

public class Tr1
{
    public string text { get; set; }
}

и

public class YandexServices
{
    private const string _addressTranslator = "https://translate.yandex.net/api/v1.5/tr.json/translate?" +
        "key=trnsl.1.1.20200226T182352Z.f50284e5823499f7.dafb6352b04e04f4d4f6d95e7d4d076fa972c2ed";
    private const string _addressDictionary = "https://dictionary.yandex.net/api/v1/dicservice.json/lookup?" +
        "key=dict.1.1.20200226T180945Z.3dff57d76cbaf934.a871b8b923f38ae1cd61bd5139c51ab39f217f83";

    private readonly HttpClient _httpClient;

    public enum TranslationDirection { RuEng, EngRu }

    public YandexServices()
    {
        _httpClient = new HttpClient();
        _httpClient.Timeout = new TimeSpan(0, 0, 30);
    }

    public async Task<YandexAnswer> GetTranslateAnswerAsync(string text, TranslationDirection direction)
    {
        var answer = new YandexAnswer
        {
            Lang = GetLang(direction),
            TranslatorAnswer = new Translator()
        };

        if (String.IsNullOrWhiteSpace(text))
        {
            answer.Text = "Нет текста для перевода";
            return answer;
        }

        var address = String.Concat(_addressTranslator,
            "&lang=", answer.Lang, "&text=", text);

        answer = await GetAnswerAsync(address, answer);
        return answer;
    }

    public async Task<YandexAnswer> GetDictionaryAnswerAsync(string text, TranslationDirection direction)
    {
        var answer = new YandexAnswer
        {
            Lang = GetLang(direction),
            DictionaryAnswer = new Dictionary()
        };
        if (String.IsNullOrWhiteSpace(text))
        {
            answer.Text = "Нет слова(ов) для перевода";
            return answer;
        }

        var address = String.Concat(_addressDictionary,
            "&lang=", answer.Lang, "&text=", text);

        answer = await GetAnswerAsync(address, answer);
        return answer;
    }

    private string GetLang(TranslationDirection direction)
    {
        switch (direction)
        {
            case TranslationDirection.RuEng:
                return "ru-en";
            case TranslationDirection.EngRu:
                return "en-ru";
            default:
                throw new ArgumentException(nameof(direction));
        }
    }

    private async Task<YandexAnswer> GetAnswerAsync(string address, YandexAnswer answer)
    {
        var response = await _httpClient.GetAsync(address);
        if (response.IsSuccessStatusCode)
        {
            var json = await response.Content.ReadAsStringAsync();
            if (answer.DictionaryAnswer != null)
            {
                answer.DictionaryAnswer = JsonConvert.DeserializeObject<Dictionary>(json);
            }
            else
            {
                answer.TranslatorAnswer = JsonConvert.DeserializeObject<Translator>(json);
            }
        }
        else
        {
            answer.Text = "Ошибка доступа к сервису Яндекса";
        }
        answer.Code = response.StatusCode.ToString();
        return answer;
    }
}

4) Создадим пустой класс MainViewModel и напишем к нему такие тесты

[TestClass()]
public class MainViewModelTests
{
    [TestMethod()]
    public async Task GetTransationAsync_WhenRusInput_ThenEngOutput()
    {
        string input = "семь раз отмерь один раз отрежь";
        string output = "measure twice cut once\r\n";
        YandexServices services = new YandexServices();
        MainViewModel sut = new MainViewModel(services);

        sut.Input = input;
        sut.SelectedLang = 0;
        await sut.GetTransationAsync();

        Assert.AreEqual(output, sut.Output);
    }

    [TestMethod()]
    public async Task GetTransationAsync_WhenEngInput_ThenRusOutput()
    {
        string input = "measure twice cut once";
        string output = "семь раз отмерь один раз отрежь";
        YandexServices services = new YandexServices();
        MainViewModel sut = new MainViewModel(services);

        sut.Input = input;
        sut.SelectedLang = 1;
        await sut.GetTransationAsync();

        Assert.IsTrue(sut.Output.Contains(output));
    }

    [TestMethod()]
    public async Task GetDictionaryAsync_WhenRusInput_ThenEngOutput()
    {
        string input = "мысль";
        string output = "thought";
        YandexServices services = new YandexServices();
        MainViewModel sut = new MainViewModel(services);

        sut.Input = input;
        sut.SelectedLang = 0;
        await sut.GetDictionaryAsync();

        Assert.IsTrue(sut.Output.Contains(output));
    }

    [TestMethod()]
    public async Task GetDictionaryAsync_WhenEngInput_ThenRusOutput()
    {
        string input = "thought";
        string output = "мысль";
        YandexServices services = new YandexServices();
        MainViewModel sut = new MainViewModel(services);

        sut.Input = input;
        sut.SelectedLang = 1;
        await sut.GetDictionaryAsync();

        Assert.IsTrue(sut.Output.Contains(output));
    }
}

5) После прохождения тестов получим такой класс

public class MainViewModel
{
    private YandexServices _yandexServices;

    public MainViewModel(YandexServices yandexServices)
    {
        _yandexServices = yandexServices ??
            throw new ArgumentNullException(nameof(yandexServices));

        Langs = new List<string>() { "с русского на английский", "с английского на русский" };
    }

    public event EventHandler OutputChanged;

    private string _Output;
    public string Output
    {
        get { return _Output; }
        set
        { 
            _Output = value;
            OutputChanged?.Invoke(this, EventArgs.Empty);
        }
    }

    public string Input { get; set; }

    public List<string> Langs { get; }

    public int SelectedLang { get; set; }

    public async Task GetTransationAsync()
    {
        var direction = GetDirection();
        var result = await _yandexServices
            .GetTranslateAnswerAsync(Input, direction);

        Output = result.Text;
    }

    public async Task GetDictionaryAsync()
    {
        var direction = GetDirection();
        var result = await _yandexServices
            .GetDictionaryAnswerAsync(Input, direction);

        Output = result.Text;
    }

    private YandexServices.TranslationDirection GetDirection()
    {
        if (SelectedLang == 0)
        {
            return YandexServices.TranslationDirection.RuEng;
        }
        else
        {
            return YandexServices.TranslationDirection.EngRu;
        }
    }
}

6) Набросаем на форму контролы и напишем привязки к нашей вьюмодели

public partial class FormMain : Form
{
    private readonly MainViewModel _viewModel;

    public FormMain()
    {
        InitializeComponent();
        _viewModel = new MainViewModel(new YandexServices());

        //привязки
        _textBoxInput.DataBindings.Add("Text", _viewModel, nameof(_viewModel.Input));
        _textBoxOutput.DataBindings.Add("Text", _viewModel, nameof(_viewModel.Output));
        _viewModel.OutputChanged += (s, e) => _textBoxOutput.DataBindings[0].ReadValue();
        _comboBoxLang.DataSource = _viewModel.Langs;
        _comboBoxLang.DataBindings.Add("SelectedIndex", _viewModel, nameof(_viewModel.SelectedLang));

        //кнопки
        _buttonTrans.Click += async (s, e) => await _viewModel.GetTransationAsync();
        _buttonDict.Click += async (s, e) => await _viewModel.GetDictionaryAsync();

    }
}

Теперь можно запустить и проверить работу программы

Пример целиком здесь