Multithread Singeleton Substitution with NSubstitute

Question

I have a public method ValidateWords inside FooService.To test the ValidateWord method, I created IAppSettingWrapper and AppSettingWrapper which returns the Instance of AppSettings.

Inside the test method, I want to substitute NotAllowedWords using NSubstitute. However, it throws an object reference exception. Is there any way for substitution? If it's not possible, how can I refactor my static instance?

public sealed class AppSettings
{
    private static object _lockObject = new object();
    private static volatile AppSettings? _instance;
    private static DateTime _cacheTime;
    private Settings[] _settings;

    public AppSettings()
    {
        try
        {

            _settings = GetSettings();
        }
        catch { }
    }
    public static AppSettings Instance
    {
        get
        {
            lock (_lockObject)
            {
                if (_instance == null)
                {
                    _instance = new AppSettings();
                }

            }
            return _instance;
        }
    }
    public List<string> NotAllowedWords
    {
        get
        {
            return new List<string>() { 
                  "index",
                  "change"
            };
        }
    }

    public T GetValues<T>(string key,T defaultValue)
    {
        T result = defaultValue;
        var settings = _settings.Where(i => i.Key == key).FirstOrDefault();
        result = (T)Convert.ChangeType(settings.Value, typeof(T));
        return result;
    }
    private Settings[]? GetSettings()
    {
        //gets data from web services
        return base.Channel.GetSettings();
    }
}
public class Settings
{
    public string Key { get; set; }
    public string Value { get; set; }
}

public interface IAppSettingsWrapper
{
    public AppSettings Instance();
}

public class AppSettingsWrapper : IAppSettingsWrapper
{
    public AppSettings Instance()
    {
        return AppSettings.Instance;
    }
}

[TestClass]
public class FooServiceTest{
    private IAppSettingsWrapper _appSettingsWrapper;

    [TestInitialize]
    public void TestInitialize(IAppSettingsWrapper appSettingsWrapper)
    {
        _appSettingsWrapper = Substitute.For<IAppSettingsWrapper>();
    }
    private FooService CreateFooService()
    {
        return new FooService(_appSettingsWrapper);
    }

    [TestMethod]
    public void Throw_Exception_When_Given_Word_Not_Allowed() {
        var service = this.CreateFooService();
        _appSettingsWrapper.Instance().NotAllowedWords.Returns(new List<string> { "index" });
        var word = "index";
        Exception ex = Assert.ThrowsException<Exception>(() => service.ValidateWords(word));
        Assert.AreEqual("this word is not allowed", ex.Message);
    }
}

public class FooService
{
    private IAppSettingsWrapper _appSettingsWrapper;
    public FooService(IAppSettingsWrapper appSettingsWrapper)
    {
        _appSettingsWrapper = appSettingsWrapper;
    }


    public void ValidateWords(string word)
    {
        if (_appSettingsWrapper.Instance().NotAllowedWords.Contains(word))
        {
            throw new Exception("this word is not allowed");
        }
    }
}

Answer 1

The AppSettings.NotAllowedWords property is not substitutable due to it not being virtual and the class being sealed. If you add NSubstitute.Analyzers to your test project it will help you find these cases. (The How NSubstitute Works documentation outlines why this is the case.)

One option is to make AppSettings implement an IAppSettings interface and inject that into FooService (rather than the wrapper). Then you can use a substitute for tests, and AppSettings.Instance for your real code.