AppSettings.cs

public class AppSettings : IAppSettings
{
    private readonly ConcurrentDictionary<string, object> _cachedValues = new ConcurrentDictionary<string, object>();

    private readonly Func<ISettingsRepository> _repositoryFactory;

    public AppSettings(Func<ISettingsRepository> repositoryFactory)
    {
        _repositoryFactory = repositoryFactory;
    }

    public string GetSetting(string key)
    {
        return GetSetting<string>(key);
    }

    public T GetSetting<T>(string key)
    {
        object value;
        if (!_cachedValues.TryGetValue(key, out value))
        {
            value = GetValueFromRepository(key, typeof(T));
            _cachedValues.TryAdd(key, value);
        }

        return (T) value;
    }

    private object GetValueFromRepository(string key, Type type)
    {
        var stringValue = _repositoryFactory.Invoke().GetSetting(key);
        if (stringValue == null)
        {
            throw new MissingSettingException(string.Format("A setting with the key '{0}' does not exist.", key));
        }

        if (type == typeof(string))
        {
            return stringValue;
        }

        return ConvertValue(stringValue, type);
    }

    private static object ConvertValue(string stringValue, Type type)
    {
        return TypeDescriptor.GetConverter(type).ConvertFromString(stringValue);
    }
}

public class MissingSettingException : Exception
{
    public MissingSettingException(string message) : base(message) { }
}

public interface ISettingsRepository
{
    string GetSetting(string key);
}

public interface IAppSettings
{
    string GetSetting(string key);

    T GetSetting<T>(string key);
}