O365Crud

ContactsRepository.cs

internal class ContactsRepository : IContactsRepository<IContact>
{
    private ExchangeClient _exchangeClient;
    private const string ExchangeResourceId = "https://outlook.office365.com";
    private const string ExchangeServiceRoot = "https://outlook.office365.com/ews/odata";
    private const int PageSize = 50;

    private async Task<ExchangeClient> EnsureExchangeClient()
    {
        if (_exchangeClient != null)
            return _exchangeClient;
     
        Authenticator authenticator = new Authenticator();
        var authInfo = await authenticator.AuthenticateAsync(ExchangeResourceId);

        _exchangeClient = new ExchangeClient(new Uri(ExchangeServiceRoot), authInfo.GetAccessToken);
        return _exchangeClient;
    }

    public async Task<IContact> GetById(string id)
    {
        var client = await EnsureExchangeClient();
        return await client.Me
            .Contacts
            .Where(c => c.Id.Equals(id))
            .ExecuteSingleAsync();
        
    }

    public async Task<IEnumerable<IContact>> Get(int pageNumber)
    {
        var skipItems = pageNumber*PageSize;
        var client = await EnsureExchangeClient();
        var contacts = await client.Me
            .Contacts
            .OrderBy(c => c.DisplayName)
            .Skip(skipItems)
            .ExecuteAsync();
            
        return contacts.CurrentPage;

    }

    public async void Create(IContact instance)
    {
        if (instance != null)
        {
            throw new ArgumentNullException("instance");
        }
        var client = await EnsureExchangeClient();
        await client.Me
            .Contacts
            .AddContactAsync(instance);
    }

    public async void Update(IContact instance)
    {
        if (instance != null)
        {
            throw new ArgumentNullException("instance");
        }
        var client = await EnsureExchangeClient();
        await instance.UpdateAsync();
    }

    public async void Delete(IContact instance)
    {
        if (instance != null)
        {
            throw new ArgumentNullException("instance");
        }
        var client = await EnsureExchangeClient();
        await instance.DeleteAsync();
    }
}

IContactsRepository.cs

internal interface IContactsRepository<T>
{
    Task<T> GetById(string id);
    Task<IEnumerable<T>> Get(int pageNumber);
    void Create(T instance);
    void Update(T instance);
    void Delete(T instance);
}