通用接口-抽象

[解释]

我需要创建一个类以通过提供程序（现在是SendGrid或Mandrill）发送电子邮件。

现在，将项目划分为“电子邮件核心”和“提供者核心”，并且已经创建了这些接口：

public interface IEmail
{
    ICollection<IContact> Recipients { get; }
    IContact From { get; set; }
    string EmailBody { get; set; }
    string Subject { get; set; }
    IContact ReplyTo { get; set; }
    ICollection<IContact> Attachments { get; }
}

这是依赖

public interface IContact
{
    string Name { get; set; }
    string Email { get; set; }
}

public interface IAttachment
{

    string FileName { get; set; }
    Stream Content { get; set; }
    string StringContent { get; }
    string ContentType { get; set; }
    string FilePath { get; set; }
}

然后，我创建了一个对象“ Email”来组装具有所有特征的电子邮件：

public class Email : IEmail
{
    #region Constructors
    public Email()
    {

    }
    #endregion

    #region Private Properties
    private readonly ICollection<Contact> _recipients = new List<Contact>();
    private readonly ICollection<Attachment> _attachments = new List<Attachment>();
    #endregion

    #region Public Properties
    public ICollection<IContact> Recipients
    { 
         get 
         {
             return (ICollection<IContact>)_recipients;
         }
     }

    public IContact From
    {
        get;
        set;
    }

    public string EmailBody
    {
        get;
        set;
    }

    public string Subject
    {
        get;
        set;
    }

    public IContact ReplyTo
    {
        get;
        set;
    }

    public ICollection<IAttachment> Attachments
    {
        get { return (ICollection<IAttachment>)_attachments ;}
    }
    #endregion

    #region Private Methods

    #endregion

    #region Public Methods
    public Email AddRecipient(IContact Recipient)
    {
        _recipients.Add((Contact)Recipient);
        return this;
    }

    public Email SetFrom(IContact from)
    {
        From = from;
        return this;
    }

    public Email SetEmailBody(string body)
    {
        EmailBody = body;
        return this;
    }

    public Email SetSubject(string subject)
    {
        Subject = subject;
        return this;
    }

    public Email SetReplyTo(IContact replyto)
    {
        ReplyTo = replyto;
        return this;
    }

    public Email AddAttachment(IAttachment attachment)
    {
        _attachments.Add((Attachment)attachment);
        return this;
    }
    #endregion
}

最后，我有一个类来创建“ Provider”对象，该对象将使用“ Email”对象，然后将其传递：

public class ProviderSendGrid : IProvider
{
    #region Constructors
    public ProviderSendGrid(string SendGridUser, string SendGridPassword)
    {
        _networdcredential = new NetworkCredential(SendGridUser, SendGridPassword);
        _sendgridweb = new Web(_networdcredential);
    }
    #endregion

    #region Propriedades Privadas
    private NetworkCredential _networdcredential;
    private SendGridMessage _message;
    private Web _sendgridweb;
    #endregion

    #region Public Properties

    #endregion

    #region Private Methods
    /// <summary>
    /// Verifica se string é e-mail
    /// </summary>
    /// <param name="Email">String que se deseja verificar se é e-mail ou não.</param>
    /// <returns>Retorna verdadeiro caso a string informada seja um e-mail e falso caso contrário.</returns>
    private bool IsValidEmail(string Email)
    {
        try
        {
            var address = new MailAddress(Email);
            return address.Address == Email;
        }
        catch
        {
            return false;
        }
    }

    private List<string> FormatedContacts(ICollection<IContact> Recipients)
    {
        if (Recipients == null)
            throw new Exception("Recipients parameter on Recipients method can't be null.");

        List<string> lstRet = new List<string>();

        Parallel.ForEach(Recipients, item =>
        {
            if (!IsValidEmail(item.Email))
                throw new Exception("Invalid e-mail informed.", new Exception("The following e-mail is not valid: " + item.Email));

            lstRet.Add(item.Name + " <" + item.Email + ">");
        });

        return lstRet;
    }
    #endregion

    #region Public Methods
    /// <summary>
    /// 
    /// </summary>
    /// <param name="Email">Objeto que implemente a interface MKM.Email.Core.Interfaces.IEmail</param>
    public async void Send(IEmail Email)
    {
        if (string.IsNullOrEmpty(Email.EmailBody) || string.IsNullOrEmpty(Email.Subject))
            throw new Exception("Email body or subject is null.");

        if (Email.From == null)
            throw new Exception(@"The property ""From"" can't be null.");

        //Compondo a mensagem
        _message = new SendGridMessage();

         //Stackoverflow Forum: The error occours in the following line, when I try to get the "Recipients" property from "Email" object
        _message.AddTo(FormatedContacts(Email.Recipients));
        _message.From = new MailAddress(Email.From.Email, Email.From.Name);
        _message.Subject = Email.Subject;
        _message.Html = Email.EmailBody;

        Parallel.ForEach(Email.Attachments, item => 
        {
            _message.AddAttachment(item.Content, item.FileName);
        });

        _message.ReplyTo = new MailAddress[]{new MailAddress(Email.ReplyTo.Email, Email.ReplyTo.Name)};

        await _sendgridweb.DeliverAsync(_message);
    }
    #endregion
}

[问题]

在“ ProviderSendGrid”类的“发送”方法中，当我尝试获取“收件人”属性时，收到以下错误：

附加信息：无法转换类型为'System.Collections.Generic.List 1[Email.Core.Contact]' to type 'System.Collections.Generic.ICollection1 [Email.Core.Interfaces.IContact]'的对象。

如果“ Email.Recipients”属性返回“列表”，为什么会发生这种情况。列表实现ICollection，而联系人实现IContact。

我不确定我的解释是否足够清楚，但是如果我不明白，请告诉我。

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