C # how to dynamically throw an object?

I am creating a helper object that has a Mailer property. In fact, the Mailer can be either System.Net.Mail.MailMessage or Mono.System.Net.Mail.MailMessage. Therefore, I would prefer only 1 ad mailer.

For example, I do not want:

private Mono.Mailing.MailMessage MonoMessage = new Mono.Mailing.MailMessage();
private System.Net.Mail.MailMessage MailMessage = new System.Net.Mail.MailMessage();

I would prefer

object mailer;

Then in the constructor

switch (software)
            {
                case EnunInternalMailingSoftware.dotnet:
                    this.mailer = new System.Net.Mail.MailMessage();
                    break;
                case EnunInternalMailingSoftware.mono:
                    this.mailer = new Mono.Mailing.MailMessage(); 
                    break;
            }

The problem is that mail has no properties at design time. Therefore, I cannot compile my code.

How can this be fixed, I am right. thanks in advance