make XmlElement optional in c#

I use REST (XML) for client-server communication. The given example should be read from my Channel.

[XmlRoot("Device")]
public class RestDevice
{
    [DataMember]
    [XmlElement("type")]
    public string Type { get; set; }
}

I would like to make the XmlElement "type" optional (so that if the root doesn't contain one, I will get null as the Type property of class RestDevice.

How can I do that?