[DataContract]

public class File

{

    // excluded from serialization

    // does not have DataMemberAttribute

    public Guid Id { get; set; }

    [DataMember]

    public string Name { get; set; }

    [DataMember]

    public int Size { get; set; }

}

File file = new File

{

    Id = Guid.NewGuid(),

    Name = "ImportantLegalDocuments.docx",

    Size =  *

};

string json = JsonConvert.SerializeObject(file, Formatting.Indented);

Console.WriteLine(json);

// {

//   "Name": "ImportantLegalDocuments.docx",

//   "Size": 51200

// }