/// <summary>
/// Defines a student.
/// </summary>
[DataContract]
public class Student
{
    /// <summary>
    /// Gets or sets the student number.
    /// </summary>
    [DataMember]
    public string StudentNumber { get; set; }

    /// <summary>
    /// Gets or sets the first name.
    /// </summary>
    [DataMember]
    public string FirstName { get; set; }

    /// <summary>
    /// Gets or sets the last name.
    /// </summary>
    [DataMember]
    public string LastName { get; set; }

    /// <summary>
    /// Gets or sets the marks obtained.
    /// </summary>
    public int MarksObtained { get; set; }
}

/// <summary>
/// A service that provides the operations for students.
/// </summary>
[ServiceContract]
public interface IStudentService
{
    //Service contract code here.
}

In above code StudentNumber, FirstName, LastName properties of Student class are explicitly marked with DataMember attribute as oppose to MarksObtained, so MarksObtained will be ignored. From ignored it means that MarksObtained wont be the part of data going across the wire to / from this service.

Below code represents an example of Opt-Out approach using Serializable and NonSerialized attributes.

/// <summary>
/// Represents a student.
/// </summary>
[Serializable]
public class Student
{
    /// <summary>
    /// Gets or sets student number.
    /// </summary>
    public string StudentNumber { get; set; }

    /// <summary>
    /// Gets or sets first name.
    /// </summary>
    public string FirstName { get; set; }

    /// <summary>
    /// Gets or sets last name.
    /// </summary>
    public string LastName { get; set; }

    /// <summary>
    /// Gets or sets marks obtained.
    /// </summary>
    [NonSerialized]
    public string MarksObtained { get; set; }
}

/// <summary>
/// A service that provides the operations for student.
/// </summary>
[ServiceContract]
public interface IStudentService
{
    //Service contract code here. Example given.
    
    /// <summary>
    /// Adds a student into the system.
    /// </summary>
    /// <param name="student">Student to be added.</param>
    [OperationContract]
    void AddStudent(Student student);
}

In above example, we explicitly marked MarksObtained property as [NonSerialized] attribute, so it will be ignored except the others. Hope this helps!