Set required datamember string in wcf

I want to set the DataMember string as required, I set "IsRequired = true", but it appears as optional in the XML request.

[MessageContract(WrapperName = "tourinforequest")]
public class TourInfoRequest
{
    public TourInfoRequest()
    {
        Request = new TourInfoRequestRequestData();
    }
    [MessageBodyMember]
    public TourInfoRequestRequestData Request;
}

public class TourInfoRequestRequestData : BaseRequest
{
    [DataMember(Order = 1, IsRequired = true ), XmlElement(ElementName = "tourcode")]
    public string TourCode { get; set; }

    [DataMember(Order = 2, IsRequired = true), XmlElement(ElementName = "depdate")]
    public DateTime DepartureDate { get; set; }
}