Why REST Services inserting null values in database (SQL)?

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WCF

REST

Hi,
I created a WCF REST service in which I m trying to insert records in SQL.I m testing it with REST client.When Im inserting values some values are getting inserted and some values are inserted as NULL eventhough I pass values from frontend.I couldn't understand what might be the reason for this...?

My code for insertion goes like this:
IService.cs:

[OperationContract]
     [WebInvoke(UriTemplate = "Appntmnts", Method = "POST")]
     void Insert(Appntmnt appointment);

Service1.cs:

public void Insert(Appntmnt appointment)
     {

SqlConnection cn = new SqlConnection();
            cn.ConnectionString = (ConfigurationManager.ConnectionStrings["c"].ConnectionString);
            string b = "";
            DateTime a = default(DateTime);
            DateTime c = default(DateTime);
 appointment.AppointmentId = Guid.NewGuid();
                a = Convert.ToDateTime(appointment.Date);
                appointment.Status = "Checkout";

                if ((appointment.DOB) != "" && appointment.DOB != null)
                {
                    c = Convert.ToDateTime(appointment.DOB);

                }
                if (appointment.Gender != null)
                {
                    b = Convert.ToString(appointment.Gender);
                }
                appointment.CreatedDateTime = DateTime.Now;
                string s1 = "Insert into Appointment(AId,CId,PId,Date,TimeStart,TimeEnd,UserId,Reason,PatientFirstName,PatientLastName,PatientDOB,PatientGender,Status,ServiceId,ReferedBy,CreatedUserId,CreatedDateTime)values('" + appointment.AppointmentId + "', '" + appointment.CompanyId + "','" + appointment.ProviderId + "', '" + appointment.Date + "', '" + appointment.TimeStart + "', '" + appointment.TimeEnd + "','" + appointment.UserId + "','" + appointment.Reason + "','" + appointment.FirstName + "','" + appointment.LastName + "','" + appointment.DOB + "','" + appointment.Gender + "','" + appointment.Status + "','"+appointment.ServiceId+"','"+appointment.ReferedBy+"','" + appointment.CreatedUserId + "','" + appointment.CreatedDateTime + "')";
                SqlCommand cm = new SqlCommand(s1, cn);
                cn.Open();
                cm.ExecuteNonQuery();
                cn.Close();
}

And while testing I am passing values from REST client like this:

<Appntmnt>
<CId>b9ca2e32-ce88-4d72-99ce-9bc592511e85</CId>
<PId>ec245cf7-b8a2-4142-8f90-8ecd2f6bb037</PId>
<Date>9/23/2012</Date>
<TimeStart>11:15</TimeStart>
<TimeEnd>11:30</TimeEnd>
<UserId>ec245cf7-b8a2-4142-8f90-8ecd2f6bb037</UserId>
<Reason>general Consulation</Reason>
<FirstName>John</FirstName>
<LastName>Williams</LastName>
<DOB>1985/7/5</DOB>
<Gender>M</Gender>
<CreatedUserId>ec245cf7-b8a2-4142-8f90-8ecd2f6bb037</CreatedUserId>
</Appntmnt>

When I test are values are getting inserted in SQL but not all the values are inserted.Eventhough I pass FirstName,LastName,Gender,Date here they are not inserted remaining all value are inserted.I want all the values which I pass to be inserted in SQL.

How can I get it resolved?

Any help would be appreciable...

Thanks ...