Calling web API using PostAsJsonAsync

I am a newbie trying to call web api method.

i has a four-sided object:

public class Quadrilateral
{
    public double SideA { get; private set; }
    public double SideB { get; private set; }
    public double SideC { get; private set; }
    public double SideD { get; private set; }

    public double AngleAB { get; private set; }
    public double AngleBC { get; private set; }
    public double AngleCD { get; private set; }
    public double AngleDA { get; private set; }

    public virtual string Name { get { return "Generic Quadrilateral"; } }
    public virtual string Description { get { return "A quadrilateral that has no specific name for its shape"; } }

    public Quadrilateral(double sideA, double sideB, double sideC, double sideD,
                        double angleAB, double angleBC, double angleCD, double angleDA)
    {
        SideA = sideA;
        SideB = sideB;
        SideC = sideC;
        SideD = sideD;
        AngleAB = angleAB;
        AngleBC = angleBC;
        AngleCD = angleCD;
        AngleDA = angleDA;
    }

}

      

        
        
        
      

    

my web mail api method does something very simple. it takes a four-sided object and returns the subtype Quadrilateral, a rectangle:

public class Rectangle : Quadrilateral
{
    public override string Name { get { return "Rectangle"; } }

    public override string Description { get { return "A quadrilateral with equal opposite sides and 4 right angles"; } }

    public Rectangle(Quadrilateral q) :
        base(q.SideA, q.SideB, q.SideC, q.SideD,
            q.AngleAB, q.AngleBC, q.AngleCD, q.AngleDA) { }

}

      

        
        
        
      

    

web api post method looks like this:

public Quadrilateral Post(Quadrilateral q)
{
    return new Rectangle(q); 
}

      

        
        
        
      

    

i create a four-sided object by assigning member values. then I am trying to call my web api using the following code:

readonly string baseUri = "http://localhost:43798/";
readonly string controller = "api/quadrilateral";

using (HttpClient client = new HttpClient())
{
    client.BaseAddress = new Uri(baseUri);
    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
    HttpResponseMessage response = await client.PostAsJsonAsync(controller, quadrilateral);

    if (response.IsSuccessStatusCode)
    {
        return await response.Content.ReadAsAsync<Quadrilateral>();
    }
    else
    {
        return null;
    }
}

      

        
        
        
      

    

shows the web api post method has hit, but the four-way object now contains only 0.0 for all of its double members. why doesn't the post method get the object I'm trying to post?