Parameterized constructor in SoapHeader class

Question

Scenario I have a WebService(.asmx) in C#.Net. I included a class which inherits from SoapHeader class. I have a parameterized constructor in it but as I forgot to include a parameterless constructor, it generated following error when I updated the WebService proxy on client side:

   Error: User cannot be serialized because it does not have a parameterless constructor.

So I immediately included the parameterless constructor and the issue ended up.

Questions
Q1. I think serialization takes place during request and response, Is it the 'class type' being serialized here (which I don't think is possible as its object which is serialized)?
Q2. I am not able to access Parameterized constructor (gives error: Does not contain a constructor that takes 1 argument ) on client side. Why is that?

Its a simple class for learning purpose:

public class User : SoapHeader
{
    private string strUid;

    public User(string id)  // cannot access
    {
        strUid = id;
    }

    public User() { }   // included later

    public string UserID
    {
        get
        {
            return strUid;
        }
        set
        {
            strUid = value;
        }
    }
}