How can i prevent to set property that client can access over web service proxy?

Question

I have a web service that i typed on c#.

I have added a class to this web service. This class has two properties. One of them has getter and setter and the other has just one getter or has private setter.

class ClassName{
    public string propGetSet{get;set;}
    public string propGet{ get; private set; }
}

class WebSer : WebService{
    [WebMethod]
    public ClassName methodName(){
        ClassName cn = new ClassName{ propGetSet = "hihi" };
        return cn;
    }
}

class Client{
    public void MethodCaller(){
        WebSer ws = new WebSer();
        ClassName cnComesFromWebService = ws.methodName();
        // I can access the propGetSet but can't access propGet
    }
}

What is the real point that i am missing?

How proxy, classes over web services work?