[RESOLVED] 'Virtual member call in constructor'

[NickThissen]

Hi,

I got an abstract class BasePage that inherits System.Web.UI.Page for a web application, and actual pages are supposed to inherit this BasePage instead of Page directly.

The BasePage needs to create some object in its constructor using a Configuration. A Configuration is a class related to NHibernation (doesn't really matter for my question).
However, as far as I know I cannot pass this configuration via the constructor because in ASP.NET you never call the constructor of a page directly (in each case, I wouldn't know how to pass parameters to the constructor of a page even if it is possible).

So I need some other way to get the information in the constructor. I thought I could use an abstract property that the derived class has to override in which it can return the Configuration object, so I did this:

public abstract class BasePage : System.Web.UI.Page
{
    public BasePage()  // cannot use parameters here
    {
        // Get the configuration it needs to use
        Configuration config = this.Configuration;
 
        // And use it
        this.CreateObjectFromConfiguration(config);
    }
 
    public abstract Configuration Configuration { get; }
 
    //...
}

My reasoning is that now, a deriving page can do supply the configuration from wherever it gets it from (for example, a session variable or whatever):

public class ActualPage : BasePage
{
    public override Configuration Configuration
    {
        get { return (Configuration)this.Session["Configuration"]; }
    }
}

While conceptually this seemed to fit, ReSharper gave me a warning on line 6 of the first code sample, saying "Virtual member call in constructor". I googled the warning and I came up with a few threads that explain why this can be a problem. If I understand it correctly, in my example, the constructor of BasePage would request the Configuration property in the deriving class (ActualPage.Configuration), but the deriving class has not been created yet. I guess that this.Session["Configuration"] would then return null?

I am not in the opportunity to test it with the actual page objects at the moment, so I created a little test where I replaced Configuration with a simple Button:

class Program
    {
        static void Main(string[] args)
        {
            var page = new ActualPage();
        }
    }
    
    public abstract class BasePage 
    {
        public BasePage()  
        {
            Button btn = this.Button;
 
            Debug.WriteLine(btn.Text);
        }
 
        public abstract Button Button { get; }
    }
 
    public class ActualPage : BasePage
    {
        private Button b;
 
        public ActualPage()
        {
            b = new Button {Text = "test"};
        }
 
        public override Button Button
        {
            get { return b; }
        }
    }

Indeed, when I run this, I get a null reference exception on the Debug.WriteLine method as 'btn' is null.



How can I solve this problem? How can I pass the Configuration (or the Button in my example) to the constructor of the BasePage without passing it via a parameter as usual?

Thanks!

[Lord_Rat]

Isn't the base constructor called before the derived constructor? If so, then the problem is simply order of operations. I'm going to do a double check online, but I believe with abstract classes, that's the case.

Edit: I am embarking on this search but perhaps before I do, I should point out the way I usually go about these things.

I create a constructor in the base class, but not the derived class. It called a virtual method that actually does my constructing. The derived class overrides the virtual method and give it innards.

class BaseClass
{
    public BaseClass(string param, int param2){
        DoInit(string param, int param2);
    }

    private abstract void DoInit(string param, int param2);
}

class DerivedClass : BaseClass
{
   private override void DoInit(string param, int param2){
      //Do Work
   }
}

Edit 2: I am indeed correct on the order of operations. Constructors are executed according to the order of inheritance.

[NickThissen]

Yes, that is what I found as well, but that is exactly the problem. The BasePage constructor is getting a property on the derived class, which is not created yet (or not 'constructed' yet, I dunno if there's a difference).

In the meantime I found a way to delay the getting and using of Configuration (until it's actually required) but I'd rather do it in the constructor.

[Lord_Rat]

The edit I added should handle that though. Just move the construction of the button to the equivelant of the "DoInit".

[NickThissen]

Yeah, I guess that could work. Thanks!

[Lord_Rat]

No problem! Sometimes, it just takes turning the problem on its side to see the solution. =)