[C#] Polymorphic oddity

Inquisitor

Inquisitor

Inquisitor

Soldato
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I've created a custom control that derives from TextBox, and overriden the Text property inherited from TextBox. The problem is that base.Text is not doing what it should do – it's just getting/setting my override of it. :confused:

Unless i'm misunderstanding polymorphism and the base keyword, this shouldn't be happening should it?

Edit: it works fine if I use the new keyword, but only when running; it causes odd behaviour in the designer (as it's getting/setting the TextBox.Text property instead).
 
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Here's the code for the property:

Code: 
public override string Text
{
    get
    {
        if (m_Empty)
        {
            return String.Empty;
        }
        else
        {
             return base.Text;
        }
    }
    set
    {
        base.Text = value;

        m_Empty = value == String.Empty;
        Display();
    }
}

And here's a screenshot illustrating the problem:

vs2005jl9.png
 
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Not a c# expert but have you tried using the 'this' keyword. If the property is public then it becomes part of the overridden class as well
 
Well, the whole point in the base keyword is to provide access to otherwise overridden methods and properties as they are implemented in the base class, but it doesn't seem to be doing that. :confused:
 
growse said:
Ho Ho!

http://blogs.msdn.com/jmstall/archive/2006/06/29/funceval_does_virtual_dispatch.aspx
Click to expand...
Well, that clears it up somewhat, but there's still a problem that the article can't explain.

The following code procuces two message boxes, both with identical text present in them (as opposed to the second being empty, as it should be):
Code: 
if (m_Text == String.Empty)
{
    MessageBox.Show(base.Text);
    base.Text = String.Empty;
    MessageBox.Show(base.Text);
}

I've tried running it straight from the binary with no debugging process attached, but it still yields the same results.

Incidentally, the code for the Text property is now:
Code: 
public override string Text
{
    get
    {
        return m_Text;
    }
    set
    {
        base.Text = m_Text = value;
        Display();
    }
}

Again, changing override to new solves the problem at runtime, but causes more problems in design view.
 
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Nope, but it shouldn't make a difference should it?

I'll try it anyway though :)

edit: yup, still getting the same behaviour in VB.NET
 
Last edited:
Inquisitor said:
vs2005jl9.png
Click to expand...


Looks ok to me! Rember it's an overridden property so if the base class uses this property it will use your version and not the base property unless you call base explicitly. In this instance you have already returned "test" so could have set the base property hence why the debugger is giving you "test".
 
geordiepaul said:
Looks ok to me! Rember it's an overridden property so if the base class uses this property it will use your version and not the base property unless you call base explicitly. In this instance you have already returned "test" so could have set the base property hence why the debugger is giving you "test".
Click to expand...
How has it set the base property? :confused:
 
very true! Controls can do some weird stuff when being loaded from viewstate etc etc. I've just tried it, and I get the same result.

Bear in mind though that when you hover over a variable like that in VS it will go away and execute the code that return whatever it is that you're hovering over. In this case it looks like it processing your method and not the base method.
 
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