Objects implicitly instantiated in vb.net?

问题:

I am maintaining an application that has both VB.NET and c# components. I thought these two languages differed only in syntax, but I have found a strange feature in VB.NET that is not present in C#.

In VB.NET, I have the following class:

Public Class bill_staff Inherits System.Windows.Forms.Form
    ....
End Class

If I want to use this class in C#, I do this:

using (var frm = new bill_staff())
    frm.ShowDialog();

However, in the VB.NET code, the class can be used like this:

bill_staff.ShowDialog();

ShowDialog is defined in the metadata like this:

Public Function ShowDialog() As System.Windows.Forms.DialogResult

So in VB.NET, it is possible to call an instance method on the class. As far as I can tell, this seems to implicitly create a new instance of the class, and then calls the method on that object. In C#, this is not possible - static methods must be called on the class, and instance methods must be called on objects.

I can't find any information about this on the Internet. What is the feature called, and is it good practice?

The project was initially converted from VB6 - is it some weird legacy feature?

回答1:

Yes that is legacy behavior. Classes did not show up in VB until v4, before that Form1.Show was The Way to show forms. In order to keep previous code compatible (VB3 was also very popular), the old method was maintained.

It is still supported in .NET as a legal means to show forms. Initially, this was added to make it easy to migrate VB6 code to VB.NET. But is also there to make it easy to get something running in VB - MS refers to it as functionality at your fingertips and similar phrases.

Basically, it provides the tinkerer an easy way to program without understanding Objects and OOP. Imagine the questions we would have here if Form1.Show threw an error.

Explicit instancing is the better method because it is object oriented and makes it less likely your code will reference - or create - a new Form2 when you actually wanted to use an existing instance.

回答2:

For forms, VB creates a default instance for you behind the scenes. e.g., the following VB:

Public Class bill_staff
    Inherits System.Windows.Forms.Form
End Class

class testclass
    sub testmethod()
        bill_staff.Show()
    end sub
end class

is equivalent to the following C#:

public class bill_staff : System.Windows.Forms.Form
{

    private static bill_staff _DefaultInstance;
    public static bill_staff DefaultInstance
    {
        get
        {
            if (_DefaultInstance == null)
                _DefaultInstance = new bill_staff();

            return _DefaultInstance;
        }
    }
}

internal class testclass
{
    public void testmethod()
    {
        bill_staff.DefaultInstance.Show();
    }
}

This only occurs in VB for forms (classes which inherit from System.Windows.Forms.Form). Personally, I think this is a terrible 'feature' of VB - it confuses the distinction between class and instance.