如果它不是静态方法，为什么我可以调用Form.Close（）

Question

Can someone explain this to me?

In Visual Studio 2010, create a VB.net Windows Forms App. Add 2 forms: Form1 and Form2. In the Form1 Load event type Form2.Close(). Now if we look in the method definition Close() is not a static (shared) method. So how is this possible to compile or to work at run time.

Furthermore, do the same thing in C# and Form2.Close(); doesn't compile.

What's going on? Why is this possible in VB.net and what is actually happening when that line of code is executed?

Answer 1

You've discovered a VB.NET-ism called "default instance".

The compiler is actually emitting this:

My.Forms.Form2.Close();

There is a nice writeup of that feature here :

The default instance is an object of that type that the VB application framework creates and manages for you.

...

If you use the default instance then you don't need to invoke a constructor explicitly. You simply access the default instance directly via the My.Forms object

Answer 2

The reason is that VB creates an automatic instance of the form if you just reference them by name, which could lead to unintended consequences at runtime if it isn't caught.

There is no setting that I have found to prevent this from happening.

However, you can "break" this behavior at compile time by changing the scope of the default constructor from Public to Friend or by removing the default constructor and adding one that requires a parameter. Either or these changes will disable the automatic form references.

Answer 3

You are correct, you cannot call Form2.Close(); when Form2 is only a class type. VB.NET, however, creates a property with the same name behind the scenes, and so you are really calling Close on an instance of Form2 . You can do the same in C#, if you manually create such a property. It looks like a static method call, but it isn't.