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Tuesday, March 27, 2012

Public Vs Shared in VB.NET OOPS Concept

public vs shared

public vs. shared -------------------------------------------------------------------------------- Public and Shared aren't comparable like protected and private and public and friend are. Those are access modifiers - they modify who has the right to access them. Public simply means that any code can access the member (function/property/field). Shared which is more like a flag (something either is shared or isn't), indicates that the member (function/property/field) doesn't behave/belong to a specific instance of the class. So your own definitions are pretty accurate, I simply want to make it clear that public and shared aren't comparable. Something can be public shared, private shared, friend shared, protected shared or simply public, private, friend or protected (then there's protected friend, but we'll ignore that for now). private shared means that only the class itself can access the field, a frequent use of a private shared field is for use with singletons: public class MyClass private shared MyClass instance = nothing public shared function GetInstance() as MyClass if instance is nothing then instance = new MyClass() end if return instance() end function private sub new() end sub ... end class from the above code you can see that MyClass can never be created directly since the constructor is private (outside code can't call it). Outside code also can't access the instance field because it too is private. Outside code can however access GetInstance because it's public. GetInstance checks to see if the private field "instance" is nothing (it can access a private field because it's all the same class), if it is, it creates the instance (again, it can access the private constructor) and returns the instance (there is a possible race condition, but that's besides the point). What's neat about the above example is that GetInstance is marked shared. IF it wasn't, no one would ever be able to call it because the constructor is private and thus an instance can't be created. Without an instance, a non-shared member can't be accessed.
Solution

Public and Shared aren't comparable like protected and private and public and friend are. Those are access modifiers - they modify who has the right to access them. Public simply means that any code can access the member (function/property/field). Shared which is more like a flag (something either is shared or isn't), indicates that the member (function/property/field) doesn't behave/belong to a specific instance of the class. So your own definitions are pretty accurate, I simply want to make it clear that public and shared aren't comparable. Something can be public shared, private shared, friend shared, protected shared or simply public, private, friend or protected (then there's protected friend, but we'll ignore that for now). private shared means that only the class itself can access the field, a frequent use of a private shared field is for use with singletons:[CODE] public class MyClass private shared MyClass instance = nothing public shared function GetInstance() as MyClass if instance is nothing then instance = new MyClass() end if return instance() end function private sub new() end sub ... end class[/CODE] from the above code you can see that MyClass can never be created directly since the constructor is private (outside code can't call it). Outside code also can't access the instance field because it too is private. Outside code can however access GetInstance because it's public. GetInstance checks to see if the private field "instance" is nothing (it can access a private field because it's all the same class), if it is, it creates the instance (again, it can access the private constructor) and returns the instance (there is a possible race condition, but that's besides the point). What's neat about the above example is that GetInstance is marked shared. IF it wasn't, no one would ever be able to call it because the constructor is private and thus an instance can't be created. Without an instance, a non-shared member can't be accessed. You might find Paul Vick's great VB.Net book useful:

http://books.google.com/books?id=ejzRTF2TL6UC&prev=http://print.google.com/print%3Fq%3Dpaul%2Bvick&pg=1&sig=ZxfWU68bu9eUj6Q910vyvVa9Gwg&hl=en

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