Can someone explain this C# static class property?

Question

I'm trying to port someone else's C# code to Xojo. There is the following definition in a class:

static cpCollisionHandler cpCollisionHandlerDefault = new cpCollisionHandler(
         cp.WILDCARD_COLLISION_TYPE, cp.WILDCARD_COLLISION_TYPE,
       DefaultBegin, DefaultPreSolve, DefaultPostSolve, DefaultSeparate, null
    );

I understand that this is static variable that can be returned from the class without instantiating it. I also understand that it is a property called cpCollisionHandlerDefault and that it returns a type of cpCollisionHandler. What I'm not sure about is does this return a new cpCollisionHandler object every time the property is requested from the class or does it return a new object the first time the property is accessed and then the same reference to that cpCollisionHandler for each subsequent request?

It will create a new instance of `cpCollisionHandle` at the startup then you'll get a reference to that instance. — The_Black_Smurf, Feb 19 '16 at 20:54

score 1 · Answer 1 · answered Feb 19 '16 at 20:54

You have it all a bit mixed up. This is a static variable cpCollisionHandlerDefault of type cpCollisionHandler which is instantiated by the new... The static instantiation takes place before all other code executes and the variable is available for the whole run of the app.

score 1 · Answer 2 · answered Feb 19 '16 at 20:54

This is a static field, not a property. From the documentation:

A static field identifies exactly one storage location. No matter how many instances of a class are created, there is only ever one copy of a static field.

So it is instantiated only once, and it does not return a new object every time it is accessed.

With C# 6.0, it is possible to return a new object every time you access a static field, with a slight syntax modification using an expression-bodied member:

static cpCollisionHandler cpCollisionHandlerDefault => new cpCollisionHandler(
     cp.WILDCARD_COLLISION_TYPE, cp.WILDCARD_COLLISION_TYPE,
   DefaultBegin, DefaultPreSolve, DefaultPostSolve, DefaultSeparate, null
);

Thanks. Very clear explanation. Extra vote for the expression-bodied member tip. — Garry Pettet, Feb 19 '16 at 22:35

Bruno · Accepted Answer · 2016-02-19T23:54:12.373

0

does this return a new cpCollisionHandler object every time the property is requested from the class

No, the program instanciates the field once, the first time the class is loaded

does it return a new object the first time the property is accessed and then the same reference to that cpCollisionHandler for each subsequent request?

Yes, it is the same reference for each request

edited Feb 19 '16 at 23:54

answered Feb 19 '16 at 20:52

Bruno

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It's the same reference, but it's created when the class is loaded, not on first access to the static member. That may be an important distinction in some circumstances. – Ian McLaird Feb 19 '16 at 20:55

NineBerry · Answer 4 · 2016-02-19T20:59:06.463

0

This is actually not a property but a field, i.e. a "variable" on class level. Fields do not have getters or setters like properties but behave exactly like local variables, they just have a different scope.

The cpCollisionHandler object is created the first time the class is loaded during runtime and lives until the application terminates or some other value is assigned to the cpCollisionHandlerDefault field.

edited Feb 19 '16 at 20:59

answered Feb 19 '16 at 20:53

NineBerry

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An attribute in C# is something different, in this case the correct term is field. – Paolo Tedesco Feb 19 '16 at 20:54
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Attribute means something entirely different, for example `[Serializable]` is a attribute. The thing the OP has is a "Field". – Scott Chamberlain Feb 19 '16 at 20:54

Can someone explain this C# static class property?

4 Answers4