I am reviewing some code and I realized I don't remember the correct terminology for something. I believe if I had the following code
pnlOne.Visible = False
Would the "visible" part be considered a method, function, or what? I am learning VB alongside JavaScript, and in JS it would be a method. Is it the same for vb?
In VB.net, that is a "property". Properties in VB.net and C# as essentially glorified methods for getting and setting a value. (They actually compile down to something like get_Visible and set_Visible methods.)
pnlOne is an instance of a class and Visible is its property
Visible could be either ..
a Property; or
a Field (called "Member Variable" in VB)
.. depending on how it is declared. Both Properties and Fields are specializations of "Members"1. See Differences Between Properties and Variables in Visual Basic.
I suspect Visible is a Property in this case, and it will be for all standard Control types .. however, to verify this either way requires knowledge of the Type of the object named by pnlOne.
1
Methods (or "Sub/Function Procedures") are a different kind of Member and it is not appropriate to call either a Property or Field a "Function" or a "Method". (Note: various references inconsistently make a distinction between a Method and a Procedure; in VB.NET they an be thought of as synonyms.)
Nit: the correct term in JavaScript would be property; properties can evaluate to function-objects and can thus also can be considered methods when they do so - usually when this is used meaningfully. In any case, the code would have to be different (e.g. jsObj.set_Visible(true)) if a method was used.
Related
Finally, when I managed to understand how to fix this, that is, how to change the value of an internal dynamic variable, the code has moved on and now it is declared in this way:
my int $is-win = Rakudo::Internals.IS-WIN;
This is a class variable declared inside class Encoding::Builtin. Makes all the sense in the world, since an OS is not something that changes during the lifetime of a variable. However, I need to test this code from other OS, so I would need to access that class variable and assign it a True value. Can I do that using the meta object protocol?
The concept of "class variable" doesn't exist in Perl 6.
The declaration being considered is of a lexical variable, and its lifetime is bound to the scope (bounded by curly braces) that it is declared within. It doesn't have any relationship with the class that's being declared, so there's no way to reach it through the MOP. (That the block in this question happens to be attached to a class declaration is incidental so far as lexical variables go.) Nor is it declared our, so it's not stored in the package either.
The only way a lexical can be accessed - aside from under a debugger - is if something inside of that lexical scope explicitly made it possible (for example, by acquiring a pseudo-package and storing it somewhere more widely visible, or by allowing EVAL of provided code). Neither is happening in this case, so the variable not possible to access.
Perl 6 is very strict about lexical scoping, and that's a very intentional part of the language design. It supports the user in understanding and refactoring the program, and the compiler in program analysis and optimization. Put another way, Perl 6 is a fairly static language when it comes to lexical things (and will likely come to do much more static analysis in future language versions), and a dynamic language when it comes to object things.
I have a COM component that has get_XXX and put_XXX methods inside it. I used it in a .NET project and a RCW was generated for it. I now see get_XXX and set_XXX methods and NOT the put_XXX one? Is that automatic or defined somewhere in IDL?
These are property accessor methods. A compiler that uses the COM server is expected to generate a call to get_Xxx() when the client program reads the property, put_Xxx() when it writes it. A special one that C# doesn't have at all is putref_Xxx(), used to unambiguously access an object instead of a value.
The normal translation performed by Tlbimp.exe is as a plain C# property. But that doesn't always work, C# is a lot more strict about what a property can look like:
The default property, the one that's annotated as DISPID_VALUE (dispid 0) must take a single argument to be compatible. This maps to the C# indexer property, the one that makes it look like you are indexing an array.
Any other property cannot take an argument, C# does not supported indexed properties other than the indexer.
C# does not have the equivalent of putref_Xxx(), the syntax ambiguity cannot occur in a C# program because of the previous two bullets. And the core reason that the C# team decided to put these restrictions in place, they greatly disliked ambiguity in the language.
So Tlbimp.exe is forced to deal with these restrictions, if the COM property accessors are not compatible then it must fall back to exposing them as plain methods instead of a property. With default names, they'll get the get_ and set_ prefixes. The latter one explains your question, they didn't pick put_ for an otherwise unclear reason.
Notable is that C# version 4 relaxed several of these restrictions, most of all to make interop with Office programs easier. Which was quite painful in earlier C# versions, to put it mildly. It extended the property syntax to lessen the pain, but only for COM interop. Very strongly recommended if you are still stuck on an old version of .NET, now is a good time to consider updating.
The properties themselves have to prefixes (put_ etc.), they have names, getter method, setter method, but no prefixes. Method table generated from type library receives prefixes to distinguish between getters and setters, hence the prefixes. Prefix string exactly depends on preference of the one who generates the names.
See also:
#pragma import attributes - raw_property_prefixes
By default, low-level propget, propput, and propputref methods are exposed by member functions named with prefixes of get_, put_, and putref_ respectively. These prefixes are compatible with the names used in the header files generated by MIDL.
I have a problem in understanding OOP...
This is it :
Sometime's you create an object with this syntax:
Object ObjectName = new Object();
But sometimes, we don't need to do that like in Android:
Textview TextviewName;
Or in J2ME:
form formName;
I already searched it and I got some information (but not sure) that this is because of static method... is it true? I think it has a relation with Polymorphism.. is it true?
Thanks all.
PS : Sory if I made some mistakes, English is not my native languange :D
Forget static methods - they're not relevant here. I'd advocate only looking at static methods / elements when you've truly grasped what objects are.
In Java, you can do this:
Object object;
Just as well as you can do this:
Object object = new Object();
In the first example, you're creating a reference but you're not populating that reference with anything, in the second example you're creating a reference and populating it with a new object, on which you can call methods, modify values and so on.
If you try and call methods on the first declaration you won't be able to - there's nothing there. Depending on the language and how you've declared it this might produce an error at runtime or a compile time error (Java does both depending on whether it's a field or a local variable.) The principle though is the same for all OO languages, you can't dereference (call methods, fields, etc.) on a reference that hasn't been populated, because in effect you're trying to call a method on something that isn't there.
you are mixing different languages and it's not the case of static methods nor polymorphism..
i suggest to read a good book of OOP beginning with the basis.. you can find "Thinking in c++" for free on the net..
Your Textview would not be initialized. Any try at using it would result in a NullReference error. In order for an object to actually be created, you have to use the new syntax or a function that returns a valid object.
However, this is a syntax-dependent issue, so first decide what language you want to study. If your Textview had been declared in C++, it would actually create an object, on the stack.
Ok, so I'm a C# coder, and I have trouble even reading VB.NET, so please forgive what may turn out to be an incredibly dumb question. But I have some code which looks like this:
Function GetName(sourceObject as Object) as String
return sourceObject.Name
End Function
So, ignoring the fact that the syntax is probably wrong - how does VB.NET get the Name property from sourceObject? Inspecting it at run-time reveals that sourceObject is of a type that supports a property called Name with a getter, but what does VB.NET do in this case? Is there some extra code that is being generated by the compiler to somehow cast this automagically at run-time?
As you may be able to tell, I'm a little confused. Thanks in advance!
This is a case of late binding. So, if the sourceObject contains a property as Name it will return the value otherwise it will just throw an error that property not found.
Late binding in C# requires reflection so it throws compile time error, whereas in vb.net it can be done without reflection.
The Visual Basic compiler performs a process called binding when an
object is assigned to an object variable. An object is early bound
when it is assigned to a variable declared to be of a specific object
type. Early bound objects allow the compiler to allocate memory and
perform other optimizations before an application executes. By
contrast, an object is late bound when it is assigned to a variable
declared to be of type Object. Objects of this type can hold
references to any object.
If you are using c# 4.0 then you can try this.
return ((dynamic)SourceObject).Name;
Because of inherent late binding, all VB variables of type Object are equivalent to c# objects which have been cast to dynamic -- or to put it another way, c# added dynamic in order to gain functional parity with VB.
So, the equivalent c# code would be ((dynamic) sourceObject).Name
I would look into doing this with Generics or an interface if possible, as thats a cleaner design.
Just be aware that VB's Late Binding bypasses type checking with your compiler. If the Object being passed in doesn't have a Name property, an exception could be raised at run time.
We are in the progress of introducing PostSharp in one of our projects. It's been working great so far! There is one thing though that we haven't managed to solve: how to fire an advice conditionally.
Details:
- we have an attribute StopWatchAttribute which makes it possible to record the time needed to run methods
- this attribute accepts an enumeration "LoggingLevel" which is set in the config file with values like 0, 1, 2 etc
- this parameter is read in a base class called BaseService during runtime: new BaseService().CurrentLoggingSettings
- we tried to set up the attribute constructor like StopWatchAttribute(new BaseService().CurrentLoggingLevel) but we get a compile error: an attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type.
--> summary: we would like the advice to be called conditionally and the condition depends on the parameter in the constructor of the attribute.
Is this possible to do?
Thanks for your help,
Andras
You cannot give variables to attributes, PostSharp or not. Since you're already reading the values from the config, just set your aspect to do the same on the Initialize() method. Override it in the aspect class and then save the value to a local field. You can use that field throughout the aspect. This compiles the value into the aspect essentially hard coding it.
Or, you can pull the value from the config from your advice method (OnMethodStart, etc) so that you can change it in the config at runtime. This is a more 'flexible' way to do it as it doesn't hard code anything.
Remember, your variables are being set at Runtime. PostSharp is a post-compile framework which means it does it's work long before your variables are even known to JIT.