Beginner question. How come I can do this:
Public Class Form1
Private StudentsInMyRoom As New ArrayList
Public Class student
Public name As String
Public courses As ArrayList
End Class
Private Sub btnCreateStudent_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCreateStudent.Click
Dim objStudent As New student
objStudent.name = "Ivan"
objStudent.courses = New ArrayList
StudentsInMyRoom.Add(objStudent)
End Sub
End Class
But I CANNOT do this:
Public Class Form1
Private StudentsInMyRoom As New ArrayList
Public Class student
Public name As String
Public courses As ArrayList
End Class
Dim objStudent As New student
objStudent.name = "Ivan"
objStudent.courses = New ArrayList
StudentsInMyRoom.Add(objStudent)
End Class
In the second example, all of the objStudent.etc get squiggly underlined and it says "declaration expected" when I hover over it. It's the same code except now it is not tied to clicking a button. Can't figure out what is the difference.
It's because the implementation needs to be in a method, the way you have it means the code couldn't possibly be executed, how would you reference this code from elsewhere?
It doesn't have to be tied to a click however:
Private Sub AnyNameYouLike
Dim objStudent As New student
objStudent.name = "Ivan"
objStudent.courses = New ArrayList
StudentsInMyRoom.Add(objStudent)
End Sub
Will work.
Rather than tell you how to fix this code directly, I'm going to explain what I think is going wrong with your thought process, so you can also do a better job writing code in the future.
What I see here is a simple misunderstanding for someone new to programming of how classes work. When you build and define a class, you are not (yet) allocating any memory in the computer, and you are not yet telling the computer to do anything. All you are doing is telling the computer about how an object might look at some point in the future. It's not until you actually create an instance of that class that anything happens:
Public Class MyClass
Public MyField As String
End Class
'Nothing has happened yet
Public myInstance As New MyClass()
'Now finally we have something we can work with,
' but we still haven't done anything
myInstance.MyField = "Hello World"
'It's only after this last line that we put a string into memory
Classes can only hold a few specific kinds of things: Fields, Properties, Delegates (events), and Methods (Subs and Functions). All of these things in the class are declarations of something, rather than the thing itself.
Looking at your samples, the code from your second example belongs inside of a method.
If you want this code to run every time you work with a new instance of your class, then there is a special method, called a constructor, that you can use. That is declared like this:
Public Class MyClass
Public MyField As String
'This is a constructor
Public Sub New()
MyField = "Hello World"
End Sub
End Class
However, even after this last example you still haven't told the computer to do any work. Again, you must create an instance of the class before the code in that constructor will run.
This is true of all code in all .Net programs anywhere. The way your program starts out is that the .Net framework creates an instance of a special object or form and then calls (runs) a specific method in that form to sort of get the ball rolling for your program. Everything else comes from there.
Eventually you will also learn about Shared items and Modules, that can (sort of) break this rule, in that you don't have to create an instance of the object before using it. But until you are comfortable working with instances, you should not worry about that too much.
Finally, I want to point out two practices in your code that professional programmers would consider poor practice. The first is ArrayLists. I can forgive this, because I suspect you are following a course of study that just hasn't covered generics yet. I only bring it up so you can know not to get too attached to them: there is something better coming. The second is your "obj" prefix. This was considered good practice once upon a time, but is no longer fashionable and now thought to be harmful to the readability of your code. You should not use these prefixes.
Related
This question already has answers here:
Classes vs. Modules in VB.NET
(8 answers)
Closed 3 years ago.
I want convert a C# code to vb.net console.
This is first time I find this two type code structure.
1.
Namespace ConsoleApp4
Module Program
Public Sub Main(ByVal args As String())
test()
End Sub
sub test()
end sub
End Module
End Namespace
2.
Namespace ConsoleApp4
Class Program
Public Shared Sub Main(ByVal args As String())
test()
End Sub
shared sub test()
end sub
End Class
End Namespace
what is the difference of this two type?
Sub Main must be shared to work as entry point of the application. It is automatically shared (or static) in modules. In classes the Shared keyword is required.
A VB module corresponds to a C# static class. Static classes and modules have only static members that can be used without having to create an object. In contrast, a non-static class must be instantiated to access its non-static (C#) or non-shared (VB) members
Module M
Public Function F(ByVal x As integer) As Integer
Return x * x
End Function
End Module
Class C
Public Function T(ByVal x As Integer) AS Integer
Return x + 10
End Function
End Class
With these declarations, you can write
Dim r1 As Integer = M.F(5) ' Or simply F(5) '
Dim o As C = New C() ' Must instantiate class, i.e., create an object.'
Dim r2 As Integer = o.T(32)
If you have variables (or properties) in a module, those exist exactly once. You can, however, create many objects from the same class and each object will contain another copy of these variables
Public Class Person
Public Property FirstName As String
Public Property LastName As String
End Class
Using this class declaration you can write
Dim list As New List(Of Person)()
list.Add( New Person With { .FirstName = "Terry", .LastName = "Adams"} )
list.Add( New Person With { .FirstName = "Lisa", .LastName = "Jones"} )
For Each p As Person In list
Console.WriteLine($"Person = {p.FirstName} {p.LastName}")
Next
Now you have two Person objects in the list having different first and last names.
Classes belong to Object-oriented programming (OOP). I suggest you to read some introductions about it, as .NET is mainly based on OOP concepts.
See also:
Object-oriented programming (Visual Basic)
Explain Object Oriented Programming in VB.NET
The difference between a Module and a Class is subtle; there is only ever one instance of a Module in all your program's life, and you cannot make multiple instances of it using the New keyword. By contrast you have to make an instance of a Class before you can use it..
In order to run, the .net framework runtime has to be able to find an available Main method without having to create an instance of a Class. This is achieved either by having the Main be kept inside a Module (and thus the Main is available because the module is available without having to instantiate a Class) or having it declared as Shared and be inside a Class (in which case you can conceive that a special thing happens that makes the shared Subs available without a class instance)
It's a hard difference to explain if you're not really well introduced to the concepts of OO programming and what "instances" actually means:
Class Person
Public Name as String
Public Sub New(n as String)
Name = n
End Sub
End Class
This declares a class of type person. Nothing about it refers to a particular person and it doesn't cause any Person object to exist in the computer memory until you use New:
Dim cj as New Person("Caius Jard")
Dim g as New Person("gxmgxm")
g.Name = "gsmgxm" 'correct a typo! this edits the name in the g object. it leaves cj's name alone
Now two instances of a Person object are in your computer memory, one named for me and one for you. You cannot do this with a Module. If we'd declared Person as Module there would only be one of them in all the entire program, accessed by the reference "Person":
Person.Name = "Caius Jard"
Person.Name = "gsmgxm" 'this overwrites my name. The program cannot remember more than one Person
and we couldn't have multiples. Consider that, at the time you launch your program, the runtime finds everything that is declared to be a Module and creates just one of them. This is somewhat vital for all sorts of advanced reasons that I won't get into, and Modules definitely do have their place in the grand scheme of things but thy aren't always incredibly useful in OO programming because more of the time we want more than one instance of a thing so we can model multiple things simultaneously.
So that's a precis on Class vs Module and why they are. In order to call any Sub or Function, you have to be able to call it on something. You have to have a DVD player before you can put a DVD in it and press Play - equally in programming, you have to have something that you can put your Main sub on, so you (or the .net runtime) can refer to it with Program.Main() and execute the instructions of the Sub.. That's how Subs and Functions work - theyre either associated with the special single instance (if it's a Module or a Shared Sub/Function of a Class), or they're associated with some object instance in the computer memory, and calling the Sub/Function acts on the object instance that was referred to:
Class Person
Public Name as String
Public Sub SetNameBlank()
Name = ""
End Sub
End Class
cj.SetNameBlank() 'the name of the cj object we declared before, is now blank
g.SetNameBlank()
By specifying the object instance name cj then the Sub name, we establish context - the actions listed in SetNameBlank will be carried out against the cj object, not the g one. If we'd declared SetNameBlank as Shared then they would have taken place in the shared context, not related to either cj or g, and Shared SetNameBlank() could be invoked without cj or g even existing
Going back to the reason you're asking your question right now, what the difference between these two (in the context of a "Main" Sub), the answer is..
..is "not much" from the perspective of getting your app going. Either approach is fine. Your Main method will have to start kicking things off my making object instances of the other Classes you have in your program and getting them to do things. It probably wont make new instances of the class that your Main is in so right now it doesn't really matter whether you put your main in a module or a class - it achieves the same end result that there is a Sub the runtime can call into to start things moving, without needing to create an instance of a Class first
https://learn.microsoft.com/en-us/dotnet/visual-basic/programming-guide/program-structure/main-procedure
I'm making a little text based adventure game in VB and I have different classes for different functions in the game (to keep organized). Well, I need two classes to reference each other. I'll explain.
So these two classes are Direct.vb and Commands.vb. I created references in the files so that they could execute methods in each other's code.
Direct.vb references Commands.vb like so:
Dim comm As New Commands
And then Commands.vb references Direct.vb in the same way.
Dim dir As New Direct
But when I run the program to see if it works, it throws a StackOverflowException. I think these classes are doing an infinite loop of references.
Is there a way I can fix this?
You can reference one class from the other, but you can't have them both create a new instance of the other.
In your current case, say you create one Commands, this creates one Direct, this creates one Commands, this creates one Direct, etc., etc.
This will fill the heap and the stack, but the stack runs out first with constructor functions.
The solution is to use a reference only, so omit the New keyword from at least one of these classes. Without knowing the specifics, it's not really possible to give more specific advise. Typically, though, this kind of construction would work:
Public Class Commands
Private dir As Direct
Public Sub New()
dir = New Direct(Me)
End Sub
End Class
Public Class Direct
Private cmd As Commands
Public Sub New(ByVal cmdInstance As Commands)
cmd = cmdInstance
End Sub
End Class
Specifically aimed at winforms development.
I suspect that the answer to this is probably No but S.O. has a nice way of introducing me to things I didn't know so I thought that I would ask anyway.
I have a class library with a number of defined methods therein. I know from personal experimentation that it is possible to get information about the application within which the class library is referenced. What I would like to know is whether it would be possible to get information about the value of a property of a control on a form when a routine on that form calls a method in my class library without passing a specific reference to that form as a parameter of the method in the class library?
So purely as an example (because it's the only thing I can think of off the top of my head). Is there a way that a message box (if it had been so designed to do so in the first place) could 'know' from which form a call to it had been made without that form being specifically referenced as a parameter of the message box in the first place?
Thanks for any insights you might have.
To address the example of the MessageBox, in many of the cases you can use the active form. You can retrieve it by using Form.ActiveForm. Of course, as regards the properties that you can request, you are limited to the properties provided by the Form or an interface that the Form implements and that the method in the other assembly also knows. To access other properties you can use Reflection, but this approach would neither be straightforward nor would it be clean.
In a more general scenario, you could provide the property value to the method as a parameter. If it is to complex to retrieve the value of the property and the value is not needed every time, you can provide a Func(Of TRESULT) to the method that retrieves the value like this (sample for an integer property):
Public Sub DoSomethingWithAPropertyValue(propValFunc As Func(Of Integer))
' Do something before
If propertyValueIsNeeded Then
Dim propVal = propValFunc()
End If
' Do something afterwards
End Sub
You call the method like this:
Public Sub SubInForm()
Dim x As New ClassInOtherAssembly()
x.DoSomethingWithAPropertyValue(Function() Me.IntegerProperty)
End Sub
I kind of question your intentions. There's no problem sending the information to a function or the constructor.
Instead of giving the information to the class, the class would ask for the information instead using an event.
Module Module1
Sub Main()
Dim t As New Test
AddHandler t.GetValue, AddressOf GetValue
t.ShowValue()
Console.ReadLine()
End Sub
Public Sub GetValue(ByRef retVal As Integer)
retVal = 123
End Sub
End Module
Class Test
Public Delegate Sub DelegateGetValue(ByRef retVal As Integer)
Public Event GetValue As DelegateGetValue
Public Sub ShowValue()
Dim val As Integer
RaiseEvent GetValue(val)
Console.WriteLine(val)
End Sub
End Class
this is my first post here.
I like to have some advice on designing a new module for our software.
It's written in vb6 and i want to rewrite it in vb.net. So I want to refactor it.
It does the following:
We got many templates for OpenOffice Documents. (about 300)
Each one has an unique identifier which is in a database and comes as the input.
Dending on this number i want to create the document. e.g. calc or writer or later something else.
Then call a method named doc[template_num] or sth.
I read about invoking methods. I think this as a good way to dynamically call methods by number. Tried to do this. I think I understood now how it works.
But I dont know how to handle the document type.
I want to reuse big parts of code due to all calc documents are created equal at the beginning. But filling the cells is just bit diffrent.
I dont know how to inherit and do method calls with invoke.
Maybe someone has a little code snip for me which can explain this to.
Or maybe some other good idea how to handle this problem.
I am thankful for any new thought on this.
You could, of course, just have a giant Select Case block, like this:
Public Sub CreateDoc(templateNum As Integer)
Select Case templateNum
Case 1
CreateDoc1()
Case 2
CreateDoc2()
Case 3
CreateDoc3()
End Select
End Sub
If you had a fairly small number of case statements that would probably be the better, simpler method. If however, as you described, you have many different possible ID's to look for, then that becomes impractical and I can appreciate your desire to invoke the correct method without a big Select Case block.
However, before I get into that, I feel like I should give you another option. Perhaps you don't really need to have that many different methods. Is it possible, for instance, that half of the template ID's actually all need to be handled in the same way, and the only difference is the template file name that needs to be different. If so, it is conceivable that the Select Case block wouldn't need to be that big. For instance:
Public Sub CreateDoc(templateNum As Integer)
Select Case templateNum
Case 1 To 100
CreateWriterDoc(GetTemplateFilePath(templateNum))
Case 101 To 200
CreateCalcDoc(GetTemplateFilePath(templateNum))
End Select
End Sub
Private Sub CreateWriterDoc(templateFilePath As String)
' Launch Writer with given template file
End Sub
Private Sub CreateCalcDoc(templateFilePath As String)
' Launch Calc with given template file
End Sub
Private Function GetTemplateFilePath(templateNum As Integer) As String
' Retrieve template file path for given ID
' (from DB, Array, List, Dictionary, etc.) and return it
End Sub
To me, that seems like a much simpler solution, if such a thing is possible. If not--if you really have entirely different logic which needs to be executed for each template ID, then there are several ways to do it. The first option would be to create a list of delegates which point to the appropriate method to call for each ID. For instance, you could store them in an array, like this:
Dim createDocDelegates() As Action =
{
AddressOf CreateDoc1,
AddressOf CreateDoc2,
AddressOf CreateDoc3
}
So now, those three delegates are indexed by number (0 through 2) in an array. You can invoke them by number, like this:
createDocDelegates(1).Invoke() ' Calls CreateDoc2
Or, if your ID's aren't sequential, you may want to use a Dictionary(Of Integer, Action), instead, like this:
Dim createDocDelegates As New Dictionary(Of Integer, Action)()
createDocDelegates.Add(1, AddressOf CreateDoc1)
createDocDelegates.Add(7, AddressOf CreateDoc7)
createDocDelegates.Add(20, AddressOf CreateDoc20)
Then you can call one by ID, like this:
createDocDelegates(7).Invoke() ' Calls CreateDoc7
Another option would be to create an Interface for an object that creates a document, like this:
Public Interface IDocCreator
Sub CreateDoc()
End Interface
Then you could implement a separate class for each type of template (each implementing that same interface). For instance:
Public Class Doc1Creator
Implements IDocCreator
Public Sub CreateDoc() Implements IDocCreator
' Do work
End Sub
End Class
Public Class Doc2Creator
Implements IDocCreator
Public Sub CreateDoc() Implements IDocCreator
' Do different work
End Sub
End Class
Then, you can create a list of those objects, like this:
Dim docCreators() As IDocCreator =
{
New DocCreator1(),
New DocCreator2()
}
Or:
Dim docCreators As New Dictionary(Of Integer, IDocCreator)()
docCreators.Add(1, New DocCreator1())
docCreators.Add(7, New DocCreator7())
docCreators.Add(20, New DocCreator7())
Then you can call one like this:
docCreators(1).CreateDoc()
The IDocCreator approach is very similar to the Delegate approach. Neither is the right or wrong way to do it. It depends on your style, what you are comfortable with, and your requirements. The main advantage of the IDocCreator approach is that you could easily add more properties and methods to it in the future. For instance, lets say that you also want to somewhere store a user-friendly, descriptive name for each template. It would be very easy to add a ReadOnly Property Name As String property to the IDocCreator interface, but if you go the list-of-delegates route, that would be more difficult and sloppy.
In any of the above examples, however, you still have to add the complete list of methods or delegates somewhere. So, while it's slightly less ugly than a giant Select Case block, it may still not be enough. If so, the technology you need to use is called Reflection. The System.Reflection namespace contains much of the reflection-related functionality. Reflection allows you to dynamically access and invoke portions of your code. For instance, you can use reflection to get a list of all of the properties or methods that are defined by a given class. Or you can use reflection to get a list of types that are defined by your assembly. Using reflection, it would be possible to get a method, by string name, and then invoke it. So, for instance, if you want to call the "CreateDoc1" method on the current object, you could do it like this:
Me.GetType().GetMethod("CreateDoc1").Invoke(Me, {})
Since you are calling it by its string name, you could build the method name via concatenation:
Dim methodName As String = "CreateDoc" & templateNum.ToString()
Me.GetType().GetMethod(methodName).Invoke(Me, {})
However, if you are going to use the reflection approach, instead of using the name of the method, like that, it would be cleaner, in my opinion, to use a custom attribute to tag each method. For instance, you could create an attribute class which allows you to decorate your methods like this:
<CreateDocMethod(1)>
Public Sub CreateDoc1()
' ...
End Sub
Then, at run-time, you can use reflection to find all of the methods that have that particular attribute and then invoke the right one.
The reason that I saved the discussion on reflection for last is because it is not as efficient and it can lead to brittle code. As such, it's best to only use reflection as a last-resort. There's nothing wrong with reflection, if you really need it, but, as a general rule, if there is another reasonable way to do something, which doesn't require reflection, then you probably ought to be doing it that other way.
Whats the difference between these two initialization methods for obj? I've seen both of these, but know know if there is an appropriate time to use one vs the other. I found this post which covers C#, but not sure if the same applies to VB.Net.
Public Class Class1
Sub New()
End Sub
Dim obj As New Object
End Class
vs
Public Class Class1
Sub New()
obj=New Object
End Sub
Dim obj As Object
End Class
My apologies ahead of time if this a duplicate.
In this case, there is no difference. The main difference would be if your constructor does other operations -
In that case, the inline initialization (Dim obj As New Object) will occur prior to any code inside the constructor. Putting the initialization in the constructor lets you choose the order of initialization.
Virtually nothing is different about these samples. In both cases you get the following order of operations
Constructor for Class1 is called
Base constructor (Object in this case) is called
Field obj is assigned a value
In all likely hood it results in identical IL being generated.
Your first version is "declarative". The advantages of it are:
It is easier to see how an field as initialized, as you don't have to look for it in the constructor.
If you have multiple constructors you don't need to repeat yourself.
Your first version is "imperative". The advantages of it are:
You control exactly when it gets created.
You can have different versions for each constructor.
I personally default to declarative style code whenever possible.