I have a little problem with an interface. A bunch of my classes implement the ILayoutObject interface. A method declares a variable as ILayoutObject (defaulting it as Nothing) and then runs some code which decides which object it should be. The problem is, the evaluation code runs in a method which receives the variable as a parameter and assigns an object to it. With objects, this would be no problem. The object would be affected by the changes in the method and everything would be OK. Howeverm, when using an interface, the variable in the calling code remains Nothing and behaves like a normal variable. Does anyone have any ideas on how to circumvent that? Alas, due to code structure I am unable to use ByRef or functions :(
Here is some code:
Protected LayoutHandler As Dictionary(Of String, Action(Of Constants.OptionsEntryStructure, ILayoutElements)) = New Dictionary(Of String, Action(Of Constants.OptionsEntryStructure, ILayoutElements)) From
{
{Constants.KeyLayoutType, AddressOf KeyLayoutType}
}
Sub MakeLayOuts
Dim LayoutElement As ILayoutElements = Nothing
Dim Value = "SomeValues"
Dim key = "Key"
LayoutHandler(key)(Value, LayoutElement)
' LayoutElement remains nothing.....
End Sub
Protected Sub KeyLayoutType(elem As Constants.OptionsEntryStructure, Layout As ILayoutElements)
Layout = New LayoutObject 'which would implement the interface
End Sub
You need to declare the parameter as ByRef if you want to alter the object to which the variable in the calling code points to:
Protected Sub KeyLayoutType(elem As Constants.OptionsEntryStructure, ByRef Layout As ILayoutElements)
Layout = New LayoutObject 'which would implement the interface
End Sub
This is true with any reference type (classes). The fact that they are referenced with an interface makes no difference.
If you can't use ByRef, and you can't use a function to return the new object, then your only other real option would be to request a type of object which has the layout object as a property. For instance:
Public Interface ILayoutElementContainer
Public Property LayoutElement As ILayoutElements
End Interface
Protected Sub KeyLayoutType(elem As Constants.OptionsEntryStructure, Container As ILayoutElementContainer)
Container.LayoutElement = New LayoutObject 'which would implement the interface
End Sub
Related
I'm working on a class to parse templates. Here's an abbreviated version of what I have:
Public Class Parser
Public Vars As New Dictionary(Of String, String)
Public Tags As New Dictionary(Of String, String)
Private Template As String
Public Sub New(ByRef _Template As String, Optional ByVal _Recursing As Boolean = False)
Recursing = _Recursing
Template = _Template
End Sub
Public Sub Go()
' Do stuff, extract a portion of the template
Dim Page as Parser = New Parser(portion, True)
Page.Go()
End Sub
End Class
As you can see, it's designed to be recursive. When the class is first instantiated, the caller passes a template name to the constructor, then sets various stuff and calls Go(). The problem comes when it recurses: Go() creates a new instance, and instead of the template name, passes a portion of the template to be worked on, which is why _Template is ByRef. The problem is, the new instance of Go() does not have access to it. VB doesn't have pointers, so is there any way to make that reference available to the rest of the class from the constructor? I've googled around and read a number of articles here, but can't find anything that answers my question. This article makes it sound like the Object data type will do what I want, so I tried
Private Template As Object
but it doesn't do what I want. I also found this which seems to be asking the same question, and seems to say that changing the parm to ByVal does the trick, but that makes no sense.
Is there a way to declare a property of type dictionary of string, string in VB.Net.
I am using this on a usercontrol to add properties via the designer.
I tried the following:
Private v As Dictionary(Of String, String)
Public Property VList As Dictionary(Of String, String)
Get
Return v
End Get
Set(ByVal value As Dictionary(Of String, String))
v = value
End Set
End Property
But when I try this the string collection editor window opens up but the add & remove buttons are disabled. What is the correct way to declare this property?
I want to add the key & value via the designer.
The Dictionary does not have a built in UITypeEditor. There are many reasons why there isn't: there are 2 Types which are generic, it also doesnt have an Item accessor, there is no simple Add method, the key must be unique and there is no built in way to serialize a Dictionary "item".
The right way is to use a Collection class inheriting from Collection<T> so you can control access to the contents (note: this is from System.Collections.ObjectModel not the horrible VB Collection!). The fast way to setup a working interface is to use a List(Of myTypeClass), but this is dangerous in production code because it allows all sorts of actions on the innerlist which you likely do not want.
<Serializable><TypeConverter(GetType(FooConverter))>
Public Class FooBar
<DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)>
Public Property Name As String
<DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)>
Public Property Value As String
' simple ctor REQUIRED for the UITypeEditor
Public Sub New()
Name = ""
Value = ""
End Sub
' ctor for the TypeConverter (NOT included)
Public Sub New(n As String, v As String)
Name = n
Value = v
End Sub
Public Overrides Function ToString
Return Name
End Sub
End Class
' must be instanced
Private myFoo As New List(Of FooBar)
' list is an object so it cant be serialized, but the CONTENTS can be
<DesignerSerializationVisibility(DesignerSerializationVisibility.Content)>
Public Property FooList As List(of FooBar)
Get
If myFoo Is Nothing Then
myFoo = New List(of FooBar)
End If
Return myFoo
End Get
Set
' do nothing
End Set
End Sub
' for designer serialization
Private Function ShouldSerializeFooList As Boolean
Return myFoo.Count > 0 ' or myFoo IsNot Nothing
End Sub
public Sub ResetMyFolist
myFoo = New List(of FooBar)
End Sub
Caveats:
It is almost always better to write a class container for the Foobar items. Usually you would inherit from Collection<T>. List<T> as shown is a container and a collection, so the contents can be cleared, reset, modified etc when exposed as shown. They are fast and easy to implement though and the basic concept is the same.
If a Dictionary is really what you want, you can write your own UITypeEditor (not UIDesigner, this is not a control) but this would probably require a great deal of work on many levels. The reason there are not gobs of them flying around is that most people make do with one of the standard collections and simply enforce unique names in other ways. (Adding "Properties" to a usercontrol, suggests that really the key or name ought to be fixed and known to the app ahead of time so it knows what it is and what to do with it(?)).
Often VS can perform designer serialization on its own with simple properties like those in FooBar. However, since they are items in a collection, you will likely need to also write a TypeConverter which can return an InstanceDescriptor, to help VS instance them. But that is a different question.
I'm used to programming in C# so I have no idea how to approach delegates and passing methods in VB
The error that I am getting is: Argument not specified for parameter 'message' of 'Public Sub ReceiveMessage(message As String)'
Here is the constructor of the class that I am trying to pass to:
Delegate Sub ReceiveDelegate(message As String)
Public ReceiveMethod As ReceiveDelegate
Sub New(ByRef receive As ReceiveDelegate)
ReceiveMethod = receive
End Sub
This is the method that I am trying to pass to that constructor:
Public Sub ReceiveMessage(message As String)
MessageBox.Show(message)
End Sub
I'm using it as such:
Dim newClass As New Class(ReceiveMessage)
The purpose of this, is that once the class receives data from a network device, it can call the corresponding method on the Form asynchronously.
You need to create the delegate object and use the AddressOf operator, like this:
Dim newClass As New Class(New ReceiveDelegate(ReceiveMessage))
However, if you don't explicitly create the delegate object, VB.NET will automatically determine the right type, based on the signature, and create it for you, so you can just do it like this:
Dim newClass As New Class(AddressOf ReceiveMessage)
The latter is obviously less typing, but the former is more explicit. So, take your pick. Both ways are perfectly acceptable and common.
Is this a bad idea? Does calling a generic private constructor within a public constructor create multiple instances, or is this a valid way of initializing class variables?
Private Class MyClass
Dim _msg As String
Sub New(ByVal name As String)
Me.New()
'Do stuff
End Sub
Sub New(ByVal name As String, ByVal age As Integer)
Me.New()
'Do stuff
End Sub
Private Sub New() 'Initializer constructor
Me._msg = "Hello StackOverflow"
'Initialize other variables
End Sub
End Class
That's perfectly valid and a commonly used way to reuse constructor code. Only one object is instantiated.
It is a valid approach. There are some caveats with where the new function can be called:
The Sub New constructor can run only once when a class is created. It
cannot be called explicitly anywhere other than in the first line of
code of another constructor from either the same class or from a
derived class.
Read more about the object lifetime on MSDN.
Chaining constructors like this will certainly not create additional object instances.
It is desirable to only write code for a certain portion of initialization once. This is a common and valid initialization pattern.
I've seen some other responses about this and they talk about interfaces but I'm pretty sure you can do this with classes and base classes but I can't this to work.
Public Class Behavior
Private _name As String
Public ReadOnly Property Name As String
Get
Return _name
End Get
End Property
Public Property EditorUpdate As Boolean
Public Sub New(ByVal name As String)
_name = name
EditorUpdate = False
End Sub
Public Overridable Sub Update()
End Sub
' runs right away in editor mode. also runs when in stand alone game mode right away
Public Overridable Sub Start()
End Sub
' runs after game mode is done and right before back in editor mode
Public Overridable Sub Finish()
End Sub
' runs right when put into game mode
Public Overridable Sub Initialize()
End Sub
' runs when the game is complete in stand alone mode to clean up
Public Overridable Sub Destroy()
End Sub
End Class
Public Class CharacterController
Inherits Behavior.Behavior
Public Sub New()
MyBase.New("Character Controller")
End Sub
Public Overrides Sub Update()
' TODO: call UpdateController()
' THINK: how can UpdateController() get the controller entity it's attached to?
' Behaviors need a way to get the entity they are attached to. Have that set when it's assigned in the ctor?
End Sub
End Class
Dim plugins() As String
Dim asm As Assembly
plugins = Directory.GetFileSystemEntries(Path.Combine(Application.StartupPath, "Plugins"), "*.dll")
For i As Integer = 0 To plugins.Length - 1
asm = Assembly.LoadFrom(plugins(i))
For Each t As Type In asm.GetTypes
If t.IsPublic Then
If t.BaseType.Name = "Behavior" Then
behaviorTypes.Add(t.Name, t)
Dim b As Behavior.Behavior
b = CType(Activator.CreateInstance(t), Behavior.Behavior)
'Dim o As Object = Activator.CreateInstance(t)
End If
End If
Next
Next
When it tries to convert whatever Activator.CreateInstance(t) returns to the base class of type Behavior I'm getting invalid cast exception. That type should be of CharacterController which is defined as a child of Behavior so why wouldn't it let me cast that? I've done something like this before but I can't find my code. What am I missing?
This may not be an answer to your question (it also might resolve your exception -- who knows), but it is something that needs to be pointed out. These lines:
If t.IsPublic Then
If t.BaseType.Name = "Behavior" Then
Should really be changed to one conditional like this one:
If t.IsPublic AndAlso (Not t.IsAbstract) AndAlso _
GetType(Behavior.Behavior).IsAssignableFrom(t) Then
Otherwise, if somebody defines a random type called "Behavior" in their own assembly and derives it from another type, your code will think it is a plugin. Additionally, if someone derives your Behavior type and then derives that type (two levels of inheritance) this code will incorrectly skip over that type. Using the IsAssignableFrom method is a quick and easy way to ensure that one type does actually derive from the specific type you want (instead of any type that shares the same name), even if there is another type in between your types in the inheritance tree. The additional check against t.IsAbstract will also ensure that you don't try to instantiate an abstract subtype of your base plugin type.
This works for me:
Dim ctor As Reflection.ConstructorInfo = _
t.GetConstructor(New System.Type() {})
Dim o As Object = ctor.Invoke(New Object() {})
Dim plugin As Plugin = TryCast(o, Plugin)
(If I find t, I invoke the parameterless constructor.)
[I just realized this is probably what Activator.CreateInstance does, so I replaced my code with yours and it worked your way -- so this probably won't help you]