Polymorphism And Shadowing Inherited Members?
Nov 17, 2009
I have a couple of small classes to represent parts in a search filter. If the searched value equals NonValue the filter is supposed to do nothing. This is defined in a Base Class:
Private Class BaseFilter
Protected NonValue As Object
Protected sQueryStringBase As String = "AND {0} {1} {2} "[code]......
When I then create a StringFilter and check for allowed value:
Dim stf As New StringFilter()
stf.CheckNonValue(MyString)
I get a NullReferenceException (NonValue = Nothing) , when I expected the NonValue object to be String.Empty. Is this a bug in my code, or am I trying to achieve polymorphism in a wrong way?
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Apr 21, 2010
When I pass an an object 'MyObject' which implements 'IMyInterface' to a method parameter declared as IMyInterface, I understand that this method parameter 'sees MyObject through the eyes of its interface'.
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Feb 3, 2010
Curious situation:
public class MyTextBox : TextBox
{
[code].....
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Jan 5, 2010
ok this is annoying, i've got a structure RInteger.
Private VAR_Value As Integer
Private VAR_Max As Integer
Private VAR_Min As Integer
[code]......
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Sep 8, 2009
In VB.NET there is a keyword 'shadows'. Let's say I have a base class called 'Jedi' and a derived class called 'Yoda' which inherits from 'Jedi'. If I declare a method in 'Jedi' called 'ForcePush' and shadow that out in 'Yoda' then when calling the method on an instance of the 'Yoda' class, it will ignore the base class implementation and use the derived class' implementation. However if I have an instance of 'Yoda' that was declared originally as of type 'Jedi', i.e. Dim j as Jedi = new Yoda(), and called the 'ForcePush' method on the instance, it will use the Jedi implementation.
Now let's say I have an event that is called 'UsingForce' which is raised when the 'ForcePush' method is called, and I shadow the event out in the derived class (this is because 'Yoda' has an interface 'IForcePowers' that declares this event) and each class raises it's respective event.
If I have an instance of 'Yoda' that is declared as type 'Jedi' (like above) and I put an event handler on the 'UsingForce' event of 'Jedi', and then the 'ForcePush' method is called in the 'Yoda' class, will this event handler be reached?
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May 4, 2011
I have a two windows forms classes, a base class and a derived class. The base class has an event handler which handles ValueChanged on some component. I have also written a different event handler for the same event on the derived class.
When I create an instance of the derived class and fire the event, I find that both event handlers run (the base class one and then the derived class one). But I want only the handler in the derived class to run.
Edit: Here is what the code looks like (can't post the actual code):
Public Class BaseForm
Inherits System.Windows.Forms.UserControl
(Windows Form Designer Generated Code)
[Code].....
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Dec 4, 2011
It works, but doesn't save what I've changed it to. In the designer everything works fine. I can change the text and have it update on my button control. BUT once I run or rebuild in any way, the text property changes back to it's default value, in my case "Button Control"These are what I've tried, and I've tried all with both Shadows and Overrides - both with identical outcomes. So I've just indicated that with Shadows/Overrides.
vbnet
<DefaultValue("Button Control"), Browsable(True)> _ Public Shadows/Overrides Property Text As String Get Return MyBase.Text End Get Set(ByVal value As String) MyBase.Text = value Me.Invalidate() End Set End Property Private _Text As String = "Button Control" <DefaultValue("Button
[code]....
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May 8, 2009
shadowing and delegate method?
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Mar 4, 2010
I'm in a project where it's pretty much my first time doing all the architecture myself,and I'm running into a frustrating situation. My architecture for forms seems to be correct from a heuristic perspective, but I don't think its implementation is correct.
My architecture is thus:
Base Class: OrderForm
Child Classes: PurchaseOrder, Invoice, Credit
[code].....
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May 13, 2011
I have created a new usercontrol. My usercontrol contains several other controls like labels, buttons and so on ...When I set the Font property of the usercontrol I want that font property value to propagate down to the child controls.
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Jun 24, 2010
I have a base form and I inherit other forms from it. However, if i go back and change say the size of the base form, I don't see those changes take effect in my inherited forms. Does anyone know why that is? Is there a solution to this issue?So as far as design is goes, what exactly gets inherited by other forms?
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Apr 4, 2011
Shadows vs. Overrides in VB.Net.What's the difference between shadowing a function in a base class in a subclass and overriding the same function? There is performance issues involved too?I know how to shadow and how to override in VB.net. My question is about when and why should I shadow a function instead override it and vice-versa.
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Jan 6, 2012
I have a function called Modify. It is delcared like so:Public Function Modify(Of SIMType As {New, DAOBase})(ByVal obj As DAOBase) As Boolean
You can see that this function is generic. It takes as a paramer a object that is a DAOBase or subclasses of DAOBase.Inside the modify function there is a call like so:
DAOToGP(obj)This is where the polymorphism comes into play. There are four or so subclasses I have created of DAOBase. I have written a DAOToGP() for each of these types. So in the Modify() function, when it calls the DAOToGP(obj), polymorphism should kick in and it should call the correct implementation of DAOToGP() depending on the type that I pass into Modify().
However, I get the following error:Error 20 Overload resolution failed because no accessible 'DAOToGP' can be called without a narrowing conversion:'Public Shared Function DAOToGP(distributor As Distributors) As Microsoft.Dynamics.GP.Vendor': Argument matching parameter 'distributor' narrows from 'SierraLib.DAOBase' to 'IMS.Distributors'.'Public Shared Function DAOToGP(product As Products) As Microsoft.Dynamics.GP.SalesItem': Argument matching parameter 'product' narrows from 'SierraLib.DAOBase' to 'IMS.Products'. C:Usersdvargo.SIERRAWOWIRESDocumentsVisual Studio 2010ProjectsSIMDev_2SIMIMSDVSIMLibGPGPSIMRunnerRunnersRunnerBase.vb 66 39 IMS
I am kind of at a loss here. I am not sure why it cant figure out which function to call.
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May 15, 2009
Okay this is a bit of a noobish question, but I ran across this today and it somewhat puzzles me WHY it would be this way.
Consider the following structure
Public Class Employee
Implements IPerson
Private _MyManager As Manager
[Code].....
Here is my question: I am wanting to have every person implement the IPerson interface, whether they are an employee, boss, or manager. However, they each have an attribute that refers to their leader (employees have a manager, managers have a boss, etc). Each implementation of their leader could be potentially differnt other than they fact that they implement the IAuthorityFigure interface. I am wanting to have the IPerson interface have a property for the IAuthorityFigure, but it throws a compiler error for me when I implement this b/c the IAuthorityFigure is not the same type as Boss or Manager (even though they implement the interface).
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Oct 29, 2011
i have assignment which using polymorphism to get answer. Below here is answer from the question.anyone give me a code using polymorphism to get this answer
i= 100, j= 200, k= 300
value for i =100
value for(i+j)/2= 150
value for(i+j+k)/3 =200
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May 20, 2011
I have a class that has two methods called "Add." One method excepts a FileInfoA object and the other excepts a DirectoryInfoA objectSo in my code, I'm adding FIAs and DIAs to property lists. But, it throws an exception because it's trying to process a FIA as a DIA, which means that the FIA is being added to the group of DIAs.I commented out the adding of DIAs to see if it would work and it did. When the DIA is commented out, the FIAs are filtered properly and the code executes properly.This is how my methods are declared:
VB.NET
''' <summary> ''' Adds a DirectoryInfoA class and adds it to one of the appropriate list. ''' </summary> ''' <param name="di">DirectoryInfo</param> '''
[code]....
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Jan 31, 2012
I wasn't quite sure how to label this but that should cover it all. I am using Linq to Sql classes to query a SQL Server table. That part is working fine. I'd like to add the records to a listbox and not just text. I can do this and it works. To get it to display something meaningful in the listbox, I have to override the ToString() function.
So, I've taken the base class tblVendor and created a new sub class tblVendorListObject that inherits tblVendor and overrides ToString to return Me.vendorName. I then use a foreach loop to iterate through the records returned by the query. Next, I create a new instance of tblVendorListObject and attempt to assign the current item in the foreach loop to it.
[Code]...
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Mar 24, 2011
Suppose I have an interface IFoo with implementation classes VideoFoo, AudioFoo, and TextFoo. Suppose further that I cannot modify any of that code. Suppose that I would then like to write a function that acts differently based on the runtime type of IFoo, such as
[Code]...
Then I have to do my "If TypeOf ... Is" again. How can I refactor this to take advantage of the polymorphism of the implementations of IFoo without manually checking the types? (in VB.NET, though my question applies to C# too)
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Sep 16, 2011
In VB.NET, say I have a function Public Function Foo(ByVal currentShape as Shape) Instead up passing in a Shape object, I pass in a subclass of Shape called Square like such: Dim square As Square = new Square() Foo(square). Do I need to convert my Square object to a Shape object before passing it in? If so, how do I do this?
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Mar 8, 2010
Can you define this :
Class
Object
Inheritance
Polymorphism
Abstraction
And could you make it easy to understand I'm having a hard time to understand those terms.
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Jan 29, 2010
In a program I am writing (in VB.NET, 2008), I have 2 classes, "Receipt" and "Group", each with similar properties, both implementing a custom interface entitled "IIDUser". In my program, I have a number of lists of "Receipt" and "Group", declared like:Friend receipts as List(Of Receipt)
I have written a few functions that apply equally to managing both lists of receipts and groups, declared like this one:Friend Function getNextAvailableUserLabel(ByRef theList As IList(Of IIDUser), ByVal startLabel As Short) As Short
Then called like this:getNextAvailableUserLabel(receipts, s)
I did all this in an attempt for interface-based polymorphism, as I thought writing 2 identical functions (except for that one would be for lists of receipts while the other for lists of groups) would be bad practice (and waste time). However, I get an "InvalidCastException" during runtime. It is explained, "Unable to cast object of type 'System.Collections.Generic.List`1[Receipt_Sorter_v2.Receipt]' to
[Code]...
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Feb 16, 2012
I'm trying to create a class for storing data on People with another class to store their Bank Transactions.Ideally, this all be hidden away and leave only simple statments, declarations and functions available to the programmer.[code]I know this is possible as these exist in the Listbox Class though can't figure out how it's done.
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Oct 24, 2011
Im trying to search the entire active directory to look for a group and then see if the current logged on user is listed, then take an action based on that result.So far i have the below code which doesnt return the users who are members of the group MyTestGroup. Could anyone assist or guide me to see how i achieve this?
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Aug 22, 2009
on the custom inherited control i created, i noticed that when i bind my custom property to a data column, if that data column returns a value other than Null it always return a datarowstate of modified even if there were no changes. to produce this issue, i have below a sample code for a test inherited control
Imports System.Windows.Forms
Imports System.ComponentModel
Public Class TestControl
[code].....
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Nov 29, 2010
According to this post the code below should compile, while it is not.
class Base
protected m_x as integer
end class
[code].....
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Oct 25, 2009
I migrate between C++ and VB.NET in my coding ventures... which leads to the occasional confusion about when something is by value or by reference in VB.NET.Let's say for example that I have an array of MyObject which is populated with a bunch of objects.
dim MyArr(5000) of MyObject. let's say that the information from this array travels throughout various data structures:
dim MyList as new List(of MyObject)
for i as integer = 0 to 5000 step 1000
Mylist.add(MyArr(i))
next
Under the above scenario, I believe everything is by reference. If I extract an entry from "MyTable" and modify its MyObject Members, I think that the original in MyArr will be modified.However, I have run into situations where I thought something was by reference, and it was by value. Are items always added to containers by reference, or are they sometimes added by value?
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Apr 16, 2010
I'm trying to change the data binding of a listbox when the value of a combo box changes.Here is the ComboBox change code - [code]...
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Jul 2, 2010
I'm having some headaches using Structures and functions that return Nothing in VB.NET. [Code] In the previous code, when I return Nothing as result of Foo function I'd expect that st is Nothing. But this is not what happens. Then I found in MSDN documentation: Assigning Nothing to a variable sets it to the default value for its declared type. If that type contains variable members, they are all set to their default values.
So I discovered that when I assign Nothing to a structure, all its members are set to their default values, instead of the structure itself.
Also, I tried to make st a Nullable type by declaring: Dim st As Nullable(Of Test) = Foo()
but, still I can't check if st is Nothing by using: If st Is Nothing Then
or If st.Equals(Nothing) Then
So, questions:
1 - Is it possible to assign Nothing to the structure and not to its members?
2 - How can I check if a return structure value is Nothing?
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Jan 23, 2011
According to a mspress book (MCTS for Exam 70-536 .NET 2.0): You might have version compatibility issues if you ever attempt to deserialize an object that has been serialized by an earlier version of your application. Specifically, if you add a member to a custom class and attempt to deserialize an object that lacks that member, the runtime will throw an exception. In other words, if you add a member to a class in version 3.1 of your application, it will not be able to deserialize an object created by version 3.0 of your application.
Now... As curious as I am I went and created a project, serialized a class, added a new member and attempted to deserialize the class to the new object. To my surprise it worked and the newly created member was set to null by default (even if it had another default value).
[Code]...
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Oct 25, 2010
I wrote some code to add the Name and Value to Combobox.
dt = New DataTable
dt.Columns.Add("Name")
dt.Columns.Add("Value")
[Code]....
Now i want to enumerate all the ValueMemer (11,22,33,44) and all the DisplayMember (AA,BB,CC,DD) from Combobox1, how to do?
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