One of my library classes exposes a property that looks somewhat like this:
Private _myCollection As New ObservableCollection(Of String)
Public ReadOnly Property MyCollection As ObservableCollection(Of String)
Get
Return _myCollection
End Get
End Property
I don't know too much about databinding, but I'm assuming that the framework will automatically notify about changes when items are added to / removed from the underlying _myCollection field (correct me if I'm wrong).
What I'd like to know is what happens when _myCollection is nulled or replaced with a different collection:
Sub WhatHappensToMyObservableCollectionWhen(updated As Collection(Of String))
' nulling underlying field
_myCollection = Nothing
' changing underlying field
_myCollection = New ObservableCollection(Of String)(updated)
End Sub
Will the framework still notify that MyCollection has changed, or do I have to implement INotifyPropertyChanged and raise PropertyChanged manually when these things occur?
EDIT
I'm asking this question with WPF developers in mind, but I guess the answer will be the same for all observers of PropertyChanged. If a GUI written in WPF binds to MyCollection, I want to make sure that it is properly updated when the _myCollection field changes in the ways that I have described.
Thanks
Related
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
I have one class implementing ISerializable interface, with one private field that is saved and retrieved:
Protected Sub New(ByVal info As SerializationInfo, ByVal context As StreamingContext)
_name= info.GetString("_name")
End Sub
Now, I want to add a new private field, but since it did not exist previously, when I try to open a serialized object I get an exception when trying to get this field, in the custom deserialization constructor. The solution I've come up with is to use a Try...Catch block, as follows:
Protected Sub New(ByVal info As SerializationInfo, _
ByVal context As StreamingContext)
_name= info.GetString("_name")
Try
_active= info.GetBoolean("_active")
Catch ex As Exception
_active= False
End Try
End Sub
It works, but it seems a bit messy to me, moreover because I will have to add a new Try...Catch block for each new field I want to add.
Is there any better way of doing this?
Thanks!
EDIT:
In this answer t0mm13b says that "There is a more neater way to do this in 2.0+ upwards". Any ideas about what is he talking about?
There is no built-in method that allows you to first determine if a particular value exists (e.g. Exists). There is also no Get... method which simply returns Nothing if the value does not exist (e.g. TryGet). However, the one way that you can do it which avoids the potential for exceptions is to use a For Each loop to iterate through all of the values in the SerializationInfo collection. For instance:
For Each i As SerializationEntry In info
Select Case i.Name
Case "_name"
_name = DirectCast(i.Value, String)
' ...
End Select
Next
Note: It may be surprising to you that you can use a For Each loop on a SerializationInfo object, since it does not implement the IEnumerable interface (nor the generic equivalent). However, For Each in VB.NET actually works with any object that has a public GetEnumerator method. So, even though it leads to some confusion, objects don't technically need to implement IEnumerable in order for you to iterate through them with a For Each loop. Since the SerializationInfo class does expose a public GetEnumerator method, you can iterate through it with a For Each loop.
I've been using classes for a while now, but I feel I may have been using them incorrectly.
When I create the properties for the class, I just use public variables so I end up with something like the following:
Class clsMyClass
Public Name As String
End Class
However, I've been reading some info on the net and they suggest that it should be set up in the following way:
Class clsMyClass
Private Name As String
Property UsersName() As String
Get
Return Name
End Get
Set(ByVal Value As String)
Name = Value
End Set
End Property
End Class
Is the way I'm doing it extremely incorrect? If so, why? I feel like the second method adds some sort of security but to be honest, it just looks like unnecessary code..?
One advantage of properties is that they let you customise the access to your private fields and enable you to do more so you can do the following (examples, it's not limited to that):
Make a property read-only for public access
Raise an even when a property is updated
Update other private fields when a property is updated
Validate the value that is being set
See below advantages of Properties over Variables from the C# in Depth article:
• There's more fine-grained access control with properties. Need it to be publicly gettable but really only want it set with protected access? No problem (from C# 2 onwards, at least).
• Want to break into the debugger whenever the value changes? Just add a breakpoint in the setter.
• Want to log all access? Just add logging to the getter.
• Properties are used for data binding; fields aren't.
Few other points:
1) You can also make properties read-only so no one from outside the class set the values but can fetch it.
2) You can do certain actions in the get and set. i.e. Append a prefix anytime set is called
3) You can also use auto-implemented property to minimize code like below:
Public Property Name As String
You are not doing anything wrong. Properties give you a shorthand basically, a syntactic sugar.
You can still use a backing private variable and do logic in get and set if you have to while using properties. Even better is the private/protected set or get, which is again another syntactic sugar so that you won't have to write all the code manually.
First of all, VB.NET allows you to use this syntax (called shorthand property declaration - I believe since VS 2010):
Public Property Name As String
Not so much different from this (called field declaration):
Public Name As String
Second, Microsoft data binding does not work well with fields. Try this example (see below).
Example. Put a listbox called ListBox1 (default name) and a button called Button1 on an empty form in an empty WinForms project. Replace your form code with this:
Public Class Form1
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim lst As New List(Of clsMyClass)
lst.Add(New clsMyClass)
ListBox1.ValueMember = "Name"
ListBox1.DisplayMember = "Name"
ListBox1.DataSource = lst
End Sub
End Class
Class clsMyClass
Public Property Name As String = "Hello"
End Class
Start the application and notice that a listbox is populated with one entry, Hello. This proves that binding worked correctly. Now replace your property declaration with a field declaration instead. Start your application one more time and notice that a listbox is showing your class type converted to String. It means that your field binding did not work, and default binding was used instead, where DisplayMember is assigned sort of classInstance.ToString().
If you are curious to learn more about what happens behind the scenes, you can put a breakpoint on .DataSource assignment, and see how DisplayMember gets reset or keeps its value depending on whether you are using fields or properties.
I am trying to create a .Net DLL basically as an abstraction layer for database connections; it is going to replace a current DLL we have that is written in VB6 and I am trying to match the current functionality as much as possible.
Anyway, the essential issue I am having is that I can't find a way to get .Net classes like DataColumnCollection or DataColumn to display in the VBA Interpreter -- It may say, for example, "Column" with the type "MarshalByValueComponent," but the value will be "No Variables".
I can get it to work if I completely re-create both classes (i.e. Fields as a collection of field, which inherits from DataColumn, and then define an interface for both), but that seems like a lot of added overhead for what (should be?) a pretty simple idea. I feel like I am just missing something very simple with the way the marshaller is handling the DataColumn class.
A lot of the stuff I am finding online is on how to convert a DataTable or DataReader to a legacy ADODB Recordset, but that also would add a lot of overhead... I'd rather leave it as a DataTable and create a COM interface to allow VBA to interact with it; that way if, for example, they want to write the table to an excel sheet, I wouldn't be duplicating work (convert to ADODB recordset, then read/write to excel sheet. You'd need to iterate the entire table twice...)
Sorry for the book-length explanation -- I felt the problem needed a bit of clarification since the root-cause is trying to match legacy functionality. Here is an example of my current interface that does not work:
Public Interface IDataTable
ReadOnly Property Column As DataColumn
End Interface
<ClassInterface(ClassInterfaceType.None)> _
<System.ComponentModel.DesignerCategory("")> _
<ComDefaultInterface(GetType(Recordset.IDataTable))> _
<Guid("E7AFBBB6-CB20-44EC-9CD2-BC70B94CD8B7")> _
Public Class Recordset : Inherits Data.DataTable : Implements IDataTable
Public ReadOnly Property Column As DataColumn Implements IDataTable.Column
Get
Return MyBase.Columns(0)
End Get
End Property
Note: I originally tried the property Columns as DataColumnCollection which returned MyBase.Columns. That came through as an Object, instead of MarshalByValueComponent, but was also empty. I know MyBase.Column(0) has a value, because I can put Msgbox(MyBase.Columns(0).ColumnName) right above the return in the get and it pops up fine (don't judge; this is way easier than using a debugger for this)...
I wouldn't mind just defining them both, but I can't inherit DataColumnCollection and the COM interface already sucks at dealing with generics. Is there any other way around this without re-inventing the wheel?
Thanks for your help!
I just spent the last 3 weeks doing something eerily similar.
I ended up making two .NET assemblies:
A pure .NET assembly that talks to the datastore (for use by .NET apps).
A "COM Interop" assembly that wraps the first assembly and adds the COM overhead (ADODB references and COM-Visible interfaces).
I call the second assembly from Excel VBA using the VSTO "AddIn.Object" property.
I ended up converting System.Data.DataTables to ADODB.Recordsets as you mentioned. Getting .NET and VBA talking about anything other than primitive types was beyond-frustrating for me. In fact, I ended up serializing some objects as JSON so the two worlds could communicate.
It does seem insane, but I reinvented the wheel.
I followed this MSDN article to make my .NET code callable by VBA.
I used this Code Project article (I'm sure you've seen) to convert to Recordset*.
I let the frameworks handle string, integers, etc.
For all other data types I used Json.Net and a custom VBA class to do JSON serialization.
*Converted article to VB.Net and added some extra error handling.
Okay, this probably isn't the most elegant (or complete, at this point) solution; but I think it's the route I am going to go.
Instead of converting the whole thing to an ADODB Recordset (and duplicating any iterations), I just threw out the DataTable class entirely and wrote my own Recordset class as a COM Wrapper for the a generic Data Reader (via the IDataReader interface) and added a new Field class to manage the type conversion and set up Fields as an array of Field (since interop hates generics)
It basically creates a forward-only ADODB Recordset (same limitations) but has the benefit of only loading one row at a time, so the bulk of the data can be handled as managed code until you know what they want to do with it (I'm going to add methods for ToArray, ToAccessDB, ToFile, etc that use the reader) while still allowing the ability to iterate through the Recordset from excel/access/vbscript/vb6 (if that's really what they want to do.. mostly needed that for legacy support anyway)
Here is an example, in case anyone else has to do this again; somewhat modified for brevity:
Public Interface IRecordset
ReadOnly Property CursorPosition As Integer
ReadOnly Property FieldCount As Integer
ReadOnly Property Fields As Field()
Function ReadNext() As Boolean
Sub Close()
End Interface
<System.ComponentModel.DesignerCategory("")> _
<ClassInterface(ClassInterfaceType.None)> _
<ComDefaultInterface(GetType(IRecordset))> _
<Guid("E7AFBBB6-CB20-44EC-9CD2-BC70B94CD8B7")> _
Public Class Recordset : Implements IRecordset : Implements IDisposable
Private _Reader = Nothing
Private _FieldCount As Integer = Nothing
Private _Fields() As Field
Public ReadOnly Property CursorPosition As Integer Implements IRecordset.CursorPosition...
Public ReadOnly Property FieldCount As Integer Implements IRecordset.FieldCount...
Public ReadOnly Property Fields As Field() Implements IRecordset.Fields...
Friend Sub Load(ByVal reader As IDataReader)
_Reader = reader
_FieldCount = _Reader.FieldCount
_Fields = Array.CreateInstance(GetType(DataColumn), _FieldCount)
For i = 0 To _FieldCount - 1
_Fields(i) = New Field(i, Me)
Next
End Sub
'This logic kinda sucks and is dumb.
Public Function ReadNext() As Boolean Implements IRecordset.ReadNext
_EOF = Not _Reader.Read()
If _EOF Then Return False
_CursorPosition += 1
For i = 0 To _FieldCount - 1
_Fields(i)._Value = _Reader.GetValue(i).ToString
Next
Return True
End Function
From here you just need to define some type like Field or Column and add an interop wrapper for that type:
Public Interface IField
ReadOnly Property Name As String
ReadOnly Property Type As String
ReadOnly Property Value As Object
End Interface
<System.ComponentModel.DesignerCategory("")> _
<ClassInterface(ClassInterfaceType.None)> _
<Guid("6230C670-ED0A-48D2-9429-84820DC2BE6C")> _
<ComDefaultInterface(GetType(IField))> _
Public Class Field : Implements IField
Private Reader As IDataReader = Nothing
Private Index As Integer = Nothing
Public ReadOnly Property Name As String Implements IField.Name
Get
Return Reader.GetName(Index)
End Get
End Property
Public ReadOnly Property Value As Object Implements IField.Value
Get
Return Reader.GetValue(Index)
End Get
End Property
Public ReadOnly Property Type As String Implements IField.Type
Get
Return Reader.GetDataTypeName(Index).ToString
End Get
End Property
Sub New(ByVal i As Integer, ByRef r As IDataReader)
Reader = r
Index = i
End Sub
End Class
All of this is rather silly, but it seems to work well.
Note: I've only been using .Net for about 4 days now, so this might be terrible, please feel free to comment on anything extremely stupid I might be doing.
I've got some classes that have properties like this, and they work perfectly because they are very normal:
Public Overridable Property CustomerLastName() As String
Get
Return m_CustomerLastName.Value
End Get
Set(ByVal Value As String)
m_CustomerLastName.Value = Value
End Set
End Property
I want to change them to work like this (and don't worry about what IField is, suffice it to say it represents a field in a table):
Public Overridable Readonly Property CustomerLastName() As IField
Get
Return m_CustomerLastName
End Get
End Property
That way, you could do Customer.CustomerLastName.PreviousValue, or Customer.CustomerLastName.IsDirty, etc.
But that doesn't bind correctly. Understandable, since databinding is supposed to be a two-way thing, and there's reflection involved, etc.
Of course it can still be a two-way street, I just need to be able to say, "Hey DataBinding! Look over here!"
So. What do I do here?
Note: Right now, all I'm trying to do is DataBind to a GridView for display purposes. But I want this to be flexible.
(Update re edit) If it is just for display purposes, you can probably just set the ToString() on the class that implements IField.
(original)
No - 2-way data binding wants either INotifyPropertyChanged, or an CustomerLastNameChanged event. You can shim this into a custom model by implementing ICustomTypeDescriptor or TypeDescriptionProvider, but then you need to write your own PropertyDescriptor implementation.
Re IsDirty - that is ShouldSerializeValue in PropertyDescriptor terms.