I'm currently working in .NET 2.0 Visual Basic. The current project is an Active Directory Wrapper class library within which I have a Searcher(Of T) generic class that I wish to use to search the underlying directory for objects.
In this Searcher(Of T) class I have the following methods:
Private Function GetResults() As CustomSet(Of T)
Public Function ToList() As CustomSet(Of T)
Public Function Find(ByVal ParamArray filter() As Object) As CustomSet(Of T)
// And some other functions here...
The one that interests me the most is the Find() method to which I can pass property and values and would like to parse my LDAP query from this filter() ParamArray parameter. Actually, all I can figure out is this:
Public Sub SomeSub()
Dim groupSearcher As Searcher(Of Group) = New Searcher(Of Group)()
Dim groupsSet as CustomSet(Of Group) = groupSearcher.Find("Name=someName", "Description=someDescription")
// Working with the result here...
End Sub
But what I want to be able to offer to my users is this:
Public Sub SomeSub()
Dim groupSearcher As Searcher(Of Group) = New Searcher(Of Group)()
Dim groupsSet As CustomSet(Of Groupe) = groupSearcher.Find(Name = "someName", Guid = someGuid, Description = "someDescription")
// And work with the result here...
End Sub
In short, I want to offer some kind of Expression feature to my users, unless it is too much work, as this project is not the most important one and I don't have like 2 years to develop it. I think that the better thing I should do is to write something like CustomExpression that could be passed in parameters to some functions or subs.
Thanks for any suggestions that might bring me to my goal!
Interesting question. This is a language dependent feature, so I don't see this happening without some clever trickery of the IDE/compiler.
You could however have optional overloads on your Find method (vb.net is good for this), then make the search string manually to obtain the result.
Finally you could make use of lambda functions, but only in .net 3.5 and above. Even still, it would require your searcher to expose a preliminary set of data so you can recover the expression tree and build up the find string.
UPDATE
I've just been playing around with Reflection to see if I can retrieve the parameters passed, and build up a string dynamically depending on if they exist. This doesn't appear to be possible, due to the fact that compiled code doesn't reference the names.
This code just used was:
'-- Get all the "parameters"
Dim m As MethodInfo = GetType(Finder).GetMethod("Find")
Dim params() As ParameterInfo = m.GetParameters()
'-- We now have a reference to the parameter names, like Name and Description
Hmm. http://channel9.msdn.com/forums/TechOff/259443-Using-SystemReflection-to-obtain-parameter-values-dynamically/
Annoyingly it's not (easily) possible to recover the values sent, so we'll have to stick with building up the string in a non-dynamic fashion.
A simple optional method would look like:
Public Sub Find( _
Optional ByVal Name As String = "", _
Optional ByVal Description As String = "")
Dim query As String = String.Empty
If Not String.IsNullOrEmpty(Name) Then
query &= "Name=" & Name
'-- ..... more go here with your string seperater.
End If
End Sub
Related
I've heard advice to change from User Defined Type (UDT) to a regular Class in order to overcome the limitations of UDT, such as not being able to use For Each with a UDT.
I've also heard advice to change from a regular Class to UDT to overcome the Class limitation where you can't pass things BYREF, like...
'Function:
Public Function RemoveArticle (ByRef strMovieTitle As String)
'Expected input is like "Terminator, The"
strMovieTitle = Left(... 'removes the article.
End Function
That works fine for this call:
Dim strMovieTitle As String
strMovieTitle = "Terminator, The"
RemoveArticle strMovieTitle
But not this call:
Dim objMovie As MovieClass
objMovie.strMovieTitle = "Terminator, The"
objMovie.strMovieGenre = "Sci-Fi"
InvertArticle objMovie.strMovieTitle
Even though MovieClass defines
strMovieTitle As String
I can't go changing RemoveArticle (and every simple little function like it) to take a MovieClass parameter instead of a String parameter because there are other UDTs or Classes and String Variables that also need to use RemoveArticle.
What do I do if I need to use For Each and I also need to pass ByRef?
Is there a way a Class can work around the parameter problem?
(Using Excel 2010.)
Now I have understood your concern.
You simply can't take that approach to meet your goal. As Tim Williams has commented in your question, your best bet would be something like this:
Dim objMovie As MovieClass
Dim strMovieTitle As String
strMovieTitle = "Terminator, The"
objMovie.strMovieTitle = InvertArticle(strMovieTitle)
However, I see that this still does not satisfy your need.
My suggestion is as follows:
make your object internal, target properties Private and expose them with Property Let and Property Get. This way you can do the modifications you want to the properties either on set or on get (from within the class... rather than fixing things from outside the class).
Aside note, in regards to create a helper class (as someone has recommended to you): you could join into one class all those functions you use widely, such as RemoveArticle or InvertArticle. However, it requires to create an instance object every time you want to use them and, therefore, does not combine well with the recommendation I am giving to you (if you want just to simplify code). So having them in a Module as you do now is fine. Just to clarify: those recommendations they gave to you are unrelated to your question here.
Example A: on set
In you class MovieClass, rename first all the instances of strMovieTitle to pStrMovieTitle and add this to your code:
Private pStrMovieTitle As String
Public Property Let strMovieTitle (strIn As String)
pStrMovieTitle = InvertArticle(strIn)
End Property
Public Property Get strMovieTitle As String
strMovieTitle = pStrMovieTitle
End Property
The usage would be something like this:
Dim objMovie As MovieClass
objMovie.strMovieTitle = "Terminator, The" ' the article position gets rectified on assignation
objMovie.strMovieGenre = "Sci-Fi"
'InvertArticle objMovie.strMovieTitle ' => you don't need to do this call
Example B: on get
To keep your original string as it comes, and do apply your helpers when you get the property value. That way you always preserve the original string. However, this approach will need more rework and it's only worthy in cases where you have lots of ways to use that String in different parts of your code.
Private pStrMovieTitleSource As String
Public Property Let strMovieTitle (strIn As String)
pStrMovieTitleSource = Trim(strIn)
End Property
Public Property Get strMovieTitleSource () As String
strMovieTitleSource = pStrMovieTitleSource
End Property
Public Property Get strMovieTitleRoot () As String
strMovieTitleRoot = RemoveArticle(pStrMovieTitleSource)
End Property
Public Property Get strMovieTitle () As String
strMovieTitle = InvertArticle(pStrMovieTitleSource)
End Property
Hope it helps
To avoid getting into the weeds on my particular program, let me just create a simplified case.
I have a generic class that should work on a variety of objects. Each of those objects must implement a certain interface.
What I WANT to say is something like:
Public Interface GenThing
Shared Function thing_name() As String ' This doesn't work! Can't be shared!
Sub FillOne(row As DataRow)
End Interface
public class Thing1
implements GenThing
public shared function thing_name() as string implements GenThing.thing_name
return "thing number one"
end function
public sub FillOne(row as DataRow) implements GenThing.MakeOne
... bunch of work ...
end sub
end class
public class ThingUtil(of T as {GenThing,New})
public function GetList(id as integer) as List(of T)
dim name=T.thing_name() ' This doesn't work!
dim ds as DataSet=GetData(name,id) ' bunch of work here that's the whole point of the class but not relevant to the question
dim my_list = new List(of T)
for each row as DataRow in ds.tables(0).rows
dim my_t = new T()
my_t.FillOne(row)
my_list.add(my_t)
next
return my_list
end function
end class
Do you get my problem? I need every class that implements the interface to have a function that returns a "name" that is used to get the data that is needed to create an instance of the object. But I need to know this name BEFORE I create the instance, because I need it to be able to create the instance. But VB doesn't allow an interface to have a shared function, so what I want to write doesn't work.
So what I've done is this:
I make thing_name not shared.
Then instead of simply "dim name=T.thing_name()", I write
dim dummy = new T()
dim name = dummy.thing_name()
Okay, it works, but it seems really ugly. I create an instance of the object, with all the overhead that that involves, just to get a piece of constant text.
Is there a better way? Or am I making a big deal out of nothing?
Update
I see that two people voted to close this question on the grounds that it is the same as "Why can't we have shared functions in an interface?"
I am not asking why I can't have a shared. I am saying, GIVEN that I can't, how do I solve this particular problem?
There's no really simple way of fixing this, no.
Depending on what thing_name does, however, you might approach things in a different way. If each implementation just returns a constant value, then it's effectively metadata about the class - and could be described in an attribute instead, which can be fetched at execution time. (See Type.GetCustomAttributes.) Unfortunately you can't then enforce all types implementing the interface to be decorated with the attribute - but you could write a unit test to check this pretty easily.
If thing_name needs to really do work at execution time, that's tougher. You could potentially look for a well-known shared method name instead and execute that via reflection (and again have unit tests to check that it's implemented properly).
I realize this is from a few years ago, but running into a similar problem, I wanted to offer a different solution. Pass a delegate as parameter to the ThingUtil constructor. You avoid having to put a shared method in an interface, and the constructor will force you to include the parameter at compile time.
You can add more delegates if needed, or to make it even simpler in this case, just pass name as a string instead of get_name as a delegate.
Define the delegate in the interface:
Public Interface GenThing
Delegate Function ThingNameDelegate() As String
Sub FillOne(row As DataRow)
End Interface
Public Class Thing1
Implements GenThing
Public Shared Function thing_name() As String 'name this whatever you want
Return "thing number one"
End Function
Public Sub FillOne(row As DataRow) Implements GenThing.FillOne
'do stuff
End Sub
End Class
In ThingUtil, add a member to store the delegate, a constructor parameter to to accept, and call it with .Invoke():
Public Class ThingUtil(Of T As {GenThing, New})
Private m_thing_name As GenThing.ThingNameDelegate
Public Sub New(thing_name As GenThing.ThingNameDelegate)
m_thing_name = thing_name
End Sub
Public Function GetList(id As Integer) As List(Of T)
Dim name = m_thing_name.Invoke()
Dim ds As DataSet = GetData(name, id) ' bunch of work here that's the whole point of the class but not relevant to the question
Dim my_list = New List(Of T)
For Each row As DataRow In ds.Tables(0).Rows
Dim my_t = New T()
my_t.FillOne(row)
my_list.Add(my_t)
Next
Return my_list
End Function
End Class
Finally, use it like this:
Dim tu as new ThingUtil(Of Thing1)(AddressOf Thing1.get_name)
tu.GetList(1)
Suppose I have this simple class:
Public Class Person
Public Property Name() As String
Public Property Married() As Boolean
End Class
I want to create a delegate to its property getters. After a bit of search, including this, I've written this code in VB.NET:
Dim p As New Person()
p.Name = "Joe"
p.Married = True
Dim propGetter1 As Func(Of Boolean) = Function() p.Married
Dim propGetter2 As Func(Of String) = Function() p.Name
And retrieve the value of the properties as follows:
Dim married as Boolean = propGetter1.Invoke
Dim name as String = propGetter2.Invoke
It works fine, but contrary to the example in C# from the link above, it does not check at compile-time the type returned by the Function, so I could just write this code:
Dim propGetter1 As Func(Of Boolean) = Function() p.Name
Getting an error at runtime.
I've also tried this code:
Dim propGetter3 As Func(Of String) = Function(p As Person) p.Name
But I get the following error:
Nested function does not have a signature that is compatible with delegate 'System.Func(Of Boolean)'
So, how can I write this delegates with type-checking at compile-time, just like in C#??
EDIT:
As I've been asked to do so, I explain what I'm trying to do with this approach. I have an application that reads the state of a system (probe mesures, flows, preassures...) each few seconds, and stores this information in some properties of the objects that model the system.
I want to offer the user a list of all the objects defined which have properties that get their values updated this way. Therefore, the user can set alarm rules, so when a property reaches a certain value, an alarm is fired.
In orden to do so, I need to save delegates to the chosen properties, so I can check its values each few seconds, and compare them to the expected ones.
Does it make sense?
This is VB.NET's in/famous dynamic typing at work:
Option Strict Off
Module Module1
Sub Main()
Dim p As New Person With {.Name = "True"}
Dim propGetter1 As Func(Of Boolean) = Function() p.Name
Dim result = propGetter1() '' Fine :)
End Sub
End Module
If you don't like surprises like this then just use Option Strict On and the compiler will tell you that you got it wrong at build time. Use Tools + Options, Projects and Solutions, VB Defaults to change the default setting. Do beware that you might have some trouble with older projects ;)
Here is an example of my function:
Public Class GlobalFunctions
Public Shared Function CreateNewDatabase(ByVal MyDomainName As String, ByVal NewDatabaseName As String, ByVal StatusBoxName As ListBox)
Try
Dim MyWorkingDirectory As String = "C:\mytest1\"
Dim MyFileName As String = "mycmd.exe"
If File.Exists(MyWorkingDirectory & MyFileName) Then
'Run mycmd
'Here are my results
Dim Result1 As String = "r2"
Dim Result2 As String = "r2"
Dim Result3 As String = "r3"
End If
Catch ex As Exception
MsgBox(ex.Message)
End Try
Return True
End Function
End Class
Basically what I need to do is run my function & then be able to use the results in my next function.. Something like:
GlobalFunctions.CreateNewDatabase(DomainName.Text, MyDbName, StatusListBox)
MsgBox(Result1)
MsgBox(Result2)
MsgBox(Result3)
Any help would be much appreciated ;)
There are two main options here. The first is to return the values you want to use later. To do this, you can either create a class with properties for the three values you want to use, or if this function will have limited use, you could consider returning a Tuple (based on your example, a Tuple(Of String, String, String))
The other option is to make class level properties and set their backing fields from the CreateNewDatabase function. The primary drawback to this is that these values will be overwritten by any future calls to the function. It also introduces some discoverability issues into your API by requiring the programmer to know that three properties are set by the method, rather than the more obvious returning an object with three properties from the function.
You should make properties Result1, Result2, Result3 in your class and set them in CreateNewDatabase(). Then you can get them as GlobalFunctions.Result1
But it looks terrible.
Well, the obviously easiest way is, declaring them as global variables, which in practice seems not to be too welcomed.
Public Class Form1
Dim Result1 As String
Dim Result2 As String
Dim Result3 As String
GlobalFunctions.CreateNewDatabase(DomainName.Text, MyDbName, StatusListBox)
MsgBox(Result1)
MsgBox(Result2)
MsgBox(Result3)
etc..
etc...
end class
Not the prettiest way to do it, but it will get the job done.
OK, that question sounds maybe a little confusing so I'll try to explain it with an example.
Pretend you have an object like this:
Class Something
Private varX As New Integer
Private varY As New String
'[..with the associated property definitions..]
Public Sub New()
End Sub
End Class
And another with:
Class JsonObject
Inherits Dictionary(Of String, String)
Public Function MakeObject() As Object 'or maybe even somethingObject
Dim somethingObject As New Something()
For Each kvp As KeyValuePair(Of String, String) In Me
'Here should happen something to use the Key as varX or varY and the Value as value for the varX or varY
somethingObject.CallByName(Me, kvp.Key, vbGet) = kpv.Value
Next
return somethingObject
End Function
End Class
I've got the 'CallByMe()' function from a previous question of myself
CallByName works different from the way you are trying to use it. Look at the documentation, it will tell you that in this particular case the correct usage would be
CallByName(Me, kvp.Key, vbSet, kpv.Value)
However, the function CallByName is part of a VB library that isn’t supported on all devices (notably it isn’t included in the .NET Mobile framework) and consequently it’s better not to use it.
Using proper reflection is slightly more complicated but guaranteed to work on all platforms.
Dim t = GetType(Something)
Dim field = t.GetField(kvp.Key, BindingFlags.NonPublic Or BindingFlags.Instance)
field.SetValue(Me, kvp.Value)