Is there any way to create class instances on the fly and refer to them later? I have a class with various methods and properties designed to hold & calculate product data. I'd like the application to be able to handle as many individual products as the user needs. This code clearly won't work but it should give you an idea of what I'm asking:
For x = 1 To howEverMany
Dim product_ & x.ToString() As New myProductClass
Next x
I appreciate this may not be the best approach (I should probably use lists or arrays to hold the product data) but I'm curious as to whether this is possible from a technical standpoint. I'm using VB.Net but answers in any .Net language will be welcome. Thanks.
Store them in a List(Of T). Then you can LINQ one or more objects in the list.
'class level
Private Products As New List(Of myProductClass)
'place where you load them
For x As Integer = 1 To howEverMany
Dim myc As New myProductClass
mpc.Id = x
'set other properties as needed
Products.Add(mpc)
Next
Get one by it's Id:
Dim mpc4 = (From p In Products Where p.Id = 4).FirstOrDefault
If Not mpc4 Is Nothing Then
'object exists
End If
You can make other similar queries on your List as well.
Related
I'm using netzero hardware to manage the contents of a number of monitors. My present solution creates a form in VB.Net that has a pixel offset corresponding to where I've placed the Monitors in display management in the control panel. Each monitor has a dedicated form, and in each form are various objects.
The annoyance is that each form must be individually created (so far as I know) at design time. I can't make an array of forms, coupled with an array of offsets and assign all the properties through code.
There ought to be a way to do this...it would simplify my coding and project management.
What I see on MSDN is either over my head or not helpful.
I haven't tested this in hardware yet, but it does compile w/o error:
Public Sub makeform()
Dim MonitorForm(21) As Form
Dim MPictureBoxes(21) As PictureBox
Dim a As Integer
For i As Integer = 0 To n 'up to 21
MonitorForm(i) = New Form
MonitorForm(i).Name = "Form" & (i + 1)
MonitorForm(i).Text = "Form" & (i + 1)
MonitorForm(i).Controls.Add(MPictureBoxes(i))
MonitorForm(i).Location= new Point (x(i), y(i))
With MPictureBoxes(i)
.Name = "Picture Box " & Convert.ToString(i)
.Image = Image.FromFile(CurrentPic(i))
.Location = New System.Drawing.Point(0, 0)
.Size = New Size(1920, 1080)
' Note you can set more of the PicBox's Properties here
End With
Next
End Sub
Where I had gone wrong in my attempts at this was trying to do it this way
Dim Monitor(21) as New Form
That doesn't work, and the difference between Dim Monitor(21) as Form followed by monitor(i)= new Form
was simply too subtle for my present understand of classes, namespaces etc.
.
Well, I've had to give up on this approach and go back to creating n forms at design time (which means that they have names of form2...form22, putting each of them at manual start positions in design mode. There just doesn't seem to be a way to do this with an array of forms. So the code I have built around the messiness of forms2...forms22 works just fine, it's just going to be messy to maintain and elaborate on.
The solution to this may lie in system.screen classes, but the documentation on this is too advanced for me and I'm not finding good code examples for anything other than extracting data about how many screens there are - nothing about writing to them.
This is very easy in code. You want to make many instances of the same form. In this case, I have created a form in the designer called frmTest and I create many instances in code called frmNew:
Public Sub Main()
For x = 100 To 400 Step 100
For y = 100 To 700 Step 200
Dim frmNew As New frmTest
frmNew.Show()
frmNew.Top = x
frmNew.Left = y
frmNew.Height = 100
frmNew.Width = 200
Next
Next
End Sub
I have just used two loops to increment x and y values, but you could do this from a database or config file easily enough.
VB2005: I've been looking for several hours on a good example and i found some but unfortunately they are for VB2008+. Im currently working in VB2005 so it seems like this was harder to do in that release.
I have a class for a point
Public Class cPoint
Public Speed As Integer
Public Alt As Integer
Public Status As String = ""
Public Err As String = ""
End Class
I populate a list of points with MyPoints=List(of cPoint). Now all i need to do is find the first match with a supplied speed and alt. I tried
Dim p As cPoint = MyPoints.Find(Function(item As cPoint) item.Speed = 85)
But that doesn't work in VB2005 much less work with more than 1 filter. I just cant seem to find a good example that works in VB2005. I could iterate through the list but its kind of big and not very efficient. Any tips on how i can do this in VB2005?
~agp
VB.Net 2005 lacks lambda support hence that style of query won't work. The simplest version that will is to manually iterate with a For Each loop.
My p As cPoint = Nothing
For Each item in MyPoints
If item.Speed = 85 Then
p = item
Exit For
End If
Next
I can't seem to find any answers that work. Here's the setup:
Info class:
Public Class ProductStageInfo
Private _ProductNumber As String
Private _ProductReference As String
Public Sub New()
End Sub
Public Property ProductNumber() As String
Get
Return _ProductNumber
End Get
Set(ByVal Value As String)
_ProductNumber = Value
End Set
End Property
End Class
and so on; I have four class declarations in the info class, the one above has fifteen different items - product number, product reference, product name, and so forth. The other's are catalogue classifications, which 'stage' of production the product is in, quality assurance questions; etc.
Then in the Controller class for DNN, I have those various info classes filled via queries to the DB DNN was deployed on; example:
Public Shared Function LoadStages(ByVal ProductNumber As String) As List(Of ProductStageInfo)
Return CBO.FillCollection(Of ProductStageInfo)(CType(DataProvider.Instance().ExecuteReader("Product_LoadStages", ProductNumber), IDataReader))
End Function
and everything works so far, I can fill a datalist using <%# DataBinder.Eval(Container.DataItem, "ProductNumber" %> and in code behind:
Dim ProductStageList As List(Of ProductStageInfo)
ProductStageList = ProductController.LoadStages(ProductNumber)
ProductStageDataList.DataSource = ProductStageList
ProductStageDataList.DataBind()
so far, so good...
but now I need to allow individuals to 'create' stages, and one of the business reqs' is that people shouldn't be able to create, for example, a delivery stage before a packaging stage.
So, how do I go about 'finding' a product number, product reference, stage number, within a collection? I thought I could fill the collection with all the stages of a certain product number, and then do an if/then stage = 0 found, stage > 5 found, etc.
If ProductStageList.Contains(strProductNumber) then
end if
gives error value of type string cannot be converted to namespace.ProductStageInfo; same thing for ProductStageList.Find...
maybe I just don't understand the whole collection/index/thing. All the examples I've found are regarding single dimension collections - 'how to find name within this collection', and the responses use strings to search through them, but somehow the Info class is being treated differently, and I'm not sure how to translate this...
any hints, tips, advice, tutorials.... appreciate it :)
thanks!
Pretty sure I just found the answer by reviewing another module; basically I need to create an empty object instead of a list object of the same class and use the two to iterate through using for/each, etc.
Dim objStages As ProductStagesInfo
Dim intStages, StageSelected As Integer
Dim intStageOption As Integer = -1
Dim blnValid As Boolean = True
Dim ProductChosen As String = lblStagesCNHeader.Text
Dim ProductStageList As List(Of ProductStagesInfo) = ProductController.LoadStages(ProductChosenNumber)
For intStages = 0 To StageList.Count - 1
objStages = StageList(intStages)
intStageOption += 1
Select objStages.StageSetNumber
Case "0"
Next
objStages._ provides me the ability to get the data I needed to do the business logic
<.<
seems so simple once you see it, wish I could just store it all in my brain
blah!
ok this is the thing I have right now which is working quite well except its a bit slow:
Public Function GetList() As List(Of SalesOrder)
Try
Dim list As New List(Of SalesOrder)
Dim ds As DataSet
ds = cls.GetSalesOrderList 'CLS is the data access class
For i = 0 To ds.Tables(0).Rows.Count - 1
Dim row As DataRow = ds.Tables(0).Rows(i)
Dim kk As SalesOrder = New SalesOrder()
kk.ID = Val(row.Item("id") & "")
kk.SalesOrderNo = row.Item("salesorderid") & ""
kk.SalesOrderDate = row.Item("OrderDate") & ""
kk.CustomerId = Val(row.Item("customerid") & "")
list.Add(kk)
Next
Return list
Catch ex As Exception
Throw ex
End Try
End Function
Now once I start retrieving more than 10000 records from the table, the loop takes long time to load values into generic class. Is there any way that I can get rid of loop? Can I do something like the following with the generic class?
txtSearch.AutoCompleteCustomSource.AddRange(Array. ConvertAll(Of DataRow, String)(BusinessLogic.ToDataTable.ConvertTo(WorkOr derList).Select(), Function(row As DataRow) row("TradeContactName")))
I would have thought the problem isn't with doing a loop but with volumes of data. Your loop method seems to process each bit of data only once so there isn't any massive efficiency crash (such as looping over the dataset once and then again for each row or that kind of thing). Any method you choose is at the end of the day going to have to loop through all your data.
Their methods might be slightly more efficient than yours but they aren't going to be that much more so I'd think. I'd look at whether you can do some refactoring to reduce your data set (eg limit it to a certain period or similar) or whether you can do whatever searching or aggregating of that list you intend in the database instead of in code. eg if you're just going to sum the values of that list then you can almost certainly do it better by having a stored procedure that will do the summing on the database rather than in the code.
I know this hasn't directly answered your question but this is mainly because I don't know of a more efficient method. I took the question as asking for optimisation in general though rather than how to do this specific one. :)
Converting the loop into some kind of LINQ construct isn't necessarily going to improve performance if you're still enumerating over every row at once. You could return IEnumerable(Of SalesOrder) if you don't need to give the consumer the ability to add/remove from the list (which it looks like might be the case), and then in that case you could create an enumerator to handle this. That way, the dataset is loaded all at once, but the items are only converted into objects when they're being enumerated over, which may be part of your performance hit.
Something like this:
Return ds.Tables(0).Rows.Select(Function(dr As DataRow) Return New SalesOrder ... );
My VB with LINQ is a little rusty, but something to that effect, where the ... is the code to instantiate a new SalesOrder. That will only create a new SalesOrder object as the IEnumerable(Of SalesOrder) is being enumerated over (lazy, if you will).
Hey Paul, You mean something like below code
Dim list As New List(Of SalesOrder)
Dim kk As SalesOrder = New SalesOrder()
Function DrToOrder(dr as datareader)
kk.ID = Val(dr.Item("id") & "")
kk.SalesOrderNo = dr.Item("salesorderid") & ""
list.Add(kk)
End function
Function LoadData()
datareader.Rows.Select(DrToOrder)
End function
Are you talking about something like above code?
I have this code to return a list of fund sources for our organization.
Dim FundSourceList As New List(Of FundSource)
Dim fs As New FundSource
If results.Count > 0 Then
For Each result In results
fs.FundID = result.Item("strFundID")
fs.FundDescription = result.Item("txtFundIDDescr")
fs.ShortFundDescription = result.Item("txtFundIDDescrShort")
FundSourceList.Add(fs)
Next
End If
Return FundSourceList
The problem is that when I loop through the resulting FundSourceList all it shows is the last value. For example, if I have three fund sources (state, federal, athletic), then when I use this code to loop through all I get listed is athletic, athletic, athletic.
For Each FundSource In FundSources
Debug.Print(FundSource.FundDescription)
Next
So I change the code to this. I moved the creation of the fs variable inside the loop.
Dim results = From result In dsResult.Tables(0) Select result
Dim FundSourceList As New List(Of FundSource)
If results.Count > 0 Then
For Each result In results
Dim fs As New FundSource
fs.FundID = result.Item("strFundID")
fs.FundDescription = result.Item("txtFundIDDescr")
fs.ShortFundDescription = result.Item("txtFundIDDescrShort")
FundSourceList.Add(fs)
Next
End If
Return FundSourceList
This works fine but now I'm creating a new class over and over again. It seems a little inefficient to me. Can I not create the class outside the loop and use it over and over again? Thanks.
If you have 3 fund sources, you need three FundSource objects. It's as simple as that. I don't know what's inefficient about it...
How can you add 3 fund sources to your list but just create one?
You're not actually creating a class - the class is the code definition for the methods and properties. When you use the New operation, you're creating an instance of that class, which results in an object. When you have a list of objects, like FundSourceList, you want the items in it to be individual objects. So yes, the solution you have at the bottom is correct. You mention efficiency concerns - when you instantiate the object, basically all that is happening (in this case) is some memory is being allocated to store the variables (and some references for the managed memory, but you don't need to worry about that here). This is necessary and is optimized under-the-hood, so you shouldn't need to worry about that either.
You can't instantiate the object outside of the loop to achieve the result you're after.
This is because your object would be a reference type.
By instantiating outside of the loop, you would create one reference to your object.
When iterating through your results and setting the properties, you'll be using that same reference over and over.
All you're adding to the list on each iteration is the same reference, which by the end of the loop, will refer to an object containing the last values in your result set.
By creating new objects inside the loop, you create new references - each pointing to a new FundSource. Your loop now writes into a fresh object, and get your desired results.