extract list of string from list of custom class - vb.net

i have a list(of custom class)
and i want to extract a list of all 'name' String, from it, through linq
I know how to do with a loop, but i need to get it with a linear, brief linq instruction.
i've checked this help
C# Extract list of fields from list of class
but i have problem in linq correct syntax
in particular because i would like to extract a New List(Of String)
Class Student
Sub New(ByVal NewName As String, ByVal NewAge As Integer)
Name = NewName
Age = NewAge
End Sub
Public Name As String
Public Age As Integer
End Class
Public Sub Main
Dim ClassRoom as New List(Of Student) From {New Student("Foo",33), New Student("Foo2",33), New Student("Foo3",22)}
Dim OneStudent as Student = ClassRoom(0)
Dim AllStudentsNames As New List(Of String) From {ClassRoom.Select(Function(x) x.Name <> OneStudent.Name).ToList}
End Sub
But something wrong...
Any help?
P.S. Since c# it's close to vb.Net, also c# helps are well welcome.

First, you don't need to create a new list From the one returned by the LINQ method. It's already in a new list at that point, so you can just set AllStudentsNames equal directly to what the ToList method returns.
Second, you are not selecting the name. You are selecting the result of the equality test to see if the names are different. In other words, when you say Select(Function(x) x.Name <> OneStudent.Name), that returns a list of booleans, where they true if the names are different and false if the names are the same. That's not what you want. You want the list of names, so you need to select the name.
Third, if you need to filter the list so that it only returns ones where the name is different, then you need to add a call to the Where method.
Dim AllStudentsNames As List(Of String) = ClassRoom.
Where(Function(x) x.Name <> OneStudent.Name).
Select(Function(x) x.Name).
ToList()

Related

VB.NET copy a 2-dimenion List into a one dimension list

In my original code I have a list object containing 2 columns, Word and Percent. I sort the list but only want to return the list containing just the Word
Here is some example code broken down into something simple:
Public Function SortMyWords() as list(of string)
Dim Words As WordsToSort
Dim ListofWords As New List(Of WordsToSort)
Words.Word = "John"
Words.Percent = "10"
ListofWords.Add(Words)
Words.Word = "Robert"
Words.Percent = "1"
ListofWords.Add(Words)
ListofWords = ListofWords.OrderBy(Function(x) x.Percent).ToList()
End Sub
Public Structure WordsToSort
Public Word As String
Public Percent As String
Public Sub New(ByVal _word As String, ByVal _percent As String)
Word = _word
Percent = _percent
End Sub
End Structure
At the end of the SortMyWords function, I want to return just the Word column back as a list, I'm not sure if I can do this direct - i.e.
Return Listofwords(column Word) or whether I need to copy my ListofWords into a new list, just containing the Column Word - something like this (which doesn't work)
Dim Newlist As New List(Of String)
Newlist.AddRange(ListofWords(Words.Word))
Return NewList
Any suggestions on whether I should do this completely differently (and better) would be really appreciated as I am trying to get my head around objects and although I use them all the time, I'm new to structures and list objects.
Thanks for any help, this has been driving me crazy for an hour now.
I think you're close. Try:
ListOfWords
.OrderBy(Function(x) x.Percent)
.Select(Function(x) x.Word)
.ToList()
If you prefer, you can also use the LINQ syntax:
(from w in ListOfWords
orderby w.Percent ascending
select w.Word).ToList()
Note that the return type is a List(Of String) and not a List(Of WordsToSort) anymore. So you cannot assign it back to the variable ListOfWords again like you do in your sample code.

Get data from a collection

I want to make a collection to have data available
Example:
Dim b As New Collection
colb = New Collection
b.Add("system", "1001", "SYSTEM")
b.Add("network", "1002", "NETWORKA")
b.Add("networksecond", "1010", "NETWORKB")
colb.Add(b, "list")
im looking for a function to get data from this collection:
I want to, based on the ID (Second number) get the first and third value
So if I search for 1010, I need to have the value Network and NETWORKA
VB6 called, they want their Collection back.
No, seriously, please consider using a Dictionary instead of the old, legacy Collection class. Behold the beauty of generics and strong typing:
Dim dic As New Dictionary(Of Integer, Tuple(Of String, String))
dic.Add(1001, Tuple.Create("system", "SYSTEM"))
dic.Add(1002, Tuple.Create("network", "NETWORKA"))
dic.Add(1010, Tuple.Create("networksecond", "NETWORKB"))
' Search
Dim t As Tuple(Of String, String) = Nothing
If dic.TryGetValue(1002, t) Then
Console.WriteLine(t.Item1) ' prints "network"
Console.WriteLine(t.Item2) ' prints "NETWORKA"
End If
As soon as you have more than two values, I suggest that you use a specialized class instead of a Tuple to increase readability.
Also, you can simply use List(Of T). In most cases this is enough. Dictionary is good for fast search out long list by a single key.
'declare model
Public Class NetworkModel
Public Property Id As Integer
Public Property Name1 As String
Public Property Name2 As String
End Class
' load list of models
Private _modelList As New List(Of NetworkModel)()
.......
' search using LINQ
Dim model As NetworkModel = _modelList.FirstOrDefault(Function(m) m.Id = 1001)
If model IsNot Nothing Then . . . . .

list all countries in two comboboxes in a form in vb.net

Imports System.Collections.Generic
Imports System.Globalization
Public Sub ListCountries(SourceCombo As System.Windows.Forms.ComboBox)
' Iterate the Framework Cultures...
For Each ci As CultureInfo In CultureInfo.GetCultures(CultureTypes.AllCultures)
Dim ri As RegionInfo
Try
ri = New RegionInfo(ci.Name)
Catch
'If a RegionInfo object could not be created don't use the CultureInfo for the country list.
Continue For
End Try
' Create new country dictionary entry.
Dim newKeyValuePair As New KeyValuePair(Of String, String)(ri.EnglishName, ri.ThreeLetterISORegionName)
' If the country is not already in the countryList add it...
If Not countryList.ContainsKey(ri.EnglishName) Then
countryList.Add(newKeyValuePair.Key, newKeyValuePair.Value)
SourceCombo.Items.Add(ri.EnglishName)
End If
Next
SourceCombo.Sorted = True
End Sub
I added three combo boxes to a form and called the above function three times for each combo boxes in the form load event.
like:
listcountries(ComboBox1)
listcountries(ComboBox2)
listcountries(ComboBox3)
but the first combobox only lists all countries and the other two are empty. please help me how to solve this.
im using vb.net 12 ultimate & windows 7
thank you
Why not return the country list object and bind the to each combobox using datasource?
Also items are added to comboxbox when countrylist doesnt contains that so need to clear countrylist. It should be comboxbox.Items.Contains()
The countryList dictionary is global to your class and it is initialized somewhere before calling this method. So the first call finds the dictionary empty and adds the infos both to the dictionary and the combo, but the second call (and the third one) find the dictionary already filled and thus doesn't add anything to the second (and third) combo
Without recreating the dictionary every time you call this method you could write
Dim countryList as SortedDictionary(Of string, String)
Public Sub ListCountries(SourceCombo As System.Windows.Forms.ComboBox)
If countryList Is Nothing Then
countryList = BuildCountryList()
End If
SourceCombo.DisplayMember = "Key"
SourceCombo.ValueMember = "Value"
SourceCombo.DataSource = New BindingSource(countryList, Nothing)
' No need to sort anything
End Sub
Public Function BuildCountryList() As SortedDictionary(Of String, String)
Dim temp = New SortedDictionary(Of String, String)
For Each ci As CultureInfo In CultureInfo.GetCultures(CultureTypes.AllCultures)
Dim ri As RegionInfo
Try
ri = New RegionInfo(ci.Name)
Catch
'If a RegionInfo object could not be created don't use the CultureInfo for the country list.
Continue For
End Try
' If the country is not already in the countryList add it...
If Not temp.ContainsKey(ri.EnglishName) Then
temp.Add(ri.EnglishName, ri.ThreeLetterISORegionName)
End If
Next
Return temp
End Function

Combo Box items - Display Member for List(Of String)?

My project is in Visual Basic. I am trying to create a custom & savable "filter" for a DataGridView using several TextBoxes. Right now, any List(Of String) that is added to the Combo Box is displayed in the box as (Collection). I want my users to be able to select the one they created, so I would like the Lists to have a display name that can be selected in the Combo Box. Here is some of the code.
Dim savedFilter As New List(Of String)
savedFilter.Add(NameTextBox.Text)
savedFilter.Add(AgeTextBox.Text)
savedFilter.Add(NotesTextBox.Text)
ComboBoxSavedFilters.Items.Add(savedFilter)
Is it possible to add a display name for a List?
Or if you are lazy use buid-in generic class Tuple From MSDN.
Create collection of Tuple(Of String, List(Of String)) and use approach suggested by #Plutonix for binding collection to ComboBox
Dim savedFilter As New List(Of Tuple(Of String, List(Of String)))()
savedFilter.Add(
Tuple.Create("default",
New List From {"filter1", "filter2", "filter3"}))
savedFilter.Add(
Tuple.Create("Blue ones",
New List From {"filter4", "filter5"}))
savedFilter.Add(
Tuple.Create("Old ones",
New List From {NameTextBox.Text, AgeTextBox.Text, NotesTextBox.Text}))
With ComboBoxSavedFilters
.DisplayMember = "Item1" 'Name of first property in Tuple type
.ValueMember = "Item2" 'Name of second property in Tuple type -List
.DataSource = savedFilter
End With
Then SelectedValue will contain currently selected filter's collection,
which can be accessed like that
Dim filter As List(Of String) =
DirectCast(Me.ComboBoxSavedFilters.SelectedValue, List(Of String))
You could setup under My.Settings a StriingCollection
Initializing (you can omit the items added if so desired)
If My.Settings.Filters Is Nothing Then
My.Settings.Filters = New StringCollection() From {"One", "Two"}
End If
Setup items in a ComboBox
ComboBox1.Items.AddRange(My.Settings.Filters.Cast(Of String).ToArray)
Adding an item
My.Settings.Filters.Add(Now.ToShortDateString)
You can remove and clear items too.
Provide a Display Member for List(Of String)
Apparently, these are less a collection of filters than a collection of criteria or clauses for one Filter:
I condensed the code in the question, but there are 14 fields that can be filtered and there are multiple filters that can be applied on one field.
For the multiples per field, I am not sure I would want to store those individually, but keep the field criteria together. So, if you want to apply a name to these, a class would not only do that but could help manage the filter elements:
Public Class SuperFilter
Public Property Name As String
Public Property Elements As SortedList
Public ReadOnly Property FilterText As String
Get
Return GetFilterText()
End Get
End Property
Public Sub New(n As String)
Name = n
Elements = New SortedList
End Sub
Public Sub AddItem(filter As String)
Elements.Add(Elements.Count, filter)
End Sub
Public Sub InsetAt(index As Int32, filter As String)
Elements.Add(index, filter)
End Sub
Private Function GetFilterText() As String
Dim els(Elements.Count - 1) As String
Elements.Values.CopyTo(els, 0)
Return String.Join(" ", els)
End Function
Public Overrides Function ToString() As String
Return String.Format("{0} ({1})", Name, Elements.Count.ToString)
End Function
End Class
You would need to add methods and properties like Remove and Count but this should be enough to demonstrate. I am not sure about the SortedList, a Dictionary using the field name might be better, but something to control the order seems worthwhile. I am also unsure I would expose the Elements collection - managing it might be better left to the class.
Hopefully, the Combo displaying a set of these (as opposed to the filter elements/clauses) is the goal.
Private filters As New List(Of SuperFilter)
Add filter items to the list:
Dim item As New SuperFilter("Default")
item.AddItem("Id = 7")
filters.Add(item)
item = New SuperFilter("Blue Ones")
item.AddItem("Color = Blue")
filters.Add(item)
item = New SuperFilter("Complex")
item.AddItem("[Name] like %Bob% OR [Name] like %Alice%")
item.AddItem("AND Color = 'Blue'")
item.AddItem("AND Active=True")
item.AddItem("AND AccessRequired < 3")
item.AddItem("AND DateAdded > #2/11/2010#")
item.AddItem("AND CreatedBy = 'ziggy'")
filters.Add(item)
cbo1.DataSource = filters
cbo1.DisplayMember = "Name"
cbo1.ValueMember = "FilterText"
The value member could be the Elements - the collection of filter clauses, or it could be the query text. The GetFilterText method joins them together for you as part of what a filter manager class could/should:
For n As Int32 = 0 To filters.Count - 1
Console.WriteLine("Name: {0} Count: {1}{2}Text:{3}", filters(n).Name,
filters(n).Elements.Count,
Environment.NewLine, filters(n).FilterText)
Next
Result:
Name: Default Count: 1
Text:Id = 7
Name: Blue Ones Count: 1
Text:Color = Blue
Name: Complex Count: 6
Text:[Name] like %Bob% OR [Name] like %Alice% AND Color = 'Blue' AND Active=True AND AccessRequired < 3 AND DateAdded > #2/11/2010# AND CreatedBy = 'ziggy'
If you use "Elements" as the ValueMember you will get back the collection.
The combo displays the Name for the user. On the right, a label displays the ValueMember in this case, it is the FilterText or joined Elements. As I said, you could get back the actual collection as the SelectedValue instead, but that is available as part of SelectedItem.
If savable means beyond the life of the application instance, that is another question, but these are very easily serialized.

How do you assign values to structure elements in a List in VB.NET?

I have a user-defined structure in a list that I am trying to change the value for in an individual element within the list of structures. Accessing the element is not a problem. However, when I try to update the value, the compiler complains:
"Expression is a value and therefore cannot be the target of the
assignment"
For example:
Public Structure Person
Dim first as String
Dim last as String
Dim age as Integer
End Structure
_
Public Sub ListTest()
Dim newPerson as Person
Dim records as List (Of Person)
records = new List (Of Person)
person.first = "Yogi"
person.last = "bear"
person.age = 35
records.Add(person)
records(0).first = "Papa" ' <<== Causes the error
End Sub
As the other comments said, when you refer to records(0), you get a copy of the struct since it is a value type. What you can do (if you can't change it to a Class) is something like this:
Dim p As Person = records(0)
p.first = "Papa"
records(0) = p
Although, I think it's just easier to use a Class.
There are actually two important concepts to remember here.
One is that, as Hans and Chris have pointed out, Structure Person declares a value type of which copies are passed between method calls.
You can still access (i.e., get and set) the members of a value type, though. After all, this works:
Dim people(0) As Person
people(0).first = "Yogi"
people(0).last = "Bear"
people(0).age = 35
So the other important point to realize is that records(0) accesses the List(Of Person) class's special Item property, which is a sugary wrapper around two method calls (a getter and setter). It is not a direct array access; if it were (i.e., if records were an array), your original code would actually have worked.
I had the same problem, and I fixed it by adding a simple Sub to the structure that changes the value of the property.
Public Structure Person
Dim first as String
Dim last as String
Dim age as Integer
Public Sub ChangeFirst(value as String)
me.first = value
End Sub
End Structure