ForEach Example
The fun starts when we use the methods. To begin with, let's implement the familiar ForEach method. The Microsoft documentation includes this usage syntax definition:
Dim instance As List
Dim action As Action(Of T)
instance.ForEach(action) Microsoft further defines action as "delegate to a method that performs an action on the object passed to it. The elements of the current List(T) are individually passed to the Action(T) delegate"
If you need more on delegates, you might want to try the About Visual Basic article, Using Delegates in Visual Basic .NET for Runtime Flexibility.
The first thing you need to code is the method that will be delegated. Misunderstanding this one key point is the source of most of the confusion of VB.NET students. This function or subroutine is where all of the customized coding for the "Of <T>" type objects is done. When you get this right, you're essentially done. In this first example, it's really simple. An entire instance of the Bottle is passed and the subroutine selects anything needed out of it. Coding the ForEach itself is simple too. Just fill in the address of the delegate using the AddressOf method.
Sub displayBottle(ByVal b As Bottle)
ResultList.Items.Add( _
b.Brand & " - " & _
b.Name & " - " & _
b.Category & " - " & _
b.Size)
End Sub
Private Sub ForEachButton_Click( ...
ResultList.Items.Clear()
ResultList.Items.Add("For Each Example")
ResultList.Items.Add("-----------------------")
Cabinet.ForEach(AddressOf displayBottle)
End Sub FindAll Example
FindAll is a little more complicated. The Microsoft documentation for FindAll looks like this:
Dim instance As List
Dim match As Predicate(Of T)
Dim returnValue As List(Of T)
returnValue = instance.FindAll(match) This syntax includes a new element, Predicate(Of T). According to Microsoft, this will represent the method "that defines a set of criteria and determines whether the specified object meets those criteria." In other words, you can create any code that will find something in the list. I coded my Predicate(Of T) to find anything in the "Beer" Category:
Instead of calling the delegate code for each item in the list, FindAll returns an entire List(T) containing only the matches that result from your Predicate(Of T). It's up to your code to both define this second List(T) and do something with it. My code just adds the items to a ListBox.
Private Sub FindAllButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles FindAllButton.Click
ResultList.Items.Clear()
ResultList.Items.Add("FindAll Example")
ResultList.Items.Add("-----------------------")
Dim sublist As List(Of Bottle)
sublist = Cabinet.FindAll(AddressOf findBeer)
For Each r As Bottle In sublist
ResultList.Items.Add( _
r.Brand & " - " & _
r.Name & " - " & _
r.Category & " - " & _
r.Size)
Next
End Sub
Function findBeer(ByVal b As Bottle) _
As Boolean
If (b.Category = "Beer") Then
Return True
Else
Return False
End If
End Function Sort Example
The final method this article examines is Sort. Again, Microsoft uses some terminology you might not be familiar with. There are actually four different overloads of the Sort method:
This lets you use sort methods defined in the .NET Framework for the list, code your own, use a system defined comparison for the type, or sort part of the collection using a starting position and count parameter. In this example, since I use the syntax ...
x.Name.x.Name.CompareTo(y.Name)(y.Name) ... to actually perform the sort, I'm using the third overload. I've coded another delegate to my own comparer. Since I want to sort by my Name, I pull just that value out of each instance of the Bottle object that is passed and use the Sort(Comparison<(Of <(T>)>)). The Sort method actually rearranges the original List(T). So that's what is processed after the method is executed.
Private Sub SortButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles SortButton.Click
ResultList.Items.Clear()
ResultList.Items.Add("Sort Example")
ResultList.Items.Add("-----------------------")
Cabinet.Sort(AddressOf sortCabinet)
For Each r As Bottle In Cabinet
ResultList.Items.Add( _
r.Name & " - " & _
r.Brand & " - " & _
r.Category & " - " & _
r.Size)
Next
End Sub
Private Shared Function sortCabinet( _
ByVal x As Bottle, ByVal y As Bottle) As Integer
Return x.Name.CompareTo(y.Name)
End Function These methods were selected to demonstrate the major ways that the Framework methods in List(T) are actually coded. There's a whole raft of other methods, however. That's what makes List(T) so useful!
If you would like to experiment with generic lists yourself, Click Here to download of the completed system above to help you get started.
