Is there a list (of string) or equivalent in VB that I can configure to only hold x entries and then overwrite oldest

Question

Is there a list (of string) or equivalent in VB that I can configure to only hold x entries and then overwrite oldest each time I add something new to it?

e.g. I can set it to hold say 25 items, and then if I add a 26th item it overwrites the first item and so on

Answer 1

HI,
you create your own list class like in following console demo. Add insert item at the top of list.

Module Module64  
  Sub Main()  
    Try  
      Call (New Demo).Execute()  
    Catch ex As Exception  
      Console.WriteLine(ex.ToString)  
    End Try  
    Console.WriteLine("Continue enter key")  
    Console.ReadKey()  
  End Sub  
  
  Friend Class Demo  
    Friend Sub Execute()  
      Dim itemList As New MyList(Of String)(5)  
      For i = 1 To 7  
        itemList.Add($"Item {i}")  
      Next  
      For Each item In itemList  
        Console.WriteLine(item)  
      Next  
    End Sub  
  End Class  
  
  Friend Class MyList(Of T)  
    Inherits List(Of T)  
  
    Private _maxCount As Integer  
    Public Sub New(maxCount As Integer)  
      Me._maxCount = maxCount  
    End Sub  
  
    Friend Shadows Sub Add(item As T)  
      Me.Insert(0, item)  
      If Me.Count > Me._maxCount Then Me.RemoveAt(Me.Count - 1)  
    End Sub  
  End Class  
  
End Module  

Result:

Answer 2

Hi
Try this example and see if it works as you want. Set the MaxItems variable to whatever you want.

' Form1 with ListBox1
' and Button1
Option Strict On
Option Explicit On
Public Class Form1
  Dim MaxItems As Integer = 6
  Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Static c As Integer = 1
    With ListBox1
      .Items.Add("Item" & c.ToString)
      c += 1
      If .Items.Count > MaxItems Then
        .Items.RemoveAt(0)
      End If
    End With
  End Sub
End Class

Answer 3

Hi,
you create your own list class based an FIFO queue like in following console demo:

Module Module1
  Sub Main()
    Try
      Call (New Demo).Execute()
    Catch ex As Exception
      Console.WriteLine(ex.ToString)
    End Try
    Console.WriteLine("Continue enter key")
    Console.ReadKey()
  End Sub

  Friend Class Demo
    Friend Sub Execute()
' instantiate List with max count 5 '
      Dim itemList As New MyList(Of String)(5)
' load list with 7 items '
      For i = 1 To 7
        itemList.Add($"Item {i}")
      Next
' display result - only 5 items '
      For Each item In itemList
        Console.WriteLine(item)
      Next
    End Sub
  End Class

  Friend Class MyList(Of T)
    Inherits Queue(Of T)

    Private _maxCount As Integer
    Public Sub New(maxCount As Integer)
      Me._maxCount = maxCount
    End Sub

    Friend Sub Add(item As T)
      Me.Enqueue(item)
      If Me.Count > Me._maxCount Then Me.Dequeue()
    End Sub
  End Class

End Module

Answer 4

Hi LesHay,

Thank you, that solution led me down the right path to do same for a List (Of String)

Public Class Form1

    Dim MaxItems As Integer = 6
    Dim mylist As New List(Of String)

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

        With mylist
            .Add("Item" & c.ToString)
            c += 1
            If .Count > MaxItems Then
                .RemoveAt(0)
            End If
        End With

        ' output to listbox to confirm working
        ListBox1.Items.Clear()

        For Each item In mylist
            ListBox1.Items.Add(item)
        Next

    End Sub

End Class

Answer 5

Hello @Darren Rose

Another option is using a class which inherits Stack

Public Class FixedSizeStack  
    Inherits Stack  
  
    Private MaxNumber As Integer  
    Public Sub New(Limit As Integer)  
        MaxNumber = Limit  
    End Sub  
  
    Public Overrides Sub Push(obj As Object)  
        If Count < MaxNumber Then  
            MyBase.Push(obj)  
        End If  
    End Sub  
End Class  

Full example using integer then string and kept it simple.

Module Module1  
  
    Sub Main()  
        Dim originalColor = Console.ForegroundColor  
  
        Dim StackData1 As New FixedSizeStack(5)  
  
        Console.ForegroundColor = ConsoleColor.White  
        Console.WriteLine("Adding")  
        Console.ForegroundColor = originalColor  
        For index As Integer = 0 To 9  
            StackData1.Push(index + 1)  
        Next index  
  
        Console.WriteLine()  
  
  
        Console.ForegroundColor = ConsoleColor.White  
        Console.WriteLine("Items in StackData")  
        Console.ForegroundColor = originalColor  
  
        Dim results = StackData1.OfType(Of Integer)().OrderBy(Function(x) x).ToList()  
        For Each result As Integer In results  
            Console.WriteLine(result)  
        Next  
  
        Dim StackData2 As New FixedSizeStack(3)  
        StackData2.Push("A")  
        StackData2.Push("B")  
        StackData2.Push("C")  
        StackData2.Push("D")  
        StackData2.Push("E")  
        StackData2.Push("F")  
        StackData2.Push("G")  
  
  
        Console.WriteLine()  
        Console.ForegroundColor = ConsoleColor.White  
        Console.WriteLine("Items in StackData2")  
        Console.ForegroundColor = originalColor  
  
        Dim results1 = StackData2.OfType(Of String).Reverse().ToList()  
  
        For Each result As String In results1  
            Console.WriteLine(result)  
        Next  
  
        Console.ReadLine()  
  
    End Sub  
  
End Module  
Public Class FixedSizeStack  
    Inherits Stack  
  
    Private MaxNumber As Integer  
    Public Sub New(Limit As Integer)  
        MaxNumber = Limit  
    End Sub  
  
    Public Overrides Sub Push(obj As Object)  
        If Count < MaxNumber Then  
            MyBase.Push(obj)  
        End If  
    End Sub  
End Class