Limit the size of List(Of T) - VB.NET

Question

I am trying to limit the size of my generic list so that after it contains a certain amount of values, it won't add any more.

I am trying to do this using the Capacity property of the List object, but this does not seem to work.

        Dim slotDates As New List(Of Date)
        slotDates.Capacity = 7

How would people advice limiting the size of a list?

I am trying to avoid checking the size of the List after each object is added.

Answer 1

There is no built-in way to limit the size of a List(Of T). The Capacity property is merely modifying the size of the underyling buffer, not restricting it.

If you want to limit the size of the List, you'll need to create a wrapper which checks for invalid size's. For example

Public Class RestrictedList(Of T)
  Private _list as New List(Of T)
  Private _limit as Integer
  Public Property Limit As Integer 
    Get 
      return _limit
    End Get
    Set 
      _limit = Value
    End Set
  End Property

  Public Sub Add(T value) 
    if _list.Count = _limit Then
      Throw New InvalidOperationException("List at limit")
    End If
    _list.Add(value)
  End Sub
End Class

Answer 2

You should implement your own list/collection if you need to restrict the maximum quantity of item in it.

Answer 3

List has no such facility.

The capacity stuff is just a performance optimisation.

You are going to have to roll your own class, derive off list and override the Add implementation.

Answer 4

You'll want to derive a new LimitedList and shadow the adding methods. Something like this will get you started.

public class LimitedList<T> : List<T>
{
    private int limit;

    public LimitedList(int limit)
    {
        this.limit = limit;
    }

    public new void Add(T item)
    {
        if (Count < limit)
            base.Add(item);
    }
}

Just realised you're in VB, I'll translate shortly

Edit See Jared's for a VB version. I'll leave this here in case someone wants a C# version to get started with.

For what it's worth mine takes a slightly different approach as it extends the List class rather than encapsulating it. Which approach you want to use depends on your situation.

Answer 5

There are several different ways to add things to a List<T>: Add, AddRange, Insert, etc.

Consider a solution that inherits from Collection<T>:

Public Class LimitedCollection(Of T)
    Inherits System.Collections.ObjectModel.Collection(Of T)

    Private _Capacity As Integer
    Public Property Capacity() As Integer
        Get
            Return _Capacity
        End Get
        Set(ByVal value As Integer)
            _Capacity = value
        End Set
    End Property

    Protected Overrides Sub InsertItem(ByVal index As Integer, ByVal item As T)
        If Me.Count = Capacity Then
            Dim message As String = 
                String.Format("List cannot hold more than {0} items", Capacity)
            Throw New InvalidOperationException(message)
        End If
        MyBase.InsertItem(index, item)
    End Sub

End Class

This way the capacity is respected whether you Add or Insert.