re: List transactions

silky — Fri, 02 Jan 2009 04:18:40 GMT

Nice idea, but calling 'Commit' on Dispose is pretty bad form. Commit should be explicitly called, like in every other transactional scenario.

Also worth noting it's not thread-safe.

An alternate version

andy — Wed, 31 Dec 2008 05:05:36 GMT

Imports System.Collections.Generic

Module ListExtensions

<Runtime.CompilerServices.Extension()> _

Function GetTransaction(Of T)(ByVal list As IList(Of T)) As ListTransaction(Of T)

Return New ListTransaction(Of T)(list)

End Function

Public Class ListTransaction(Of T)

Implements IDisposable, IList(Of T)

Private _list As IList(Of T)

Private _clone As List(Of T)

Public Sub New(ByVal list As IList(Of T))

_list = list

End Sub

Public Sub Add(ByVal item As T) Implements ICollection(Of T).Add

If _clone Is Nothing Then _clone = New List(Of T)(_list)

_clone.Add(item)

End Sub

Public Function Remove(ByVal item As T) As Boolean Implements ICollection(Of T).Remove

If _clone Is Nothing Then _clone = New List(Of T)(_list)

Return _clone.Remove(item)

End Function

Public Sub Clear() Implements ICollection(Of T).Clear

If _clone Is Nothing Then

_clone = New List(Of T)

Else

_clone.Clear()

End If

End Sub

Public Function Contains(ByVal item As T) As Boolean Implements ICollection(Of T).Contains

If _clone Is Nothing Then

Return _list.Contains(item)

Else

Return _clone.Contains(item)

End If

End Function

Public Sub CopyTo(ByVal array() As T, ByVal arrayIndex As Integer) Implements ICollection(Of T).CopyTo

If _clone Is Nothing Then

_list.CopyTo(array, arrayIndex)

Else

_clone.CopyTo(array, arrayIndex)

End If

End Sub

Public ReadOnly Property Count() As Integer Implements ICollection(Of T).Count

Get

If _clone Is Nothing Then

Return _list.Count

Else

Return _clone.Count

End If

End Get

End Property

Public ReadOnly Property IsReadOnly() As Boolean Implements ICollection(Of T).IsReadOnly

Get

Return _list.IsReadOnly

End Get

End Property

Public Function GetEnumerator() As IEnumerator(Of T) Implements IEnumerable(Of T).GetEnumerator

If _clone Is Nothing Then

Return _list.GetEnumerator

Else

Return _clone.GetEnumerator

End If

End Function

Private Function mGetEnumerator() As System.Collections.IEnumerator Implements System.Collections.IEnumerable.GetEnumerator

If _clone Is Nothing Then

Return DirectCast(_list, System.Collections.IEnumerable).GetEnumerator

Else

Return DirectCast(_clone, System.Collections.IEnumerable).GetEnumerator

End If

End Function

Public Function IndexOf(ByVal item As T) As Integer Implements IList(Of T).IndexOf

If _clone Is Nothing Then

Return _list.IndexOf(item)

Else

Return _clone.IndexOf(item)

End If

End Function

Public Sub Insert(ByVal index As Integer, ByVal item As T) Implements IList(Of T).Insert

If _clone Is Nothing Then _clone = New List(Of T)(_list)

_clone.Insert(index, item)

End Sub

Default Public Property Item(ByVal index As Integer) As T Implements IList(Of T).Item

Get

If _clone Is Nothing Then

Return _list.Item(index)

Else

Return _clone.Item(index)

End If

End Get

Set(ByVal value As T)

If _clone Is Nothing Then _clone = New List(Of T)(_list)

_clone.Item(index) = value

End Set

End Property

Public Sub RemoveAt(ByVal index As Integer) Implements IList(Of T).RemoveAt

If _clone Is Nothing Then _clone = New List(Of T)(_list)

_clone.RemoveAt(index)

End Sub

Public Sub Commit()

If _clone Is Nothing Then Return

_list.Clear()

Dim genericList As List(Of T) = TryCast(_list, List(Of T))

If genericList IsNot Nothing Then

genericList.Capacity = _clone.Count

End If

For Each loItem As T In _clone

_list.Add(loItem)

End Sub

Public Sub Rollback()

_clone = Nothing

End Sub

Public Sub Dispose() Implements IDisposable.Dispose

Commit()

GC.SuppressFinalize(Me)

End Sub

End Class

End Module

re: List transactions

andy — Wed, 31 Dec 2008 04:36:25 GMT

You could get around the add/remove/add problem by maintaining a single list of actions (i.e. merging _adds and _removes together) and then Commit performing them in order.

It might be nice to support Contains too; in fact having ListTransaction implement IList would be really cool.

Bill McCarthy: List transactions… « notgartner

Bill McCarthy: List transactions… « notgartner — Wed, 31 Dec 2008 03:14:44 GMT

Pingback from Bill McCarthy: List transactions… « notgartner

re: List transactions

bill — Fri, 19 Dec 2008 02:40:30 GMT

Hi Martin,

Try it. You'll find OjbA is in the list, at least with List(Of T).

If you had a unique list, then this might be a problem.

re: List transactions

MartinJ — Thu, 18 Dec 2008 17:09:32 GMT

There's an edge case that will fail.

Add(objA)

Remove(objA)

Add(objA)

Commit

objA will not be in the list after Commit is called. The best way that I can think to do this is to maintain two lists. One contains the committed items. The other contains the working list. Commit will replace the commited list with the working one. Rollback will replace the working one with the commited list.

This could get tricky.