Collection was modified when trying to remove sele

问题:

by following this answer I coded almost same way. Below is part of my code.

XAML binding:

<cst:CustomDataGrid x:Name="grdItems"   ItemsSource="{Binding myItems, IsAsync=True}"   SelectedItemsList="{Binding SelectedItems, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"                            

ViewModel:

   Protected Overrides Sub DeleteCurrent()
        Dim msg As New UserMessage()
        If SelectedItems IsNot Nothing Then
            For Each SelectedItem In SelectedItems
                  myItems.Remove(SelectedItem)
            Next

            RaisePropertyChanged("Items")

        End If

    End Sub


Private m_myItems As ObservableCollection(Of item)
    Public Property myItems() As ObservableCollection(Of item)
        Get
            Return m_myItems
        End Get
        Set(value As ObservableCollection(Of item))
            m_myItems = value
        End Set
    End Property


    Private m_SelectedItem As IList = New List(Of item)()
    Public Property SelectedItems() As IList
        Get
            Return m_SelectedItem
        End Get
        Set(value As IList)
            m_SelectedItem = value
            '   RaisePropertyChanged("SelectedItems")
        End Set
    End Property

Selection works perfectly fine. But when I am looping to remove selected each item, I am getting exception that "Collection was modified. Enumeration operation may not execute.". For example, I select 3 rows and press delete keyboard button, when I observe my SelectedItems object, count is 3 as expected but right after executing the line with myItems.Remove(SelectedItem), count goes down to 1. why is that happening, i couldnt figure out because I am not modifing SelectedItems but myItems.

PS; customdatagrid is exactly same code as the original post above. thats why i didnt add it here.

回答1:

just copy selecteditems to new collection when doing foreach loop, e.g using ToArray linq extension method:

For Each SelectedItem In SelectedItems.ToArray()
    myItems.Remove(SelectedItem)
 Next

回答2:

" i couldnt figure out because I am not modifing SelectedItems but myItems"

You're removing item from myItems which used as ItemsSource of the data grid. Removing item from ItemsSource automatically followed by removing it from SelectedItems, and that behavior makes perfect sense (you can't keep an item selected when it is no longer in the data grid).

That's why the error. You can't let the collection changed while enumerating through it.

One possible way to accomplish this is by copying selected items to temporary collection, to a temporary array, for example. Then enumerate the array instead of the original collection :

Dim copyOfSelectedItems(SelectedItems.Count-1) As Object
SelectedItems.CopyTo(copyOfSelectedItems,0)
For Each SelectedItem In copyOfSelectedItems
      myItems.Remove(SelectedItem)
Next