"Items collection must be empty before using ItemsSource."

Question

"Items collection must be empty before using ItemsSource."

.net binding entity-framework vb.net wpf

Question

I'm trying to get images to display in a WPF ListView styled like a WrapPanel as described in this old ATC Avalon Team article: How to Create a Custom View.

WPF ListView WrapPanel Images

When I try to populate the ListView with a LINQ-to-Entities queried collection of ADO.NET Entity Framework objects I get the following exception:

Exception

Items collection must be empty before using ItemsSource.

My code…

Visual Basic

Private Sub Window1_Loaded(...) Handles MyBase.Loaded
    ListViewImages.ItemsSource = From g In db.Graphic _
                                 Order By g.DateAdded Ascending _
                                 Select g
End Sub

XAML

<ListView Name="ListViewImages"
          SelectionMode="Single"
          ItemsSource="{Binding}">
    <local:ImageView />
</ListView>

I put a breakpoint on that line. ListViewImages.ItemsSource is Nothing just before the LINQ assignment.

Zack Peterson

Popular Answer

I had this same error for a while in a slightly different scenario. I had

<wpftoolkit:DataGrid
    AutoGenerateColumns="False"
    ItemsSource="{Binding Path=Accounts}" >
    <wpftoolkit:DataGridTextColumn 
        Header="Account Name" 
        Binding="{Binding Path=AccountName}" />
</wpftoolkit:DataGrid>

which I fixed to be

<wpftoolkit:DataGrid
    AutoGenerateColumns="False"
    ItemsSource="{Binding Path=Accounts}" >
    <wpftoolkit:DataGrid.Columns>
        <wpftoolkit:DataGridTextColumn 
            Header="Account Name" 
            Binding="{Binding Path=AccountName}" />
    </wpftoolkit:DataGrid.Columns>
</wpftoolkit:DataGrid>

kenwarner

View more on Stack Overflow

Accepted Answer

The reason this particular exception gets thrown is that the content of the element gets applied to the ListView's Items collection. So the XAML initialises the ListView with a single local:ImageView in its Items collection. But when using an ItemsControl you must use either the Items property or the ItemsSource property, you can't use both at the same time. Hence when the ItemsSource attribute gets processed an exception is thrown.

You can find out which property the content of an element will get applied to by looking for the ContentPropertyAttribute on the class. In this case it's defined higher in the class hierarchy, on the ItemsControl:

[ContentPropertyAttribute("Items")]

The intention here was that the ListView's View be set to a local:ImageView so the fix is to explicitly indicate the property to be set.

Fix the XAML and the exception goes away:

<ListView Name="ListViewImages"
          SelectionMode="Single"
          ItemsSource="{Binding}">
    <ListView.View>
        <local:ImageView />
    </ListView.View>
</ListView>

It was missing that <ListView.View> tag.

"Items collection must be empty before using ItemsSource."

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