"Operation is not valid while ItemsSource is in use. Access and modify elements with ItemsControl.ItemsSource instead." with no known cause

Will Pittenger 281

I have a data bound System.Windows.Controls.ItemsControl that's giving me a mysterious runtime exception. It's acting as though I'm attempting to add items with its Items property. But I'm not. Unfortunately, the ItemsControl in question is one of a series of nested ItemsControl instances via their ItemTemplate property. The exception I'm getting doesn't specify which one. If I could tell what the DataSource value was, I might be able to figure it out. But when I look at the entries in the call stack at the point of the exception being thrown, most of the relevant values appear to be unavailable to the Debugger. I'm using Visual Studio 2022. My project is targeting .NET 6.0.

What's wrong? How do I track down which source is the problem?

Hui Liu-MSFT 40,266 Reputation points Microsoft Vendor

2022-04-04T01:25:58.863+00:00

Hi,@Will Pittenger . Is it possible to show the complete minimal code that reproduces the error? Could you show a screenshot of the line of code where the error occurs and the error message？
Will Pittenger 281 Reputation points

2022-04-04T01:28:49.677+00:00

No. I don't know what portion of the program to send you. It doesn't show up until runtime. It's a binding error that happens then. If I knew how to find out which item control was the problem, I might not have posted. :(

Pravin Chaudhary 1

I am getting the same error.

I am populating a TreeView:

        <TreeView ItemsSource="{Binding Topics}">  
            <TreeView.ItemTemplate >  
                <HierarchicalDataTemplate>  
                    <TreeViewItem Header="{Binding Name}"  ItemsSource="{Binding Partitions}">  
                        <HierarchicalDataTemplate>  
                            <TreeViewItem Header="{Binding Name"}/>  
                        </HierarchicalDataTemplate>  
                    </TreeViewItem>  
                </HierarchicalDataTemplate>  
            </TreeView.ItemTemplate>  
        </TreeView>

Here, Topics is an ObservableCollection. Each topic contains Partitions, which is also an ObservableCollection.

1 answer

Nils Berghs 1

I had the exact same error message, and spent way to much time figuring it out, because of the confusing error message.

The cause of the problem was that I defined the "dataTemplate" in an ItemsControl as content:

<ItemsControl ItemsSource="{Binding SomeList}">  
    <Grid>  
        ... template ...  
    </Grid>  
</ItemsControl>

Fixed it by proparly declaring the template

<ItemsControl ItemsSource="{Binding SomeList}">  
    <ItemsControl.ItemTemplate>  
        <DataTemplate>  
             <Grid>  
                  ... template ...  
             </Grid>  
        </DataTemplate>  
    </ItemsControl.ItemTemplate>  
</ItemsControl>

It something that I have already done a hundred times, but I was completely fooled by the message and started looking in the wrong file...

Will Pittenger 281 Reputation points

2022-09-13T14:10:03.783+00:00

As stated, I'm already using ItemTemplate.

Steve Dinn (sdinn) 0

I am having the same issue, specifically with a TreeView, the same as the original poster. As well, the ItemsSource is an ObservableCollection<T>.

<TreeView
    x:Name="FilesystemHierarchy"
    Margin="10,0,0,0"
    HorizontalAlignment="Stretch"
    VerticalAlignment="Stretch"
    MinHeight="175"
    ItemsSource="{Binding FileSystemHierarchy}">
    <TreeView.ItemTemplate>
        <HierarchicalDataTemplate
            DataType="x:Type local:FileSystemItem"
            ItemsSource="{Binding Children}">
            <TextBlock
                Text="{Binding Name}"
                Margin="3" />
        </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
    <Style TargetType="TreeViewItem">
        <Setter Property="IsExpanded" Value="{Binding IsExpanded}" />
        <Setter Property="IsSelected" Value="{Binding IsSelected}" />
        <Setter Property="FontWeight" Value="Normal" />
        <Style.Triggers>
            <Trigger Property="IsSelected" Value="True">
                <Setter Property="FontWeight" Value="Bold" />
            </Trigger>
        </Style.Triggers>
    </Style>
</TreeView>