Question

my xaml file

<ListBox Height="522" HorizontalAlignment="Left" Margin="20,162,0,0" Name="listBox1" VerticalAlignment="Top" Width="448" ItemsSource="{Binding}">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <StackPanel Orientation="Horizontal">
                <TextBlock Text="{Binding Text}" Foreground="#FFC8AB14" FontSize="36" />
            </StackPanel>
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

xaml.cs file

       listBox1.Items.Clear();
       for (int i = 0; i < tasks.Count(); i++) {
            List<Taskonlistbox> dataSource = new List<Taskonlistbox>();
            dataSource.Add(new Taskonlistbox() {Text = "Blalalalala"} );
            this.listBox1.ItemsSource = dataSource; // visual stdio shows error here:
        }

Taskonlistbox:

public class Taskonlistbox
{
    public string Text { get; set; }
}

Error: “Items collection must be empty before using ItemsSource”
whats a problem?

Answer 1

You want to create the list only once and assign the data source only once! Therefore, create the list before the loop and assign the data source after the loop

// Clear the listbox.
// If you never add items with listBox1.Items.Add(item); you can drop this statement.
listBox1.Items.Clear();

// Create the list once.
List<Taskonlistbox> dataSource = new List<Taskonlistbox>(); 

// Loop through the tasks and add items to the list.
for (int i = 0; i < tasks.Count(); i++) { 
    dataSource.Add(new Taskonlistbox {Text = "Blalalalala"} ); 
}

// Assign the list to the `ItemsSouce` of the listbox once.
this.listBox1.ItemsSource = dataSource;