WPF design time error Object reference not set to an instance of an object

dependency-properties, design-time, observablecollection, wpf, xaml

Solution

I usually check to see if i am in design time before wiring up events etc..

It could be that your Container is null in design mode.

public class Utils
{
    public static bool IsInDesignerMode
    {
        get
        {
            return ((bool)(DesignerProperties.IsInDesignModeProperty
                .GetMetadata(typeof(DependencyObject)).DefaultValue));
        }
    }

}

Maybe in your constructor you should do this..

public UC()    
{    
    InitializeComponent();    
    if (!Utils.IsInDesignerMode)
    {
        Labels = new ObservableCollection<Label>();    

        Labels.CollectionChanged += new         System.Collections.Specialized.NotifyCollectionChangedEventHandler(Labels_CollectionChanged);    
    }
}

On another note though i think you would be better using an ItemsControl

Problem

Ok I am getting sick of this problem. I am trying to make UserControl where I can populate its content from XAML. Previously I have created ObservableCollection DependencyProperty. It was working at runtime but at design time there was an error: Object reference not set to an instance of an object. Now I did simpler version: ``` public partial class UC : UserControl { public UC() { Labels = new ObservableCollection<Label>(); InitializeComponent(); Labels.CollectionChanged += new System.Collections.Specialized.NotifyCollectionChangedEventHandler(Labels_CollectionChanged); } void Labels_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e) { foreach (Label uc in e.NewItems) Container.Children.Add(uc); } private ObservableCollection<Label> _lables = new ObservableCollection<Label>(); public ObservableCollection<Label> Labels { get { return _lables; } set { _lables = value; } } } ``` This is how I like to use my UserControll ``` <Window x:Class="WpfApplication1.MainWindow" xmlns:gsh="clr-namespace:WpfApplication1" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="MainWindow" Height="350" Width="525"> <Grid Margin="0,0,0,30"> <gsh:UC x:Name="uC1"> <gsh:UC.Labels> <Label Content="qwerty1" /> <Label Content="qwerty2" /> </gsh:UC.Labels> </gsh:UC> </Grid> ``` However it still gives me the error at design time: Object reference not set to an instance of an object. So if anyone can help me please. How can I make UserControl that I can use like native controls which I can populate with collection of elements? I am trying to find the answer second day already.

Original source