Reset DataContext on ComboBox

[=?Utf-8?B?V29ua28gdGhlIFNhbmU=?=]

Hello,

I have a ComboBox that has a DataContext set on it. When a PropertyChanged
event is fired (based on a static instance of some class), I'd like to update
the DataContext on the combo, but it seems that I need to set it to null
first:

Combo1.DataCont ext = null;
Combo1.DataCont ext = dc;

If I don't set it to null first, the combo's Items count is 0.

Thanks,
WtS

[Ji Zhou [MSFT]]

RE: Reset DataContext on ComboBox

Hello Paul,

Thanks for using Microsoft Newsgroup Support Service, my name is Ji Zhou
[MSFT] and I will be working on this issue with you.

Based on my understanding, the current issue is that we use the data
binding in WPF and update the comboBox's DataContext, but the data in the
comboBox does not get updated unless we set the DataContext to null first
and set it back. If any misunderstandin g here, please feel free to correct
me.

Actually, if we update the DataContext's content, the comboBox will not
know it's DataContext has been changed. So, we will not see the combox
items' updating. The way to work around this problem is to implement
INotifyProperty Changed for the data in the DataContext. I write the
following codes on my side which works fine:

In the WPF xaml file:

<Window x:Class="WpfApp lication1.Windo w1"
xmlns="http://schemas.microso ft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microso ft.com/winfx/2006/xaml"
Title="Window1" Height="300" Width="300" Loaded="Window_ Loaded">
<StackPanel>
<ComboBox Name="comboBox1 " IsSynchronizedW ithCurrentItem= "True"

ItemsSource="{B inding}" DisplayMemberPa th="CustomerNam e"/>
<Button Name="button1" Click="button1_ Click">Button</Button>
</StackPanel>
</Window>

I create a MyCustomer class which implements the INotifyProperty Changed
interface:

public class MyCustomer : INotifyProperty Changed
{
private string customerName = String.Empty;

public event PropertyChanged EventHandler PropertyChanged ;

private void NotifyPropertyC hanged(String info)
{
if (PropertyChange d != null)
{
PropertyChanged (this, new PropertyChanged EventArgs(info) );
}
}

public MyCustomer(stri ng name)
{
this.customerNa me = name;
}

public string CustomerName
{
get { return this.customerNa me; }
set
{
if (value != this.customerNa me)
{
this.customerNa me = value;
NotifyPropertyC hanged("custome rName");
}
}
}
}

In the Window1.xaml.cs , I have the following codes. Whenever the button is
clicked, I update the dc's content and my combobox items get updated in the
UI.

public partial class Window1 : Window
{
ArrayList dc = new ArrayList();

public Window1()
{
InitializeCompo nent();
dc.Add(new MyCustomer("Ere ka"));
dc.Add(new MyCustomer("Col bert"));
}

private void Window_Loaded(o bject sender, RoutedEventArgs e)
{
this.comboBox1. DataContext = dc;
}

private void button1_Click(o bject sender, RoutedEventArgs e)
{
(dc[0] as MyCustomer).Cus tomerName = "Tom";
}
}

Please let me know if the above suggestion works for your scenario or not.
And if you have any questions or concerns about this, please feel free to
post.

Best regards,
Ji Zhou (v-jzho@online.mic rosoft.com, remove 'online.')
Microsoft Online Community Support

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[=?Utf-8?B?V29ua28gdGhlIFNhbmU=?=]

RE: Reset DataContext on ComboBox

Hi Zhou,

Thank you - that essentially answers my question. However, I do see an
issue if I take your example a little further. I added some buttons with
event handlers:


[Window1.xaml]
<StackPanel>
<ComboBox Name="comboBox1 " IsSynchronizedW ithCurrentItem= "True"

ItemsSource="{B inding}" DisplayMemberPa th="CustomerNam e"/>

<Button Name="button1" Click="button1_ Click">Change First
Item</Button>
<Button Name="button2" Click="button2_ Click">Add One Item</Button>
<Button Name="button3" Click="button3_ Click">Add Items, No
Null</Button>
<Button Name="button4" Click="button4_ Click">Add Items, Null
dc</Button>
</StackPanel>

[Window1.xaml.cs]
private void button2_Click(o bject sender, RoutedEventArgs e)
{
dc.Add(new MyCustomer("Yin "));
this.comboBox1. DataContext = dc;
}

private void button3_Click(o bject sender, RoutedEventArgs e)
{
dc.Add(new MyCustomer("Smo oshie"));
dc.Add(new MyCustomer("Jac ko"));
dc.Add(new MyCustomer("Fis her"));
this.comboBox1. DataContext = dc;
this.comboBox1. SelectedIndex = 3;
}

private void button4_Click(o bject sender, RoutedEventArgs e)
{
dc.Add(new MyCustomer("Ral ph"));
dc.Add(new MyCustomer("Spe ncer"));
dc.Add(new MyCustomer("Lul u"));
this.comboBox1. DataContext = null;
this.comboBox1. DataContext = dc;
this.comboBox1. SelectedIndex = 4;
}


If I press button 2 to add a single customer, and then press button 3 to add
other customers, I will see the customer as specified in the SelectedIndex.
However, if you drop down the combo, only the original 3 (sometimes, only the
original 2) items are shown. None of the new customers added in the button3
handler are shown.

If you run the application again, and this time choose button 4 instead of
button 3, you will see all of the customers when expanding the combo. The
only difference is that I set the DataContext to null before setting it to
the updated ArrayList.

Thanks,
Paul