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Unable to bind collection to ComboBox in silverlight

I've a ObservableCollection, when I want to display this ObservableCollection with CheckBox I am simply binding the contents to CheckBox but when I want to display the same collection by using ComboBox I am unable to do that. Any suggestions?
XAML: Display collection using CheckBox --WORKS
<ItemsControl ItemsSource="{Binding Synonyms}">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel Orientation="Horizontal" Margin="0,5,0,0" />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemTemplate>
<DataTemplate>
<!--Display items in CheckBox-->
<CheckBox Content="{Binding Display}" Margin="10,0,0,0" /> // Display is the collection.
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
Output:
XAML: Display collection using ComboBox --NOTHING OVER HERE
<ItemsControl ItemsSource="{Binding Synonyms}">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel Orientation="Horizontal" Margin="0,5,0,0" />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemTemplate>
<DataTemplate>
<!--Display items in ComboBox.-->
Approach - 1
<ComboBox>
<ComboBoxItem Content="{Binding Display}"/>
</ComboBox>
Approach - 2
<ComboBox ItemsSource="{Binding Path=Synonyms}" DisplayMemberPath="Display"/>
Approach - 3
<ComboBox >
<ComboBox.ItemTemplate>
<DataTemplate>
<Border BorderBrush="Green" BorderThickness="1" Padding="5">
<TextBlock Text="{Binding Path=Display,StringFormat='Display: {0}'}" />
</Border>
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
Output:
I want to display items(one,two,three, etc.) inside oneComboBox with Select All option. I've tried several approaches but nothing. What am I missing here?

The checkbox is not designed to hold multiple items unlike the Combobox so implementing a system designed for the checkbox is not relevant for the combobox.
To solve your issue, remove the combobox from the ItemsControl and have it stand on its own:
<ComboBox ItemsSource="{Binding Synonyms}" DisplayMemberPath="Display"/>
Which tells the combobox to bind its ItemsSource to the data context which is unspecified, which is fine, so it then gets it's parent's data context. That process works its way up to each parent until it finds a bound data context (most likely the page's datacontext to a VM instance).
Assuming that the data context is valid at some point in the visual tree, it will bind to that instance and look for a property named Synonyms. From the Synonyms property it will use that as a list to display items.
To show (display) text in the combobox (instead defaulting to the item's ToString()) the combobox will show the string from the item's property Display.
Giving a list of items in one drop down.

The short answer, is you should use a ComboBox as the root element, not ItemsControl. CompboBox is just a specialized version of ItemsControl.
<ComboBox ItemsSource="{Binding Synonyms}" DisplayMemberPath="Display"/>
The longer answer.
ComboBox derives from ItemsControl, so you get all the features the base class, plus additional features.
ItemsControl (and its derived classes) provides a way of repeating a set of data in the UI. The DataTemplate is where you specify what UI you want for each "row" of data in the Synonyms source.
What you are doing is asking Silverlight to create a separate ComboBox for each underlying data row.
You can still use a DataTemplate within the ComboBox. Like this.
<ComboBox ItemsSource="{Binding Synonyms}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<!--Display items in CheckBox-->
<TextBox Text="{Binding Display}"
Margin="10,0,0,0"
FontWeight="Bold" />
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>

Use the DataTemplate to display the content in the exact same way as not using a DataTemplate

My ListBox currently binds as expected using
<ListBox ItemsSource="{Binding ChildDuplicate, UpdateSourceTrigger=PropertyChanged}" DataContext="{Binding}" Height="Auto" >
Where ChildDuplicate is an ObservableCollection<MyUserControl>
The problem I am facing is I need to add a button for every item within the ListBox (so it shows both my MyUserControl and the Button control).
This button cannot live within the MyUserControl for several reasons.
I was hoping to use a DataTemplate but this doesn't seem to work (XmlParser error is thrown), I've tried
<ListBox ItemsSource="{Binding ChildDuplicate, UpdateSourceTrigger=PropertyChanged}" DataContext="{Binding}" Height="Auto" >
<ListBoxItem>
<DataTemplate>
<ContentControl Content="{Binding}"></ContentControl>
</DataTemplate>
</ListBoxItem>
</ListBox>
I also tried using the ItemsControl control in the same way, the same issue.
Any idea how I use the DataTemplate to display the content in the exactly same way as not using a DataTemplate?

Try this instead:
<ListBoxItem>
<ListBoxItem.Template>
<ControlTemplate>
<ContentControl Content="{Binding}"></ContentControl>
</ControlTemplate>
</ListBoxItem.Template>
</ListBoxItem>

Optimal way to populate a large number of textboxes in MVVM

I'm currently creating a WPF application using MVVM. I have a large number of textboxes in a window (about 20) that need to be bound to specific elements in a list and need to be populated all at once. Normally I'd push them into an array and populate them that way but I can't do so without breaking the MVVM model. Is there a quick and efficient way I can do this while still adhering to MVVM?

You could bind your list to an ItemsControl and change it's item template to be a TextBox.
<ItemsControl ItemSource={Binding aList}>
<ItemsControl.ItemTemplate>
<DataTemplate>
<TextBox Text="{Binding Text}" />
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>

I don't see why strictly this would break MVVM, if instead of using an Array you used a List, put it in your ViewModel and then use indexed binding to bind to specific elements.
Something like:
<StackPanel>
<TextBox Text="{Binding MyViewModelList[0]}">
<TextBox Text="{Binding MyViewModelList[1]}">
<TextBox Text="{Binding MyViewModelList[2]}">
</StackPanel>
or if you want something more dynamic, instead of List, put an ObservableCollection in your VM, and bind to it in an ItemsControl with a DataTemplate.
<ItemsControl ItemsSource="{Binding Path=MyViewModelObsCol}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<TextBox Text="{Binding}"/>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
Syntax might not be 100% as I don't have an IDE to test, but something along these lines might be what you're after.

If you are trying to populate textbox on the bases of selection in list box try this
Another option is creating a COllection view source which i don't think you will require here

Separate items in ListView using line in wpf c#

How to separate between items BY Lines in ListView control in WPF ?

You can change the ItemTemplate for the ListBox,
given the little information you have provided, just a quick example:
<ListBox>
<ListBox.ItemTemplate>
<DataTemplate>
<Border BorderBrush="Black" BorderThickness="0.5">
<TextBlock Text={Binding}/>
</Border>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
You will have to modify it to suit your needs.

WPF Databinding stackpanel

Im a beginner in WPF programming, coming from .NET 2.0 C#.
Im trying to make a horizontal StackPanel which should be filled with data from a table in a database. The problem is that I want it to display an image with some text from the table below and then stack those two items horizontally.
Here's some pseudo-code to display what I want to do:
<StackPanel Orientation="horizontal" ItemsSource="{Binding Path=myTable}">
<StackPanel>
<Image Source="User.png"/>
<Label HorizontalAlignment="Center" Content="{Binding Path=UserName}"></Label>
</StackPanel>
</StackPanel>
I simply cannot figure oout how to do this.

Julien's answer is correct for your written description, however, looking at your XAML, it appears you are looking for something like the following:
<DataTemplate x:Key="UserDataTemplate">
<StackPanel>
<Image Source="User.png"/>
<Label HorizontalAlignment="Center" Content="{Binding Path=UserName}" />
</StackPanel>
</DataTemplate>
<ItemsControl x:Name="UserList" ItemTemplate="{StaticResource UserDataTemplate}" >
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<VirtualizingStackPanel Orientation="Horizontal"/>
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
</ItemsControl>
You definately need an ItemsControl (or some derivation of) to bind your source to. Then you can change the the orientation by setting it's items panel (which I believe is a VirtualizingStackPanel with Vertical orientation by default) so just set it to a VirtualizingStackPanel with Horizontal Orientation. Then you can set the ItemsTemplate for each of your items to the layout you desire (an image stacked on top of text bound from your database).

Basically, you want to use a control capable of displaying an enumeration of objects. The control capable of this is the class ItemsControl and all of its descendants (Selector, ListBox, ListView, etc).
Bind the ItemsSource property of this control to a list of objects you want, here a list of users you've fetched from the database. Set the ItemTemplate of the control to a DataTemplate that will be used to display each item in the list.
Sample code:
In a Resources section (for example Window.Resources):
<DataTemplate x:Key="UserDataTemplate">
<StackPanel Orientation="Horizontal">
<Image Source="User.png"/>
<Label HorizontalAlignment="Center" Content="{Binding Path=UserName}" />
</StackPanel>
</DataTemplate>
In your Window/Page/UserControl:
<ItemsControl x:Name="UserList" ItemTemplate="{StaticResource UserDataTemplate}" />
In your code behind:
UserList.ItemsSource = ... // here, an enumeration of your Users, fetched from your db