Creating ContextBinding XAML - wpf

Question

I'm having problems setting the DataContext for all fields that are inside a stackPanel. What I would like to do is set the Data Context as vm:ViewModel. But it's not working and when I ask VS for assistance with DataBinding it Displays the TextBox.DataContext inside the TextBox. Is there a way to only set it once or do I have to set it for each control?
<StackPanel DataContext="vm:ViewModel">
<TextBox Text="{Binding FirstNumber}" HorizontalAlignment="Left" Height="23" Margin="206,45,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="120">
<TextBox.DataContext>
<vm:ViewModel/>
</TextBox.DataContext>
</TextBox>
</StackPanel>

Answer 1

You need to get your DataContext to refer to an instance of your ViewModel.
DataContext="vm:ViewModel" is not creating an instance of the ViewModel...it's just setting a string.
Use Property Element syntax instead:
<StackPanel>
<StackPanel.DataContext>
<vm:ViewModel/>
</StackPanel.DataContext>
<TextBox Text="{Binding FirstNumber}" HorizontalAlignment="Left" Height="23" Margin="206,45,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="120"/>
</StackPanel>
And if you do that, then there's no need/in fact it's probably wrong to create another one in your TextBox.DataContext...you just want to inherit the DataContext of the StackPanel.
I'll just show you another way to create a ViewModel and to refer to the instance of it so you have a bigger picture.....that is to create it in resources, and then refer to that resource in the binding...here's an example:
<Window x:Class="WpfApplication8.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:vm="clr-namespace:WpfApplication8"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<vm:ViewModel x:Key="myviewmodel"/>
</Window.Resources>
<StackPanel x:Name="stackp" DataContext="vm:ViewModel">
<TextBox Text="{Binding Source={StaticResource myviewmodel}, Path=FirstNumber}" HorizontalAlignment="Left" Height="23" Margin="206,45,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="120"/>
</StackPanel>
</Window>
Another example that sets the DataContext at a higher level in the tree:
<Window x:Class="WpfApplication8.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:vm="clr-namespace:WpfApplication8"
Title="MainWindow" Height="350" Width="525">
<Window.DataContext>
<vm:ViewModel/>
</Window.DataContext>
<StackPanel>
<TextBox Text="{Binding FirstNumber}" HorizontalAlignment="Left" Height="23" Margin="206,45,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="120"/>
</StackPanel>
</Window>