Overriding parent control's datacontext with local datacontext - wpf

Question

I have a Groupbox in which i have multiple Textboxes. All these Textbox derive their Datacontext from that of Groupbox but one of the Textbox in the group needs a different Datacontext.
<GroupBox Header="My Group" Height="150" Width="1132" DataContext="{Binding ContextA}" >
<Grid>
<Label x:Name="lblA" Content="Policy Number:" Margin="6,12,970,92" />
<TextBox x:Name="txtbA" Margin="155,12,0,0" HorizontalAlignment="Left" Height="24" TextWrapping="Wrap" Text="{Binding ValueA}" VerticalAlignment="Top" Width="278" Grid.ColumnSpan="2"/>
<Label x:Name="lblB" Content="Policy Type:" Margin="612,10,334,88" Height="30"/>
<TextBox x:Name="txtbB" Margin="801,12,0,0" HorizontalAlignment="Left" Height="24" TextWrapping="Wrap" DataContext="{Binding ContextB}" Text="{Binding ValueB}" VerticalAlignment="Top" Width="278"/>
</Grid>
</GroupBox>
In the above code txtbA uses the Datacontext same as that of Groupbox.
I want txtbB to have a separate Datacontexti.e. ContextB
But the ContextB is not getting assigned to txtbB. How to solve the problem?
Note:
ContextAand ContextB= list of Entity Framework models.

Answer 1

WPF binding engine look for property in current DataContext. So, in your case binding engine is looking for property ContextB in class ContextA since textBox is inheriting DataContext from parent GroupBox.
What you can do is use more verbose definition for ContextA like this:
<GroupBox Header="My Group" Height="150" Width="1132"
DataContext="{Binding}"> <-- HERE Or can remove setting DC altogether.
<Grid>
<Label x:Name="lblA" Content="Policy Number:" Margin="6,12,970,92" />
<TextBox x:Name="txtbA" Margin="155,12,0,0" HorizontalAlignment="Left"
Height="24" TextWrapping="Wrap"
Text="{Binding ContextA.ValueA}" <-- HERE
VerticalAlignment="Top"
Width="278" Grid.ColumnSpan="2"/>
<Label x:Name="lblB" Content="Policy Type:" Margin="612,10,334,88"
Height="30"/>
<TextBox x:Name="txtbB" Margin="801,12,0,0" HorizontalAlignment="Left"
Height="24"
TextWrapping="Wrap" DataContext="{Binding ContextB}"
Text="{Binding ValueB}" VerticalAlignment="Top" Width="278"/>
</Grid>
</GroupBox>