Binding of a dependency property in a userControl does't work!

Question

public static readonly DependencyProperty myTBProperty =
DependencyProperty.Register("myTB", typeof(decimal?), typeof(TB2), new FrameworkPropertyMetadata() { BindsTwoWayByDefault = true });
public decimal? myTB
get { return (decimal?)GetValue(myTBProperty); }
set { SetValue(myTBProperty, value); }
<TextBox Grid.Column="1" Text="{Binding myTB,Mode=TwoWay}" Width="{Binding TBWidth}"/>
The above code is for a UserControl. 
Now I used this UserControl into another UserControl like this:
<c:TB2 myText="OutSide Diameter" myTB="{Binding Di,Mode=TwoWay}" TBWidth="120"/>
private decimal? _Di;
public decimal? Di
get { return _Di; }
set
_Di = value;
NotifyPropertyChanged();
But when I change the Value of Di, nothing happens. I have also tried with an ordinary TextBox and it works. 

Answer 1

You cannot explicitly set the DataContext of the TB2 UserControl.
Give TB2 a name in the XAML markup:
<UserControl x:Class="WpfApplication1.TB2"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:WpfApplication22"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300" Name="tb2">
...and bind to the myTB property of the UC using the ElementName syntax:
<TextBox Grid.Column="1" Text="{Binding myTB,Mode=TwoWay, ElementName=tb2}" Width="{Binding TBWidth}"/>
Do not set the DataContext of TB2 to itself programmatically because then you cannot bind to the Di source property in the other UC.
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