Strange UserControl Issue (Struck with the easiest)

Hi Guys,

Today I am going to discuss about one of the most silly issue that I faced very recently. Its all about building a Composite UserControl using WPF. Let me discuss how I build the control.


TextCalc is a control that takes numeric input from the user. Traditionally, we use TextBox to do this, but here I have implemented one control that allows you to choose numeric digits to input into the control. I will not discuss on the actual implementation of the control; which is fairly simple;  rather I emphasis here only on the problem I have faced while creating the control.

The Implementation

The TxtCalc control has few components :

  1. Border : This element wraps around all the controls inside it. I have created this just to ensure that the control has a firm border around the controls inside it. 
  2. TextBox : This is the main textbox, where the contents will be put and the data bound to this control will primarily be displayed over this TextBox. 
  3. ToggleButton : The ToggleButton is used to display the calculator. Just like what ComboBox has, it opens up a Popup below it and displays the Calculator UI.
  4. Popup : This control holds the UI for the calculator. Calculator holds a number of buttons from 0 to 9 and few calculation buttons etc.
Let us see how the code looks like (I intentionally removed the code of all the buttons, you will find in the demo application )  :

<usercontrol loaded="UserControl_Loaded" 
x:class="TextCalc.txtCalc" x:name="txtCalculator" xmlns:x="" xmlns="">
    <border horizontalalignment="Stretch" x:name="brdCalculator">
                <columndefinition width="*">
                <columndefinition width="Auto">
            <textbox borderthickness="0" 
text="{Binding ElementName=txtCalculator, Path=Message, Mode=TwoWay}" textalignment="Right" x:name="txtObj">
            <togglebutton click="ToggleButton_Click" 
ischecked="{Binding ElementName=pupCalculator,Path=IsOpen}" name="btnDropdown" template="{Binding ElementName=txtCalculator, Path=CalculatorComboButton}"></togglebutton>
            <popup allowstransparency="True" horizontaloffset="-5" placement="Bottom" 
PlacementTarget="{Binding ElementName=txtObj}" staysopen="False" verticaloffset="10" x:name="pupCalculator">
                    <grid background="Transparent" cliptobounds="True" margin="1">

From the above code you can see I have created a textbox, which is bound to a property Message. There is a popup which displays the calendar, which is opened whenever the Toggle Button is opened.  All these is wrapped around using a Border.

In the code Behind I have created a Dependancy property MessageProperty and associated with my control txtCalc. A wrapper property is also created which allows you to call the property whenever Source value is modified. The code page looks like :

public readonly static DependencyProperty MessageProperty = 
DependencyProperty.Register("Message", typeof(string), typeof(txtCalc), new UIPropertyMetadata("0"));
  public string Message
            get { return this.GetValue(MessageProperty) as string; }
                string oldValue = this.Message;
                this.SetValue(MessageProperty, value);
DependencyPropertyChangedEventArgs(MessageProperty, oldValue, value));
private static void PropertyChangedCallback(DependencyObject obj, 
DependencyPropertyChangedEventArgs args)
            txtCalc tcalc = obj as txtCalc;
            tcalc.txtObj.Text = tcalc.Message;
            if (tcalc != null)


Everything looks fairly simple but as I said, it was not working for me. The problem here is Binding generally defaults to TwoWay for any control. But in case of UserControl when I use the my custom binding statements, it is essential to explicitely define the Binding Mode to TwoWay, otherwise, it defaults to OneTime.

So the Culprit here with my implementation is :
Text="{Binding ElementName=txtCalculator, Path=Message, Mode=TwoWay}"

Here if you forget to mention the Mode=TwoWay, the binding will not be proper. And hence any changes will not be notified to the Observer when used with collection.

So I have solved the issue just by mentioning {Binding Mode=TwoWay}. Pity on me. :)

You can try out the sample application from : (80 KB)

Thank you for reading.
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Rajesh Singh said...

There are lots of things that we face which doesnt corresponds to what we think.

So dont worry.. carry on with your work.

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