What is the use of ConvertBack method in IValueConverter interface?

Question

What is the use of ConvertBack method in the IValueConverter interface.

When will it be called?

Or what is the order of invocation of the Convert and ConvertBack methods?

I have asked the question here because: I have bound one property of codebehind to TEXTBOX’s TEXT Property and am using convertor for that property. The first Convert Method invokes and when I change TEXT in TEXTBOX nothing happens... but as soon as I close the form the ConvertBack method invokes.

Solution 1

IMO, the ConvertBack method is used to convert your visual representation of the data to the specific DataType.

For example: you use a Converter to convert a boolean true to the string "TrueBoolean". This text will be displayed in your TextBox. When you change the value of the TextBox, the ConvertBack method will be called as soon as the binding fires again (default OnFocusLost). Now your ConvertBack method will try to convert the new value to the datatype you want it to be. So you will have to implement logic to convert "FalseBoolean" to false.

public class Converter : IValueConverter
{
    object IValueConverter.Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return (bool) value ? "TrueBoolean" : "FalseBoolean";
    }

    object IValueConverter.ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var s = (string) value;
        if (s.Equals("TrueBoolean",StringComparison.CurrentCultureIgnoreCase))
            return true;
        if (s.Equals("FalseBoolean", StringComparison.CurrentCultureIgnoreCase))
            return false;
        throw new Exception(string.Format("Cannot convert, unknown value {0}", value));
    }
}

This technique is used a lot in DataGrids if I'm not mistaken.

Hope this is a bit clear...

UPDATE
About you question in the comment:
To overwrite the default OnFocusLost binding behavior you have to change your binding like this:

<TextBox Text="{Binding MyText, UpdateSourceTrigger=PropertyChanged}"/>
<!--syntax might differ, can't access VS at the moment.-->

Solution 2

ConvertBack is when you have mode=TwoWay on your binding.

It converts the user input back into the datatype that you're binding to and gets invoked when the TextBox (say) loses focus.

For example (if this wasn't built in), if you have a number that represents a price, the Convert method would take the value and format it into a string with the correct currency symbol, decimal separator etc.

If the user types in a new value - including possible currency symbol, thousands separator etc. you'd use the ConvertBack method to parse the string and extract the numeric value.

Another example could be credit card number input. The user could enter the number as a single string of digits or groups of digits separated by spaces or dashes. The ConvertBack method would take all these possible inputs and convert them to the single format you require.

Solution 3

check the UpdateSourceTrigger Property on your Binding. http://msdn.microsoft.com/library/system.windows.data.binding.updatesourcetrigger.aspx

The textbox will by default update the source if it looses focus, using the UpdateSourceTrigger you can set this behaviour to immediate update of the source if the textbox content changes.

HTH Dominik

Comments

[Aryan SuryaWansi]

@Roel, ya but in my case ConvertBack is fire when i closing the window, is n't it a strange?

[RoelF]

no, I think this is normal behavior: Before the window is closed, the OnFocusLost event of the TextBox will be fired, or at least the binding will update.

[Aryan SuryaWansi]

@Roel, what should i have to do if i need to have call ConvertBack method as soon as any change make in TEXT of TEXTBOX? i could n't find update source trigger property as @Dominik says...

[Max Mazur]

@Roel, How about INotifyPropertyChanged interface? Do you need to implement that on your property in order for the "UpdateSourceTrigger=propertychanged" to work? and when is a "Mode=TwoWay" binding relevant? You seem to know your way around this area.

[RoelF]

@MaxMazur yes you need to implement INotifyPropertyChanged. And TwoWay binding is the default for most controls (you can check BindsTwoWayByDefault on the control to confirm). It is relevant for TextBoxes and Checkboxes for example, where it is important that the value can be upated either via code or via user input.