A Value Converter functions as a bridge between a target and a source and it is necessary when a target is bound with one source, for instance you have a text box and a button control. You want to enable or disable the button control when the text of the text box is filled or null.

In this case you need to convert the string data to Boolean. This is possible using a Value Converter. To implement Value Converters, there is the requirement to inherit from IValueConverter in the System.Windows.Data namespace and implement the two methods Convert and ConvertBack.
Note: In WPF, Binding helps to flow the data between the two WPF objects. The bound object that emits the data is called the Source and the other (that accepts the data) is called the Target.

Code Example1: Declaration of Value Converter class

  1. using System;
  2. using System.Collections.Generic;
  3. using System.Linq;
  4. using System.Text;
  5. using System.Threading.Tasks;
  6. using System.Windows.Data;
  8. namespace ValueConverters
  9. {
  10. public class ValueConverter:IValueConverter
  11. {
  12. public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
  13. {
  14. bool isenable = true;
  15. if (string.IsNullOrEmpty(value.ToString()))
  16. {
  17. isenable = false;
  18. }
  19. return isenable;
  20. }
  21. public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
  22. {
  23. throw new NotImplementedException();
  24. }
  25. }
  26. }
In the preceding code I have created a class named Value Converter that will convert the value from the source to the target. This class does the conversion of the WPF value by implementing IValueConverter. There are two methods in the Value Converter class that are implemented from the IValue Converter interface. The Convert method helps to do the conversion from the target to the source and ConvertBack converts from the Source to the Target.

Here in the preceding code I have written code in the convert method to enable or disable the button control and have not written any code in the ConvertBack function. To use the convert back I have another example.

Let's say you have a check box control bound the text with the text of text box control. You want to determine whether the check box is checked when the text is married and unchecked when the text is unmarried.

Code Example2
  1. using System;
  2. using System.Collections.Generic;
  3. using System.Linq;
  4. using System.Text;
  5. using System.Threading.Tasks;
  6. using System.Windows.Data;
  8. namespace ValueConverters
  9. {
  10. class CheckBoxCheckConverter:IValueConverter
  11. {
  12. public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
  13. {
  14. if (value.ToString().ToUpper() == "MARRIED")
  15. {
  16. return true;
  17. }
  18. else
  19. {
  20. return false;
  21. }
  22. }
  24. public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
  25. {
  26. bool married = System.Convert.ToBoolean(value);
  27. if (married == true)
  28. return "Married";
  29. else
  30. return "Unmarried";
  31. }
  32. }
  33. }

In the preceding code, I am checking the text of text box. If it is married then the convert function returns true that will make the check box checked otherwise it will uncheck the checkbox. The convert back function is doing as said above, if the check box is checked it is setting the text of the TextBox to Married and otherwise Unmarried.

Use of Converter

To use the converter you need to implement the interface of the converter class in the XAML page of WPF and need to declare the resource. After declaring the resource there is the need to use it with binding. See the following XAML code example.

XAML Example: The code example uses a converter in WPF XAML.
  1. <Window x:Class="ValueConverters.MainWindow"
  2. xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  3. xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  4. xmlns:local="clr-namespace:ValueConverters"
  5. Title="MainWindow" Height="350" Width="525">
  6. <Window.Resources>
  7. <local:ValueConverter x:Key="valueconverter"></local:ValueConverter>
  8. <local:CheckBoxCheckConverter x:Key="checkBoxCheckConverter"></local:CheckBoxCheckConverter>
  9. </Window.Resources>
  10. <Grid>
  11. <TextBlock Text="Value Converter Exampkle" HorizontalAlignment="Stretch" VerticalAlignment="Top" TextAlignment="Center"></TextBlock>
  12. <TextBox Name="txtFirstName" HorizontalAlignment="Left" VerticalAlignment="Top" Height="36" Width="255" Margin="136,38,0,0" ></TextBox>
  13. <Button Content="Click" HorizontalAlignment="Left" VerticalAlignment="Top" Height="23" Width="50" Margin="230,101,0,0" IsEnabled="{Binding Path=Text, ElementName=txtFirstName,Converter={StaticResource valueconverter}}"></Button>
  14. <CheckBox Content="Married" HorizontalAlignment="Left" VerticalAlignment="Top" Margin="307,108,0,0" IsChecked="{Binding Path=Text, ElementName=txtFirstName,Converter={StaticResource checkBoxCheckConverter}}"></CheckBox>
  15. <TextBlock Text="MultiValue Converter Exampkle" HorizontalAlignment="Stretch" VerticalAlignment="Top" TextAlignment="Center" Margin="10,146,-10,0"></TextBlock>
  16. </Grid>
  17. </Window>
Summery

I hope you have gotthe the idea of Value Converters and how to use them.