Introduction

If you want to do data binding between  incompatible types, you need to use a converter. A converter converts the value from source type  to target type and vice versa. A value converter is a class, that implements the simple interface IValueConverter with the two methods
1) object Convert(object value)
2) object ConvertBack(object value).

Implementing a ValueConverter :

Add a class to the project and and  make it public and implement the IValueConverter interface like below :

public class Converter : IValueConverter
{
    public object Convert(object value, Type targetType, 
        object parameter, CultureInfo culture)
    {
        // Do the conversion from bool to visibility
    }
 
    public object ConvertBack(object value, Type targetType, 
        object parameter, CultureInfo culture)
    {
        // Do the conversion from visibility to bool
    }
}

Implementing a ValueConverter in XAML

First we need to  map the namespace of the converter to a XAML namespace. Then we  create an instance of a value converter in the resources of the view and give it a name.To reference the converter we use  {StaticResource}

<Window x:Class="VirtualControlDemo.Window1"
    ...
    xmlns:l="clr-namespace:VirtualControlDemo"
    ...>
    <Window.Resources>
        <l:BoolToVisibilityConverter x:Key="converter" />
    </Window.Resources>
    <Grid>
        <Button Visibility="{Binding HasFunction, 
            Converter={StaticResource converter}}" />
    </Grid>
</Window>

Many times in WPF applications we need to show an auto increment counter in the begining of items in ListBox: Following is a converter to implement the same:

 public class IndexConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            try
            {
                ListBoxItem item = value as ListBoxItem;
                ListBox view = ItemsControl.ItemsControlFromItemContainer(item) as ListBox;
                int index = view.ItemContainerGenerator.IndexFromContainer(item);
                return (index + 1).ToString() + ". ";
            }
            catch (Exception)
            {
                throw;
            }

        }
        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }

Hope the blog helped in understanding converters better.  Converters are powerful constructs used extensively in WPF.