Binding的数据转换（Data Converter）

    当Source端Path所关联的数据与Target端目标属性数据类型不一致时，添加数据转换器（Data Converter),
简单的数据转换（如double转string）WPF类库会自动转换。
    手动转换时需要手动写Converter，创建一个类，并实现IValueConverter接口。

public interface IValueConverter
{
    object Convert(object value, Type targetType, object parameter, CultureInfo culture);//当数据从Binding的Source流向Target时调用
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture);//反之调用ConvertBack
}

例：
用于在UI上显示图片和将Source的三个值映射到CheckBox控件的IsChecked属性值（bool?类型）

//种类，用于显示图片
public enum Category
{
    Bomber,
    Fighter
}
//状态 CheckBox对应的三种状态
public enum State
{
    Available,
    Locked,
    Unknown
}
//飞机类
public class Plane
{
    public Category Category { set; get; }
    public string Name { set; get; }
    public State State { set; get; }
}

提供两个Converter用于转换数据，一个将Category单项转为string（XAML自动解析为图片），另一个实现State与bool？的双向转换。

public class CategoryToSourceConverter :IValueConverter
{
    //将Category转换为Uri
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        Category c = (Category)value;
        switch (c)
        {
            case Category.Bomber:
                return @"\Icons\Bomber.png";//数据在项目Icons目录里
            case Category.Fighter:
                return @"\Icons\Fighter.png";
            default:
                return null;
        }
    }
    //不被调用
    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}
//将Stae转换为bool?
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        State s = (State)value;
        switch (s)
        {
            case State.Available:
                return true;
            case State.Locked:
                return false;
            case State.Unknown:
            default:
                return null;
        }
    }

    //将bool?转换为State
    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        bool? nb = (bool?)value;
        switch (nb)
        {
            case true:
                return State.Available;
            case false:
                return State.Locked;
            case null:
            default:
                return State.Unknown;
        }
    }

之后再XAML里消费这些Converter

<Window x:Class="Windows6.Window6_4_2"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:Windows6.BLL"
        Title="Window6_4_2" Height="266" Width="300">
    <Window.Resources>
        <local:CategoryToSourceConverter x:Key="cts"/>
        <local:StateToNullableBollConverter x:Key="stnb"/>
    Window.Resources>
    <StackPanel Background="LightBlue">
        <ListBox x:Name="listBoxPlan" Height="160" Margin="5">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal">
                        <Image Width="20" Height="20"
                               Source="{Binding Path=Category, Converter={StaticResource cts}}"/>
                        <TextBlock Text="{Binding Path=Name}" Width="60" Margin="80,0"/>
                        <CheckBox IsThreeState="True"
                                  IsChecked="{Binding Path=State, Converter={StaticResource stnb}}"/>
                    StackPanel>
                DataTemplate>
            ListBox.ItemTemplate>
        ListBox>
        <Button x:Name="buttonLoad" Content="Load" Height="25" Margin="5,0"
                Click="buttonLoad_Click"/>
        <Button x:Name="btnSave" Content="Save" Height="25" Margin="5,5"
                Click="btnSave_Click"/>
    StackPanel>
Window>

最后实现两个按钮的点击事件

private void buttonLoad_Click(object sender, RoutedEventArgs e)
{
    List planeList = new List()
    {
        new Plane() {Category = Category.Bomber, Name = "B-1", State = State.Unknown},
        new Plane() {Category = Category.Fighter, Name = "F-22", State = State.Unknown}
    };
    this.listBoxPlan.ItemsSource = planeList;
}

private void btnSave_Click(object sender, RoutedEventArgs e)
{
    StringBuilder sb = new StringBuilder();
    foreach (Plane p in listBoxPlan.Items)
    {
        sb.AppendLine(string.Format("Category={0},Name={1},State={2}", p.Category, p.Name, p.State));
    }
    File.WriteAllText(@"D:\test\PlaneList.txt", sb.ToString());
}