Binding的數據轉換（Data Converter）

    當Source端Path所關聯的數據與Target端目標屬性數據類型不一致時，添加數據轉換器（Data Converter),
簡單的數據轉換（如double轉string）WPF類庫會自動轉換。
    手動轉換時需要手動寫Converter，創建一個類，並實現IValueConverter接口。

public interface IValueConverter
{
    object Convert(object value, Type targetType, object parameter, CultureInfo culture);//當數據從Binding的Source流向Target時調用
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture);//反之調用ConvertBack
}

例：
用於在UI上顯示圖片和將Source的三個值映射到CheckBox控件的IsChecked屬性值（bool?類型）

//種類，用於顯示圖片
public enum Category
{
    Bomber,
    Fighter
}
//狀態 CheckBox對應的三種狀態
public enum State
{
    Available,
    Locked,
    Unknown
}
//飛機類
public class Plane
{
    public Category Category { set; get; }
    public string Name { set; get; }
    public State State { set; get; }
}

提供兩個Converter用於轉換數據，一個將Category單項轉爲string（XAML自動解析爲圖片），另一個實現State與bool？的雙向轉換。

public class CategoryToSourceConverter :IValueConverter
{
    //將Category轉換爲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";//數據在項目Icons目錄裏
            case Category.Fighter:
                return @"\Icons\Fighter.png";
            default:
                return null;
        }
    }
    //不被調用
    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}
//將Stae轉換爲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;
        }
    }

    //將bool?轉換爲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;
        }
    }

之後再XAML裏消費這些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>

最後實現兩個按鈕的點擊事件

private void buttonLoad_Click(object sender, RoutedEventArgs e)
{
    List<Plane> planeList = new List<Plane>()
    {
        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());
}