[WPF] DataTemplate Binding to Interface

在WPF内可以使用DataTemplate，来辅助完成对对象集合做DataBinding的工作。并且透过DataTemplate的DataType属性，来让对象集合中不同的对象，经过DataBinding之后能有「不同的外观」。简单的范例如下：不同的车辆类型，会依照车辆类型，呈现不同的详细数据。

namespace BindingInterfaceSample
{
    public class Car
    {
        public string Name { get; set; }
    }

    public class Truck : Car
    {
        public int MaxLoad { get; set; }
    }

    public class Roadster : Car
    {
        public int MaxSpeed { get; set; }
    }
}

<Window x:Class="BindingInterfaceSample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:clk="clr-namespace:BindingInterfaceSample"
        Title="MainWindow" Height="350" Width="525">
    <ItemsControl >
        <ItemsControl.Items>
            <clk:Truck Name="Truck01" MaxLoad="100" />
            <clk:Truck Name="Truck02" MaxLoad="500" />
            <clk:Roadster Name="Roadster01" MaxSpeed="99" />
        </ItemsControl.Items>
        <ItemsControl.Resources>
            <DataTemplate DataType="{x:Type clk:Truck}">
                <WrapPanel>
                    <TextBlock Text="Truck" />
                    <TextBlock Text="{Binding Path=MaxLoad}" />
                </WrapPanel>
            </DataTemplate>
            <DataTemplate DataType="{x:Type clk:Roadster}">
                <WrapPanel>
                    <TextBlock Text="Roadster" />
                    <TextBlock Text="{Binding Path=MaxSpeed}" />
                </WrapPanel>
            </DataTemplate>
        </ItemsControl.Resources>
    </ItemsControl>
</Window>

反过来说，也可以经由DataTemplate的DataType属性，来让对象集合中不同的对象，经过DataBinding之后能有「相同的外观」。简单的范例如下：只要是车，都只呈现车的数据。

namespace BindingInterfaceSample
{
    public class Car
    {
        public string Name { get; set; }
    }

    public class Truck : Car
    {
        public int MaxLoad { get; set; }
    }

    public class Roadster : Car
    {
        public int MaxSpeed { get; set; }
    }
}

<Window x:Class="BindingInterfaceSample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:clk="clr-namespace:BindingInterfaceSample"
        Title="MainWindow" Height="350" Width="525">
    <ItemsControl >
        <ItemsControl.Items>
            <clk:Truck Name="Truck01" MaxLoad="100" />
            <clk:Truck Name="Truck02" MaxLoad="500" />
            <clk:Roadster Name="Roadster01" MaxSpeed="99" />
        </ItemsControl.Items>
        <ItemsControl.Resources>
            <DataTemplate DataType="{x:Type clk:Car}">
                <Button Content="{Binding Path=Name}" />
            </DataTemplate>
        </ItemsControl.Resources>
    </ItemsControl>
</Window>

而在让对象集合中不同的对象，经过DataBinding之后能有「相同的外观」。这个使用情景中，DataTemplate的DataType如果设定为Interface型别，在WPF中会出现意外的执行结果。简单的范例如下：DataType设定为Interface型别，会无法对应DataTemplate。

namespace BindingInterfaceSample
{
    public interface ICar
    {
        string Name { get; set; }
    }

    public class Truck : ICar
    {
        public string Name { get; set; }

        public int MaxLoad { get; set; }        
    }

    public class Roadster : ICar
    {
        public string Name { get; set; }

        public int MaxSpeed { get; set; }
    }
}

<Window x:Class="BindingInterfaceSample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:clk="clr-namespace:BindingInterfaceSample"
        Title="MainWindow" Height="350" Width="525">
    <ItemsControl >
        <ItemsControl.Items>
            <clk:Truck Name="Truck01" MaxLoad="100" />
            <clk:Truck Name="Truck02" MaxLoad="500" />
            <clk:Roadster Name="Roadster01" MaxSpeed="99" />
        </ItemsControl.Items>
        <ItemsControl.Resources>
            <DataTemplate DataType="{x:Type clk:ICar}">
                <Button Content="{Binding Path=Name}" />
            </DataTemplate>
        </ItemsControl.Resources>
    </ItemsControl>
</Window>

遇到必须设定DataTemplate的DataType为Interface型别的情景。解决的方法很简单：只要改写数据系结容器使用的DataTemplateSelector，增加以Interface型别做索引，查询Resource里的DataTemplate，再用这个DataTemplate来做DataBinding。简单的范例如下：只要是有实做车接口，都只呈现车的数据。

namespace BindingInterfaceSample
{
    public interface ICar
    {
        string Name { get; set; }
    }

    public class Truck : ICar
    {
        public string Name { get; set; }

        public int MaxLoad { get; set; }
    }

    public class Roadster : ICar
    {
        public string Name { get; set; }

        public int MaxSpeed { get; set; }
    }
}

<Window x:Class="BindingInterfaceSample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:clk="clr-namespace:BindingInterfaceSample"
        Title="MainWindow" Height="350" Width="525">
    <ItemsControl>
        <ItemsControl.Items>
            <clk:Truck Name="Truck01" MaxLoad="100" />
            <clk:Truck Name="Truck02" MaxLoad="500" />
            <clk:Roadster Name="Roadster01" MaxSpeed="99" />
        </ItemsControl.Items>        
        <ItemsControl.Resources>
            <DataTemplate DataType="{x:Type clk:ICar}">
                <Button Content="{Binding Path=Name}" />
            </DataTemplate>
        </ItemsControl.Resources>
        <ItemsControl.ItemTemplateSelector>
            <clk:StandardDataTemplateSelector />
        </ItemsControl.ItemTemplateSelector>
    </ItemsControl>
</Window>

namespace BindingInterfaceSample
{
    public class StandardDataTemplateSelector : DataTemplateSelector
    {
        // Methods
        public override DataTemplate SelectTemplate(object item, DependencyObject container)
        {
            #region Require

            if (item == null) throw new ArgumentNullException();
            if (container == null) throw new ArgumentNullException();

            #endregion

            // Result
            DataTemplate itemDataTemplate = null;

            // Base DataTemplate
            itemDataTemplate = base.SelectTemplate(item, container);
            if (itemDataTemplate != null) return itemDataTemplate;

            // Interface DataTemplate
            FrameworkElement itemContainer = container as FrameworkElement;
            if (itemContainer == null) return null;

            foreach (Type itemInterface in item.GetType().GetInterfaces())
            {
                itemDataTemplate = itemContainer.TryFindResource(new DataTemplateKey(itemInterface)) as DataTemplate;
                if (itemDataTemplate != null) break;
            }

            // Return
            return itemDataTemplate;
        }
    }
}

参考数据
Style、DataTemplate 和隐含索引键
DataTemplateKey 类别

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