Question

我有这个标记：

   <GroupBox BorderThickness="2">
    <GroupBox.BorderBrush>
        <SolidColorBrush x:Name="Border">
            <SolidColorBrush.Color>
                <MultiBinding Converter="{StaticResource ConnectionAndLoggedInToBorderBrush}">
                    <Binding Path="IsConnected"/>
                    <Binding Path="IsLoggedIn"/>
                </MultiBinding>
            </SolidColorBrush.Color>
        </SolidColorBrush>
    </GroupBox.BorderBrush>

在后面的代码中，我在window_loaded方法中有这一行：

DataContext = uiManager;

uiManager是UIManager类型，它有两个名为IsConnected和IsLoggedIn的公共属性。

此代码在启动时失败，因为Multibinding调用的Converter中的values数组未填充布尔值，但值为DependencyProperty.UnsetValue。

当我使用下面的标记（并更改转换器的返回类型）时，它确实有效。

   <GroupBox BorderThickness="2">
    <GroupBox.BorderBrush>
         <MultiBinding Converter="{StaticResource ConnectionAndLoggedInToBorderBrush}">
              <Binding Path="IsConnected"/>
              <Binding Path="IsLoggedIn"/>
         </MultiBinding>
    </GroupBox.BorderBrush>

似乎通过后面代码中的DataContext的绑定集在第一个示例中失败，但在第二个示例中工作。为什么呢？

为了UIManager类下面的完整性：

public class UIManager:IUIManager
    {

        #region Implementation of IUIManager

        private const string IsLoggedInProperty = "IsLoggedIn";
        private bool loggedIn;
        private readonly object loggedInLock = new object();
        public bool IsLoggedIn
        {
            get
            {
                lock (loggedInLock)
                {
                    return loggedIn;
                }
            }
            set
            {
                lock (loggedInLock)
                {
                    if(value==loggedIn)return;
                    loggedIn = value;
                    OnPropertyChanged(IsLoggedInProperty);
                }
            }
        }

        private void OnPropertyChanged(string property)
        {
            if(PropertyChanged!=null)PropertyChanged(this,new PropertyChangedEventArgs(property));
        }

        private const string IsConnectedProperty = "IsConnected";
        private bool isConnected;
        private object isConnectedLock = new object();
        public bool IsConnected
        {
            get
            {
                lock (isConnectedLock)
                {
                    return isConnected;
                }
            }
            set
            {
                lock (isConnectedLock)
                {
                    if(value==isConnected)return;
                    isConnected = value;
                    OnPropertyChanged(IsConnectedProperty);
                }
            }
        }

        #endregion

        #region Implementation of INotifyPropertyChanged

        public event PropertyChangedEventHandler PropertyChanged;

        #endregion
    }

编辑：失败的XAML的转换方法（在转换为bool值[0]时失败：

public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            var is_connected = (bool) values[0];
            var is_loggedin = (bool) values[1];
            return is_loggedin
                       ? is_connected
                             ? Colors.YellowGreen
                             : Colors.Red
                       : Colors.Gray;
        }

用于工作XAML：

public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            var is_connected = (bool) values[0];
            var is_loggedin = (bool) values[1];
            return is_loggedin
                       ? is_connected
                             ? Brushes.YellowGreen
                             : Brushes.Red
                       : Brushes.Gray;
        }

Answer 1

问题与MultiBinding或您的转换器无关。 DependencyProperty.UnsetValue表示绑定没有任何价值。事实上，如果您在调试模式下运行，您可以在Output窗口中看到绑定错误：

System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:Path=IsConnected; DataItem=null; target element is 'SolidColorBrush' (HashCode=17654054); target property is 'Color' (type 'Color')
System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:Path=IsLoggedIn; DataItem=null; target element is 'SolidColorBrush' (HashCode=17654054); target property is 'Color' (type 'Color')

所以让我们稍微简化标记并应用一些诊断：

<GroupBox>
    <GroupBox.BorderBrush>
        <SolidColorBrush>
            <SolidColorBrush.Color>
                <Binding Path="GroupColor" PresentationTraceSources.TraceLevel="High"/>
            </SolidColorBrush.Color>
        </SolidColorBrush>
    </GroupBox.BorderBrush>
</GroupBox>

应用附加的依赖项属性PresentationTraceSources.TraceLevel会产生更多输出：

System.Windows.Data Warning: 52 : Created BindingExpression (hash=17654054) for Binding (hash=44624228)
System.Windows.Data Warning: 54 :   Path: 'GroupColor'
System.Windows.Data Warning: 56 : BindingExpression (hash=17654054): Default mode resolved to OneWay
System.Windows.Data Warning: 57 : BindingExpression (hash=17654054): Default update trigger resolved to PropertyChanged
System.Windows.Data Warning: 58 : BindingExpression (hash=17654054): Attach to System.Windows.Media.SolidColorBrush.Color (hash=52727599)
System.Windows.Data Warning: 60 : BindingExpression (hash=17654054): Use Framework mentor <null>
System.Windows.Data Warning: 63 : BindingExpression (hash=17654054): Resolving source 
System.Windows.Data Warning: 65 : BindingExpression (hash=17654054): Framework mentor not found
System.Windows.Data Warning: 61 : BindingExpression (hash=17654054): Resolve source deferred
System.Windows.Data Warning: 91 : BindingExpression (hash=17654054): Got InheritanceContextChanged event from SolidColorBrush (hash=52727599)
System.Windows.Data Warning: 63 : BindingExpression (hash=17654054): Resolving source 
System.Windows.Data Warning: 66 : BindingExpression (hash=17654054): Found data context element: GroupBox (hash=51393439) (OK)
System.Windows.Data Warning: 67 : BindingExpression (hash=17654054): DataContext is null
System.Windows.Data Warning: 91 : BindingExpression (hash=17654054): Got InheritanceContextChanged event from SolidColorBrush (hash=52727599)
System.Windows.Data Warning: 63 : BindingExpression (hash=17654054): Resolving source 
System.Windows.Data Warning: 65 : BindingExpression (hash=17654054): Framework mentor not found
System.Windows.Data Warning: 63 : BindingExpression (hash=17654054): Resolving source 
System.Windows.Data Warning: 65 : BindingExpression (hash=17654054): Framework mentor not found
System.Windows.Data Warning: 63 : BindingExpression (hash=17654054): Resolving source  (last chance)
System.Windows.Data Warning: 65 : BindingExpression (hash=17654054): Framework mentor not found
System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:Path=GroupColor; DataItem=null; target element is 'SolidColorBrush' (HashCode=52727599); target property is 'Color' (type 'Color')

我们发现绑定找不到DataContext并且绑定失败。当我更改windows的构造函数以便在初始化内容之前设置DataContext绑定有效：

public Window1()
{
    DataContext = ...;
    InitializeComponent();
}

这很奇怪，因为在其他地方绑定并不重要。不知道为什么它不起作用所以我只能提供解决方法。例如，使用绑定创建画笔作为资源（该资源也可以是GroupBox的本地资源）：

<GroupBox BorderBrush="{DynamicResource resbrush}">
    <GroupBox.Resources>
        <SolidColorBrush x:Key="resbrush">
            <SolidColorBrush.Color>
                <MultiBinding Converter="{StaticResource ConnectionAndLoggedInToBorderBrush}">
                    <Binding Path="IsConnected"/>
                    <Binding Path="IsLoggedIn"/>
                </MultiBinding>
            </SolidColorBrush.Color>
        </SolidColorBrush>
    </GroupBox.Resources>
</GroupBox>

如果您的MultiBinding课程是某种DataContext {{1}，我建议您放弃UIManager并在MVVM中进行一些预处理}。

Answer 2

我的理论。颜色是struct（不能为null），因此SolidColorBrush.Color = null是错误的。 WPF无法创建SolidColorBrush，您将获得异常。

 <GroupBox.BorderBrush>
     <SolidColorBrush x:Name="Border">
         <SolidColorBrush.Color>
             <MultiBinding Converter="{StaticResource 
                           ConnectionAndLoggedInToBorderBrush}">
                 <Binding Path="IsConnected"/>
                 <Binding Path="IsLoggedIn"/>
             </MultiBinding>
         </SolidColorBrush.Color>
     </SolidColorBrush>
 </GroupBox.BorderBrush>

BorderBrush是object（可以为null），所以GroupBox.BorderBrush = null就可以了。

 <GroupBox.BorderBrush>
      <MultiBinding Converter="{StaticResource 
                    ConnectionAndLoggedInToBorderBrush}">
           <Binding Path="IsConnected"/>
           <Binding Path="IsLoggedIn"/>
      </MultiBinding>
 </GroupBox.BorderBrush>

此SolidColorBrush不是一个对象，而是一个FACTORY。它仅在需要时进行实例化，此时您已经附加了DataContext。

 <GroupBox.Resources>
      <SolidColorBrush x:Key="resbrush">
           <SolidColorBrush.Color>
                <MultiBinding Converter="{StaticResource 
                              ConnectionAndLoggedInToBorderBrush}">
                     <Binding Path="IsConnected"/>
                     <Binding Path="IsLoggedIn"/>
                </MultiBinding>
           </SolidColorBrush.Color>
      </SolidColorBrush>
 </GroupBox.Resources>

只需2美分。

如果您需要一些奇怪的Bindings或Animations与奇怪的转换器，请阅读我的文章，顺便说一下，这可能很有用。 http://www.codeproject.com/KB/WPF/BindingHub.aspx

Answer 3

出于这样的原因，您可能需要考虑学习MVVM。这种模式可以帮助您抽象模型和绑定，这样您就不必非常依赖DP - 您可以简单地绑定到视图模型中的可通知属性。

关于MVVM有几篇优秀的文章，所以我建议你先阅读Karl Shifflett，Josh Smith，Marlon Grech和Sacha Barber的作品。

WPF MultiBinding失败。为什么？

3 个答案: