CommandParameter is always NULL

本文关键字：NULL always is CommandParameter | 更新日期: 2023-09-27 17:49:23

我想在WPF窗口中实现一个简单的关闭按钮。窗口基本上是这样的:

    <Window x:Class="MyApplication.MainWindow"
            xmlns=""....
            ...."
            Title="MainWindow" WindowState="Maximized" WindowStartupLocation="CenterScreen" x:Name="mainWindow">
<DockPanel LastChildFill="True">
        <Ribbon DockPanel.Dock="Top">
            <Ribbon.ApplicationMenu>
                <RibbonApplicationMenu SmallImageSource="Resources/menu_16x16.png">
                    <RibbonApplicationMenu.FooterPaneContent>
                        <RibbonButton Label="Beenden" 
                                      Command="{Binding CmdCloseApp}" 
                                      CommandParameter="{Binding ElementName=mainWindow}"
                                      SmallImageSource="Resources/ende_16x16.png" 
                                      HorizontalAlignment="Right"/>
                    </RibbonApplicationMenu.FooterPaneContent>
                </RibbonApplicationMenu>
            </Ribbon.ApplicationMenu>
        </Ribbon>
</DockPanel>
    </Window>

此窗口的DataContext在其代码中设置为MainWindowViewModel的实例

MainWindowViewModel: 

public class MainWindowViewModel
{
        public ICommand CmdCloseApp { get; set; }
        public MainWindowViewModel()
        {
            CmdCloseApp = new RelayCommand<Window>(CloseApp);
        }
        public void CloseApp(Window w)
        {
            w.Close();
        }
}

在CloseApp中w总是空的。允许其他命令使用字符串参数等。工作完美-我唯一的问题是，我没有得到窗口元素没有找到我的视图模型的方法。

谢谢你的帮助!

编辑我很抱歉，我尝试了一个简单的按钮，它工作-问题只发生在RibbonButton

CommandParameter is always NULL

编辑:在您更改应用程序菜单RibbonButton后，您的问题是Microsoft RibbonControlLibrary使用弹出菜单来保持按钮，并且MainWindow不是弹出菜单的可视树的一部分(父)，因此您的ElementName绑定无法找到" MainWindow "窗口，因此它分配null给CommandParameter
您可以使用静态应用程序。当前获取主窗口，然后它将工作。
注! MainWindow是应用程序的属性名，而不是你的窗口名

<Window x:Class="WpfRelayCommandParameter.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:wpfRelayCommandParameter="clr-namespace:WpfRelayCommandParameter"
    xmlns:ribbon="http://schemas.microsoft.com/winfx/2006/xaml/presentation/ribbon"
    mc:Ignorable="d"
    d:DataContext="{d:DesignInstance wpfRelayCommandParameter:MainWindowViewModel}"
    Title="MainWindow" Height="350" Width="525"
    x:Name="mainWindow">
<DockPanel LastChildFill="True">
    <DockPanel LastChildFill="True">
        <ribbon:Ribbon DockPanel.Dock="Top">
            <ribbon:Ribbon.ApplicationMenu>
                <ribbon:RibbonApplicationMenu SmallImageSource="Resources/AppMenu.png">
                    <ribbon:RibbonApplicationMenu.FooterPaneContent>
                        <ribbon:RibbonButton Label="Beenden" 
                                  Command="{Binding CmdCloseApp}" 
                                  CommandParameter="{Binding MainWindow, Source={x:Static Application.Current}}"
                                  SmallImageSource="Resources/Exit.png" 
                                  HorizontalAlignment="Right" Click="ButtonBase_OnClick"/>
                    </ribbon:RibbonApplicationMenu.FooterPaneContent>
                </ribbon:RibbonApplicationMenu>
            </ribbon:Ribbon.ApplicationMenu>
        </ribbon:Ribbon>
    </DockPanel>
</DockPanel>


我个人更喜欢通过一个接口来回调窗口，这个接口可以根据你的视图模型需求定制，并且可以在单元测试中模拟:
<Window x:Class="WpfRelayCommandParameter.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:wpfRelayCommandParameter="clr-namespace:WpfRelayCommandParameter"
    mc:Ignorable="d"
    d:DataContext="{d:DesignInstance wpfRelayCommandParameter:MainWindowViewModel}"
    Title="MainWindow" Height="350" Width="525"
    x:Name="mainWindow">
<Grid>
    <Button Content="Close Window" 
            Command="{Binding CmdCloseApp}" 
            VerticalAlignment="Top"
            HorizontalAlignment="Left" />
</Grid>

代码
public partial class MainWindow : Window, IMainWindow
{
    public MainWindow()
    {
        DataContext = new MainWindowViewModel(this);
        InitializeComponent();
    }
}
public interface IMainWindow
{
    void Close();
}
public class MainWindowViewModel
{
    private readonly IMainWindow _mainWindow;
    public MainWindowViewModel() : this(null)
    {
        // Design time
    }
    public MainWindowViewModel(IMainWindow mainWindow)
    {
        _mainWindow = mainWindow;
        CmdCloseApp = new RelayCommand(CloseApp);
    }
    public ICommand CmdCloseApp { get; set; }
    public void CloseApp(object parameter)
    {
        _mainWindow.Close();
    }
}

注意在RelayCommand上CanExecute的type和null可能存在的问题

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