DelegateCommand 抛出 "Specified cast is not valid"

Question

我想使用带有布尔值作为参数的 PRISM 委托命令。这是相关代码：

public class ChartViewModel : BindableBase
{
    public DelegateCommand<bool?> ChangeZoomPanCommand { get; private set; }

    private bool isInRealtimeMode;
    public bool IsInRealtimeMode
    {
        get { return isInRealtimeMode; }
        set
        {
            SetProperty(ref isInRealtimeMode, value);
            ChangeZoomPanCommand.RaiseCanExecuteChanged();
        }
    }

    private bool dragToZoom;
    public bool DragToZoom
    {
        get { return dragToZoom; }
        set { SetProperty(ref dragToZoom, value); }
    }


    public ChartViewModel()
    {
        ChangeZoomPanCommand = new DelegateCommand<bool?>(ExecuteChangeZoomPan, CanExecuteChangeZoomPan);
        IsInRealtimeMode = true;
        DragToZoom = true;
    }

    private bool CanExecuteChangeZoomPan(bool? arg)
    {
        return !IsInRealtimeMode;
    }

    private void ExecuteChangeZoomPan(bool? enableZoom)
    {
        if (enableZoom.HasValue)
        {
            DragToZoom = enableZoom.Value;
        }
    }
}

当我在 CanExecuteChangeZoomPan 中设置断点时，它永远不会被击中。问题发生在 ChangeZoomPanCommand.RaiseCanExecuteChanged().

之后

这是堆栈跟踪：

>

    at Prism.Commands.DelegateCommand`1.<>c__DisplayClass1_0.<.ctor>b__1(Object o)
   at Prism.Commands.DelegateCommandBase.CanExecute(Object parameter)
   at Prism.Commands.DelegateCommandBase.System.Windows.Input.ICommand.CanExecute(Object parameter)
   at MS.Internal.Commands.CommandHelpers.CanExecuteCommandSource(ICommandSource commandSource)
   at System.Windows.Controls.Primitives.ButtonBase.UpdateCanExecute()
   at System.Windows.Controls.Primitives.ButtonBase.HookCommand(ICommand command)
   at System.Windows.Controls.Primitives.ButtonBase.OnCommandChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
   at System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
   at System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
   at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
   at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType)
   at System.Windows.DependencyObject.InvalidateProperty(DependencyProperty dp, Boolean preserveCurrentValue)
   at System.Windows.Data.BindingExpressionBase.Invalidate(Boolean isASubPropertyChange)
   at System.Windows.Data.BindingExpression.TransferValue(Object newValue, Boolean isASubPropertyChange)
   at System.Windows.Data.BindingExpression.Activate(Object item)
   at System.Windows.Data.BindingExpression.AttachToContext(AttachAttempt attempt)
   at System.Windows.Data.BindingExpression.MS.Internal.Data.IDataBindEngineClient.AttachToContext(Boolean lastChance)
   at MS.Internal.Data.DataBindEngine.Task.Run(Boolean lastChance)
   at MS.Internal.Data.DataBindEngine.Run(Object arg)
   at MS.Internal.Data.DataBindEngine.OnLayoutUpdated(Object sender, EventArgs e)
   at System.Windows.ContextLayoutManager.fireLayoutUpdateEvent()
   at System.Windows.ContextLayoutManager.UpdateLayout()
   at System.Windows.ContextLayoutManager.UpdateLayoutCallback(Object arg)
   at System.Windows.Media.MediaContext.InvokeOnRenderCallback.DoWork()
   at System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()
   at System.Windows.Media.MediaContext.RenderMessageHandlerCore(Object resizedCompositionTarget)
   at System.Windows.Media.MediaContext.AnimatedRenderMessageHandler(Object resizedCompositionTarget)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)

如果我将参数类型更改为字符串，一切似乎都有效。然后我当然得到一个 "False" 字符串作为参数，因为那是我的 xaml 命令参数。为什么这不适用于任何 T，因为 T 似乎没有限制。我发现 T 必须是对象或可为空的困难方法，但即使可为空似乎也不合适。如何使用 bool 参数？

谢谢

Answer 1

相关的命令参数属性通常被键入为对象，因此任何文字都被解释为字符串。如果 Prism 无法自动转换为您的类型，您将必须传递正确的类型，但在 XAML 中使用泛型（如 Nullable<T>）通常很痛苦，因此我不建议这样做.

要转换为 XAML 中的简单类型，您通常应该能够使用这样的绑定：{Binding Source=(sys:Boolean)true}，其中 sys 是到 System 命名空间当然在 mscorlib 中。

Answer 2

您可以只创建一个 DelegateCommand<object> 而不是 DelegateCommand<bool?>，然后像这样处理它：

void ExecuteChangeZoomPan(object obj)
{
    if (obj is bool?)
    {
        bool? arg = obj as bool?;

        // The rest of the code goes here.
    }
}

这样，ExecuteChangeZoomPan可以很灵活，接受像System.DateTime一样复杂的类型！

Answer 3

这就是我创建 DelegateCommand 的新实例的方式

ICommand ConnectCommand = new DelegateCommand<object>(ConnectExecute,ConnectCanExecute);

然后如果你想提高零钱

 ((DelegateCommand<object>)ConnectCommand).RaiseCanExecuteChanged();

DelegateCommand 抛出 "Specified cast is not valid"

DelegateCommand throws "Specified cast is not valid"

wpf

prism

delegatecommand