这个XAML例外来自哪里？

Question

我使用绑定组来检查输入参数。突然间，我在XAML中抛出异常。好的，我们在这里使用一些我认为可能相关的代码。首先是XAML的一部分：

<Grid
     x:Name="TopGrid"
     DockPanel.Dock="Top"
     Height="269">
     <Grid.BindingGroup>
          <BindingGroup
               Name="LengthBindingGrp"
               NotifyOnValidationError="True">
               <BindingGroup.ValidationRules>
                  <c:LengthValidationRule
                     ValidationStep="ConvertedProposedValue" />
               </BindingGroup.ValidationRules>
           </BindingGroup>
     </Grid.BindingGroup>
     <Grid.RowDefinitions>

这是验证程序：

     public class LengthValidationRule : ValidationRule
        {
            public override ValidationResult Validate(object value, CultureInfo cultureInfo)
            {
                BindingGroup bindingGroup = (BindingGroup)value;

                CableViewModel cvm = ((CableViewModel)bindingGroup.Items[0]);            
                double x = cvm.RHSupportX;

                if (x < 0)
                    return new ValidationResult(false, Resources.RHSupport_Right);
                if (cvm.RHSupportY < 0)
                    return new ValidationResult(false, Resources.RHSupportNotHigher);

                ObservableCollection<LoadCombi> loads = cvm.Loads;

                foreach (LoadCombi ld in loads)
                {
                    if (ld.Position < 0 || (ld.Position > x && !ld.Fixed) || (ld.Fixed && ld.Position > cvm.Ucl))
                        return new ValidationResult(false, String.Format(Resources.LoadPosition, ld.Position ));
                }

                return ValidationResult.ValidResult;
            }
}

然后我在XAML中看到了这个错误屏幕：

    InvalidCastException: Unable to cast object of type '_.di1.CableCalc.CableViewModel' to type 'CableCalc.CableViewModel'.

Stack Trace
    at CableCalc.LengthValidationRule.Validate(Object value, CultureInfo cultureInfo)
       at System.Windows.Controls.ValidationRule.Validate(Object value, CultureInfo cultureInfo, BindingGroup owner)
       at System.Windows.Data.BindingGroup.CheckValidationRules()
       at System.Windows.Data.BindingGroup.UpdateAndValidate(ValidationStep validationStep)
       at System.Windows.Controls.DataGridRow.DelayedValidateWithoutUpdate(Object arg)
       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)

错误似乎起源于

行

CableViewModel cvm = ....

因为我有一个演员。

但不知怎的，这在以前有效。我怎么做才能在XAML中获得此异常？