无法将“ MultiBinding”类型的实例添加到“ DoubleCollection”类型的集合中

Question

Error: I am trying to use a IMultiValueConverter I wrote, but Intellisense is giving me this error "Cannot add instance of type 'MultiBinding' to a collection of type 'DoubleCollection'. Only items of type 'double' are allowed."

Q: I do not understand what this error means. I've used other converters to modify the StrokeDashArray properties in other paths; however, I am not sure how to use a multibinding. Can someone explain why I am getting this error, and how I remove this error?

Details: The converter is named "DashedWhenValue1ArrayEqualsValue2ArrayConverter" and I have defined it above as a StaticResource. Here are the interesting parts of my converter

. . . 
public DoubleCollection DoubleCollectionWhenEqual { get; set; }
public DoubleCollection DoubleCollectionWhenNotEqual { get; set; }
public DoubleCollection DoubleCollectionWhenValueIsNull { get; set; }

. . .

public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{

  if (values == null || values.Length < 2)
  {
    return this.DoubleCollectionWhenValueIsNull;
  }

  if ( !(values[0] is Array) || !(values[1] is Array) )
  {
    return this.DoubleCollectionWhenNotEqual;
  }

  object[] array1 = values[0] as object[];
  object[] array2 = values[1] as object[];

  if (array1.Length != array2.Length)
  {
    return this.DoubleCollectionWhenNotEqual;
  }

  for (int i = 0; i < array1.Length; i++)
  {
    if (array1[i] != array2[i])
    {
      return this.DoubleCollectionWhenNotEqual;
    }
  }

  return this.DoubleCollectionWhenEqual;

}
. . .

I use my converter in the view here

<Path x:Name="some_Path" Data="M7,4.167 L7,162.08887" HorizontalAlignment="Right" Margin="0,0,-391.5,-871" StrokeStartLineCap="Square" StrokeEndLineCap="Square" Stroke="#FF33CC33" StrokeThickness="10" Width="10.5" RenderTransformOrigin="0,0" Height="167.167" VerticalAlignment="Bottom" >
      <Path.StrokeDashArray>
        <MultiBinding Converter="{StaticResource DashedWhenValue1ArrayEqualsValue2ArrayConverter}"> <!-- Error starts here -->
          <Binding Path="ModelViewProperty1" />
          <Binding Path="ModelViewProperty2" />
        </MultiBinding> <!-- Error ends here -->
      </Path.StrokeDashArray>
    </Path>

Answer 1

I was able to replicate your error in Visual Studio 2015. It does appear to be simply a bug with the XAML editor, not an actual problem with your XAML or converter. When I run the app, I get different dashing depending on whether ModelViewProperty1 and ModelViewProperty2 both have values and are equivalent.

So, my advice would be to ignore this particular error.

PS Just for fun, here is a more succinct way to write your Convert method using LINQ:

public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
    var arrays = values
        .OfType<Array>()
        .Select(x => x.OfType<object>())
        .ToList();
    if (arrays.Count != 2) return DoubleCollectionWhenValueIsNull;
    return arrays[0].SequenceEqual(arrays[1]) ? DoubleCollectionWhenEqual : DoubleCollectionWhenNotEqual;
}