CODEWITHSUNDEEP
c#wpfxamldata-bindingmultibinding
Alex P.
Alex P.

Reputation: 3787

WPF MultiBinding VS designer exception

Visual Studio 2010 designer says that unhandled exception occurred in MultiValueConverter but I can build my program and it works fine (multibinding also works).

enter image description here

XAML (I set window.DataContext in constructor):

            <ComboBox Name="cbbProfile" DisplayMemberPath="Name" Grid.Row="1" Margin="10,5" Grid.ColumnSpan="3" ItemsSource="{Binding ProfilesData.ProfilesItems}" SelectionChanged="cbbProfile_SelectionChanged" >
                <ComboBox.IsEnabled>
                    <MultiBinding Converter="{StaticResource multiEnabledToEnabled}">
                        <Binding Path="ProfilesData.ProfilesItems.Count" Converter="{StaticResource itemsCountToEnabled}" />
                        <Binding Path="State" Converter="{StaticResource stateToControlsEnabled}" />
                    </MultiBinding>
                </ComboBox.IsEnabled>
            </ComboBox>

Converters:

public class MultiEnabledToEnabled : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    { 
        foreach (object val in values)
        {
            if (!(bool) val)     // <-- EXCEPTION (line 176) HERE 
                return false;
        } 

        return true;
    }    

public class ItemsCountToEnabled : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return (int)value == 0 ? false : true;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

public class StateToControlsEnabled : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var val = (ProgramState)value;
        switch (val)
        {
            ...
            default:
                return true;
        }

    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

VS Exception text:

System.InvalidCastException Specified cast is not valid. at myassemblyname.MultiEnabledToEnabled.Convert(Object[] values, Type targetType, Object parameter, CultureInfo culture) in C:...\Converters.cs:line 176 at System.Windows.Data.MultiBindingExpression.TransferValue() at System.Windows.Data.MultiBindingExpression.Transfer() at System.Windows.Data.MultiBindingExpression.UpdateTarget(Boolean includeInnerBindings) at System.Windows.Data.MultiBindingExpression.AttachToContext(Boolean lastChance) at System.Windows.Data.MultiBindingExpression.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 System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs) at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)

Upvotes: 1

Views: 2327

Answers (2)

Rafal
Rafal

Reputation: 12629

VS designer is a hard beast to work with and I've come to conclusion that it is not worth the effort. But you can use:

if(DesignerProperties.GetIsInDesignMode(Application.MainWindow))

To provide default value for your converter. This will remover the error.

DesignerProperties.GetIsInDesignMode method at msdn

Upvotes: 2

Louis Kottmann
Louis Kottmann

Reputation: 16648

My best guess is that the binding happens before some initialization, and at least one value in the collection of object is DependencyProperty.UnsetValue, making the cast invalid.

Now, assuming you have a design time viewmodel set up, you could check beforehand if all the values are indeed booleans:

if(values.All(v => v is bool))
{
   //Do regular computation
}
else
{
   //Handle edge case
}

But as soon as any view gets complicated, the designer breaks, and it is painful to get it working again.

Expression Blend handles this better, if you absolutely want a designer but can't be bothered to set up a design time environment, go for it.

Otherwise do it like most people: forget about the designer.

Upvotes: 2

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