CODEWITHSUNDEEP
wpfivalueconverter
Josep B.
Josep B.

Reputation: 587

Binding error when setting "DependencyProperty.UnsetValue;"

I have this variable binding in my View:

<Image Source="{Binding Path=GrappleTypeVar, Source={StaticResource CustomerData}, Converter={StaticResource GrappleDataConverter}}" Width="40" Height="40"/>

And then, this converter:

public class GrappleDataConverter : IValueConverter
{
    public object Convert(object value, System.Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value != null)
        {
            int currentType = (int)value;

            if (Enum.IsDefined(typeof(GrappleType), currentType))
            {
                switch ((GrappleType)currentType)
                {
                    case GrappleType.Hydraulic:
                        return String.Empty;
                    case GrappleType.Parallel:
                        return "/GUI;component/Images/040/SensorSoft.png";
                }
            }
        }
        // Not defined... Set unknown image
        return String.Empty;
    }

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

Using that code, my result windows returned a lot of binding errors of the type:

System.Windows.Data Information: 10 : Cannot retrieve value using the binding and no valid fallback value exists; using default instead. BindingExpression:Path=GrappleTypeVar; DataItem='CustomerData' (HashCode=50504364); target element is 'Image' (Name=''); target property is 'Source' (type 'ImageSource') System.Windows.Data Error: 23 : Cannot convert '' from type 'String' to type 'System.Windows.Media.ImageSource' for 'en-US' culture with default conversions; consider using Converter property of Binding. NotSupportedException:'System.NotSupportedException: ImageSourceConverter cannot convert from System.String.

Reviewing that error, I found that solution: ImageSourceConverter error for Source=null

And I changed in my code:

case GrappleType.Hydraulic:
    return String.Empty;

for 

case GrappleType.Hydraulic:
    return DependencyProperty.UnsetValue;

Now the application runs smoother, but in the result windows it appears the following binding error: System.Windows.Data Information: 10 : Cannot retrieve value using the binding and no valid fallback value exists; using default instead. BindingExpression:Path=GrappleTypeVar; DataItem='CustomerData' (HashCode=62171008); target element is 'Image' (Name=''); target property is 'Source' (type 'ImageSource')

Can anyone provide me some help? Is it possible to solve this error?

Thanks!

Upvotes: 0

Views: 673

Answers (1)

Clemens
Clemens

Reputation: 128097

Your converter should return a value that matches the type of the target property, i.e. an instance of a type derived from ImageSource. This is typically a BitmapImage or a BitmapFrame. In case no image shall be displayed, it should return null:

public object Convert(
    object value, System.Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
    object result = null;

    if (value is GrappleType)
    {
        switch ((GrappleType)value)
        {
            case GrappleType.Hydraulic:
                break;

            case GrappleType.Parallel:
                result = new BitmapImage(new Uri(
                    "pack://application:,,,/GUI;component/Images/040/SensorSoft.png"));
                break;
        }
    }

    return result;
}

Please note how the converter uses a Resource File Pack URI to create a BitmapImage.

Upvotes: 1

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