WPF BindingExpression路径错误

Question

I have model BaseModel with fields as Id , Created , Deleted and Name etc.

From this model I have derived models Category and Brand . Model Brand has field Image .

Also I have class Node (Title as Name and Value as all object)

public class Node : INotifyPropertyChanged
{
    private string _title;
    private BaseModelDto _value;
    private bool _isSelected;

    #region ctor

    public Node(string title, BaseModelDto value)
    {
        Title = title;
        Value = value;
    }

    #endregion

    #region Properties

    public string Title
    {
        get { return _title; }
        set
        {
            _title = value;
            NotifyPropertyChanged("Title");
        }
    }

    public BaseModelDto Value
    {
        get { return _value; }
        set
        {
            _value = value;
            NotifyPropertyChanged("Value");
        }
    }

    public bool IsSelected
    {
        get { return _isSelected; }
        set
        {
            _isSelected = value;
            NotifyPropertyChanged("IsSelected");
        }
    }

    #endregion

    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

I use class Node for ComboBox . So I have ComboBox for Category and for Brand. Cause Category and Brand are derived from BaseModel I use for them same class Node

In <ComboBox.ItemTemplate> I want to display Image if it exists. So I wrote next code:

<Image MaxHeight="30" Margin="15,0,0,0" HorizontalAlignment="Right" Name="ImageCheckBox" Grid.Column="1">
    <Image.Style>
        <Style TargetType="{x:Type Image}">
            <Setter Property="Source" Value="{Binding Value.Image.FileLocation, Converter={StaticResource ImagePathConverter}}" />
        </Style>
    </Image.Style>
</Image>

It works, it displays images only for Brand items, cause only they have Image .

But in output window I see next message:

System.Windows.Data Error: 40 : BindingExpression path error: 'Image' property not found on 'object' ''Category' (HashCode=56044044)'. BindingExpression:Path=Value.Image.FileLocation; DataItem='Node' (HashCode=65381042); target element is 'Image' (Name='ImageCheckBox'); target property is 'Source' (type 'ImageSource')

As I read before, any exception in Binding can have influence on WPF App performance. How can I solve this issue?

Answer 1

You can set the ItemTemplateSelector of the ComboBox to something like this:

public class CategoryBrandItemTemplateSelector : DataTemplateSelector
{
    public DataTemplate CategoryItemTemplate { get; set; }
    public DataTemplate BrandItemTemplate { get; set; }

    public override DataTemplate SelectTemplate(object item, DependencyObject container)
    {
        if(item is Category)
            return CategoryItemTemplate;

        if(item is Brand)
            return BrandItemTemplate;

        return base.SelectTemplate(item, container);
    }
}

In XAML:

<ComboBox>
    <ComboBox.ItemTemplateSelector>
        <local:CategoryBrandItemTemplateSelector BrandItemTemplate="{StaticResource BrandTemplate}"
                                                 CategoryItemTemplate="{StaticResource CategoryTemplate}" />
    </ComboBox.ItemTemplateSelector>
</ComboBox>

Where BrandTemplate is a DataTemplate resource declared somewhere (this will be the same that you are now using as the ItemTemplate ) and the CategoryTemplate will be the one without the Image and the failing binding.