.net – ‘System.Windows.Data.Binding’类型的对象无法转换为

[TemplatePart(Name = InformationBubble.InformationBubbleTitlePart,Type = typeof(TextBlock))]
[TemplatePart(Name = InformationBubble.InformationBubbleProductImagePart,Type=typeof(Image))]

public class InformationBubble : Control 
{
    #region Template Parts Name Constants

    /// <summary>
    /// Name constant for the Information Bubble Title Part
    /// </summary>
    public const string InformationBubbleTitlePart = "InformationBubbleTitleText";

    /// <summary>
    /// Name constant for the Information Bubble Product Image Part
    /// </summary>
    public const string InformationBubbleProductImagePart = "InformationBubbleProductImage";

    #endregion

    #region TemplateParts

    private TextBlock _Title;

    internal TextBlock Title
    {
        get { return _Title; }
        private set
        {
            _Title = value;

            if (_Title != null)
            {
                _Title.Text = this.ProductTitleText;       
            }
        }
    }

    private Image _ProductImage;

    internal Image ProductImage
    {
        get { return _ProductImage; }
        private set
        {
            _ProductImage = value;

            if (_ProductImage != null)
            {
                _ProductImage.Source = this.ProductImageSource;
            }
        }
    }

    #endregion

    #region Public String Product Title 

    // Dependency properties declaration
    public static readonly DependencyProperty ProductTitleProperty = DependencyProperty.Register(
        "ProductTitle",typeof(string),typeof(InformationBubble),new PropertyMetadata(string.Empty,new PropertyChangedCallback(OnProductTitleChanged)));

    public static void OnProductTitleChanged(DependencyObject sender,DependencyPropertyChangedEventArgs e)
    {
        InformationBubble iBubble = sender as InformationBubble;

        if (iBubble.Title != null)
        {
            iBubble.Title.Text = e.NewValue as string;
        }
    }

    public string ProductTitleText
    {
        get { return GetValue(ProductTitleProperty) as string; }
        set { SetValue(ProductTitleProperty,value); }
    }

    #endregion

    #region Public Image Source Product Image

    public static readonly DependencyProperty ProductImageSourceProperty = DependencyProperty.Register(
        "ProductImageSource",typeof(ImageSource),new PropertyMetadata(null,new PropertyChangedCallback(OnProductImageSourceChanged)));

    public static void OnProductImageSourceChanged(DependencyObject sender,DependencyPropertyChangedEventArgs e)
    {
        InformationBubble iBubble = sender as InformationBubble;

        if (iBubble.ProductImage != null)
        {
            iBubble.ProductImage.Source = e.NewValue as ImageSource;
        }
    }

    public ImageSource ProductImageSource
    {
        get { return GetValue(ProductImageSourceProperty) as ImageSource; }
        set { SetValue(ProductImageSourceProperty,value); }
    }

    #endregion

    public InformationBubble()
    {
         this.DefaultStyleKey = typeof(InformationBubble);
    }

    #region Overrides

    public override void OnApplyTemplate()
    {       
        base.OnApplyTemplate();

        Title = GetTemplateChild(InformationBubble.InformationBubbleTitlePart) as TextBlock;
        ProductImage = GetTemplateChild(InformationBubble.InformationBubbleProductImagePart) as Image;
    }

    #endregion

    #region Private Methods

    private void GoToState(string stateName,bool useTransitions)
    {
        VisualStateManager.GoToState(this,stateName,useTransitions);
    }

    #endregion
}

现在,如果我在我的xaml中的某个地方使用此控件,那么如果我执行此操作：

<controls:InformationBubble 
        ProductImageSource="{Binding SelectedItem.NormalImageSource}"
        ProductTitleText="Test Title"
        "/>

但是,如果我尝试将数据绑定到ViewModel中的SelectedItem对象的title属性的数据：

<controls:InformationBubble 
            ProductImageSource="{Binding SelectedItem.NormalImageSource}"
            ProductTitleText="{Binding SelectedItem.Title,Mode=TwoWay"
            "/>

我得到’System.Windows.Data.Binding’类型的对象不能转换为’System.String’类型. TextBlock的text属性是DependencyProperty,所??以我必须在这里遗漏一些明显的东西.

非常感谢任何帮助或见解.

短剑的一种