Question

I am try to Make a Image Button User control. But When I set image at window Initialize then throw this exception. Here is my Code

EXAM.

<UserControl.Resources>
    <Style x:Key="ImageButton" TargetType="{x:Type Button}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type Button}">
                    <StackPanel>
                        <Image x:Name="ButtonImage" Stretch="None"/>
                        <ContentPresenter Margin="0,8,0,0" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" HorizontalAlignment="Center" VerticalAlignment="Center"/>
                    </StackPanel>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsFocused" Value="True"/>
                        <Trigger Property="IsDefaulted" Value="True"/>
                        <Trigger Property="IsMouseOver" Value="True"/>
                        <Trigger Property="IsPressed" Value="True"/>
                        <Trigger Property="IsEnabled" Value="False"/>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</UserControl.Resources>
<Button x:Name="ProcestaImageButton" Content="TextBlock" Style="{DynamicResource ImageButton}"/>

C# Code.

    public ImageSource ButtonImage
    {
        get { return ((this.ProcestaImageButton.Template).FindName("ButtonImage", this.ProcestaImageButton) as Image).Source; }
        set { ((this.ProcestaImageButton.Template).FindName("ButtonImage", this.ProcestaImageButton) as Image).Source = value; }
    }

Please tell me where i am wrong.Please Provide me a Example.
Thank`s for Help.

Answer 1

For a start, I’d expand that property out – each statement is rather complicated at the moment. Here’s the expanded version:

public ImageSource ButtonImage
{
    get
    { 
        var template = ProcestaImageButton.Template
                                .FindName("ButtonImage", ProcestaImageButton);

        var image = template as Image;
        return image.Source;
    }
    set
    {
        var template = ProcestaImageButton.Template
                                .FindName("ButtonImage", ProcestaImageButton);
        var image = template as Image;
        image.Source = value;
    }
}

I would then probably extract the common code to a helper property:

private Image ProcestaImageButtonImage
{
    get
    {
        var template = ProcestaImageButton.Template
                                FindName("ButtonImage", ProcestaImageButton);

        return template as Image;
    }
}

public ImageSource ButtonImage
{
    get { return ProcestaImageButtonImage.Source; }
    set { ProcestaImageButtonImage.Source = value; }
}

Next I would change the as to a cast:

private Image ProcestaImageButtonImage
{
    get
    {
        var template = ProcestaImageButton.Template
                                .FindName("ButtonImage", ProcestaImageButton);

        return (Image) template;
    }
}

public ImageSource ButtonImage
{
    get { return ProcestaImageButtonImage.Source; }
    set { ProcestaImageButtonImage.Source = value; }
}

At this point, you might still have a NullReferenceException or you might have a InvalidCastException – and suddenly you have a lot more information about what’s wrong.

A cast is better here as you’re always expecting FindName to return an Image, aren’t you? If it’s an error condition for the type conversion to fail, you should use a cast – only use as when it’s valid for the expression not to be of the target type, and you’re intending to handle that case separately.