Question

When I am working on a button template I just encountered a binding StringFormat issue. Here it is.

— I setup a button with a given ToolTip as a string. I like to use it to specify the image name within binding. Then I apply it to a button style called ImageButton.

<Button ToolTip="Minimize" Height="34" Style="{StaticResource ImageButton}"  Margin="220,36,125,371" />;

— This is the code in style. I like to use the image name from the binding and come up with a full image path. It is supposed to be working like this, <Image x:Name="ButtonImage" Source="Images/Minimize-Normal.png" />. Yes, I do have such a image in place and it works great if I just put the xml code without binding.

<Image x:Name="ButtonImage" Source="{Binding ToolTip, RelativeSource={RelativeSource TemplatedParent}, StringFormat={}Images/{0}-Normal.png}" />

But the binding did not work as I expected. The image cannot be shown correctly. If I put the Image full name in ToolTip then it worked correct. It looks like the StringFormat had been ignored.

Any help will be appreciated. Thanks in advance.

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Thanks for dbaseman’s response. I know I can achieve it by using Converter. But as I think StringFormat is easier to apply, that’s why I tried this.

I also found a reply here, it says – StringFormat is used when binding to a string property, while the Text property in your control is of type object, hence StringFormat is ignored.

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It should be by design. Below code works. Because the Text property is string type.

<TextBlock Text="{Binding ToolTip, RelativeSource={RelativeSource Self}, StringFormat={}Images/{0}-Normal.png}" ToolTip="Minimize" />

Answer 1

That’s interesting — I’m not sure if it’s a bug, or by design. You can work around this problem, though, by using a Converter instead of StringFormat.

public class ImageNameConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return string.Format("Images/{0}-Normal.png", value);
    }

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

And then of course:

<local:ImageNameConverter x:Key="ImageNameConverter" />

And:

<Image x:Name="ButtonImage" 
    Source="{Binding ToolTip, 
                RelativeSource={RelativeSource TemplatedParent}, 
                Converter="{StaticResource ImageNameConverter}}" />

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Edit I think it’s by design. From the docs for BindingBase.StringFormat:

Gets or sets a string that specifies how to format the binding if it displays the bound value as a string.

So (apparently) the string-format conversion won’t happen unless the binding source is to actually be rendered as a text string.