Xamarin.Forms UWP - InvalidCastException when trying to set TextDecorations for TextBlock

Question

In a Xamarin.Forms project, I'm trying to allow underlined Labels. So I have a custom renderer, and I'm trying to do something simple:

Control.TextDecorations = TextDecorations.Underline;

It compiles just fine, but when the app launches, I'm getting an InvalidCastException on that line which says:

System.InvalidCastException: 'Unable to cast object of type 'Windows.UI.Xaml.Controls.TextBlock' to type 'Windows.UI.Xaml.Controls.ITextBlock5'.'

Here's a screenshot of the exception:

Also, when inspecting the Control, I noticed there are a ton of InvalidCastException exceptions on other properties of the TextBlock control as well - here's a small sample:

Why is it trying to cast to a ITextBlock5 type? Is this a UWP bug? Is there a workaround to get underline to work?

Answer 1

According to the Microsoft documentation, the TextDecorations property wasn't introduced until version 15063. You're probably getting that exception because you're on an earlier version of Windows.

As a workaround, you can create an Underline() object and add a Run() object to the Underline object's Inlines collection, like this:

// first clear control content
Control.Inlines.Clear();

// next create new Underline object and
// add a new Run to its Inline collection
var underlinedText = new Underline();
underlinedText.Inlines.Add(new Run { Text = "text of xamarin element here" });

// finally add the new Underline object
// to the Control's Inline collection
Control.Inlines.Add(underlinedText);

Inside the custom renderer's OnElementChanged() override method, it would look something like this:

protected override void OnElementChanged(ElementChangedEventArgs<Label> e)
{
    base.OnElementChanged(e);
    var view = e.NewElement as LabelExt; // LabelExt => name of PCL custom Label class
    var elementExists = (view != null && Control != null);
    if (!elementExists)
    {
        return;
    }

    // add underline to label
    Control.Inlines.Clear();
    var underlinedText = new Underline();
    underlinedText.Inlines.Add(new Run { Text = view.Text });
    Control.Inlines.Add(underlinedText);
}