UWP Custom Markup Extension Example

Calc.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Windows.UI.Xaml.Markup;

namespace CustomMarkupExtensionRS3
{
    [MarkupExtensionReturnType(ReturnType = typeof(int))]
    public sealed class Calc : MarkupExtension
    {
        private string _text;

        public string Eq
        {
            get { return _text; }
            set { _text = value; }
        }

        public Calc()
        {
        }

        protected override object ProvideValue()
        {
            int.TryParse(Eq, out int value);
            return 5 + value;
        }
    }
}

MainPage.xaml

<Page
    x:Class="CustomMarkupExtensionRS3.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:CustomMarkupExtensionRS3"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <StackPanel Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
        <Rectangle Fill="Red" Width="100" Height="{local:Calc Eq=10}" />
    </StackPanel>
</Page>