.NET WPF Binding markup extension supporting designer specific value

BindingWithDesignSupport.cs

using System;
using System.ComponentModel;
using System.Windows;
using System.Windows.Data;
using System.Windows.Markup;
using static System.Convert;

namespace MarkupExtensions
{
    /// <summary>
    /// Markup extension that provides a binding that supports designer specific value.
    /// </summary>
    /// <remarks>
    /// The binding is used in place of a typical binding. Within the designer, the specified <see cref="DesignTimeValue"/> will be used
    /// instead of the binding specified.
    /// <p/>
    /// Typically simple values are passed into the extension as strings. <see cref="DesignTimeValueType"/> can specify a type to which to
    /// convert the value through <see cref="ChangeType(object, TypeCode)"/>.
    /// <p/>
    /// Concept borrowed from: <see href="https://stackoverflow.com/a/38011434/84522"/>
    /// </remarks>
    /// <example>
    /// <code><![CDATA[
    /// <RowDefinition MaxHeight="{cm:BindingWithDesignSupport 
    ///                             {Binding ElementName = RealTimeView, Path = ActualHeight, Mode = OneTime},
    ///                             DesignTimeValue=300, DesignTimeValueType={x:Type sys:Double}}"/>
    /// ]]></code>
    /// </example>
    /// <seealso cref="MarkupExtension" />
    public class BindingWithDesignSupport : MarkupExtension
    {
        private static readonly bool IsInDesignMode = DesignerProperties.GetIsInDesignMode(new DependencyObject());
        private readonly object _unsetObject = new object();

        /// <summary>
        /// Initializes a new instance of the <see cref="BindingWithDesignSupport" /> class.
        /// </summary>
        public BindingWithDesignSupport()
        {
            DesignTimeValue = _unsetObject;
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="BindingWithDesignSupport" /> class.
        /// </summary>
        /// <param name="binding">The binding.</param>
        public BindingWithDesignSupport(BindingBase binding)
            : this()
        {
            Binding = binding ?? throw new ArgumentNullException(nameof(binding));
        }

        /// <summary>
        /// Gets or sets the binding (required).
        /// </summary>
        public BindingBase Binding { get; set; }

        /// <summary>
        /// Gets or sets the design time value (required).
        /// </summary>
        public object DesignTimeValue { get; set; }

        /// <summary>
        /// Gets or sets the type of the design time value (optional).
        /// </summary>
        public Type DesignTimeValueType { get; set; }

        /// <inheritdoc />
        /// <param name="serviceProvider">The parameter is not used.</param>
        public override object ProvideValue(IServiceProvider serviceProvider)
        {
            if (Binding == null)
            {
                throw new InvalidOperationException($"Required property '{nameof(Binding)}' is unset.");
            }

            if (DesignTimeValue == _unsetObject)
            {
                throw new InvalidOperationException($"Required property '{nameof(DesignTimeValue)}' is unset.");
            }

            if (DesignTimeValueType != null)
            {
                DesignTimeValue = ChangeType(DesignTimeValue, DesignTimeValueType);
            }

            return IsInDesignMode ? DesignTimeValue : Binding.ProvideValue(serviceProvider);
        }
    }
}

Gist referenced here: https://stackoverflow.com/a/38011434/84522