ConsoleApp0.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net472</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <Reference Include="PresentationFramework" />
    <Reference Include="System.Xaml" />
  </ItemGroup>

</Project>

Driver.cs

using System;
using System.ComponentModel;
using System.Globalization;

// WPF markup types
using WpfXamlReader = System.Windows.Markup.XamlReader;
using WpfXamlWriter = System.Windows.Markup.XamlWriter;

// System.Xaml types
using SystemXamlServices = System.Xaml.XamlServices;

static class Driver
{
    static void Main()
    {
        Ser obj = Ser.CreateFrom("some payload");

        // System.Windows.Markup
        {
            string s = WpfXamlWriter.Save(obj);
            Ser obj2 = (Ser)WpfXamlReader.Parse(s);
            Console.WriteLine($"obj.Payload:  {obj.Payload}");
            Console.WriteLine($"obj2.Payload: {obj2.Payload}");
        }

        // System.Xaml
        {
            // this line throws immediately:
            string s = SystemXamlServices.Save(obj);

            Ser obj2 = (Ser)SystemXamlServices.Parse(s);
            Console.WriteLine($"obj.Payload:  {obj.Payload}");
            Console.WriteLine($"obj2.Payload: {obj2.Payload}");
        }
    }
}

[TypeConverter(typeof(TC))]
public sealed class Ser
{
    private Ser(string s) => this.Payload = s;

    public string Payload { get; }

    public static Ser CreateFrom(string payload) => new Ser(payload);
}

public sealed class TC : TypeConverter
{
    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
    {
        return sourceType == typeof(string) || base.CanConvertFrom(context, sourceType);
    }

    public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
    {
        return value is string s
            ? Ser.CreateFrom(s)
            : base.ConvertFrom(context, culture, value);
    }

    public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
    {
        return destinationType == typeof(string) || base.CanConvertTo(context, destinationType);
    }

    public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
    {
        return destinationType == typeof(string)
            ? ((Ser)value).Payload
            : base.ConvertTo(context, culture, value, destinationType);
    }
}