using System;
using System.IO;
using System.Reflection;
using System.Xaml;
using System.Xaml.Schema;

namespace Sandbox
{

    public class Foo
    {
        public object DataContext { get; set; }
    }

    class Program
    {
        private static XamlDirective DesignContextDirective = new XamlDirective(
            new[] { "http://schemas.microsoft.com/expression/blend/2008" },
            "DataContext", XamlLanguage.Object, null, AllowedMemberLocations.Attribute);

        class CustomContext : XamlSchemaContext
        {
            public bool IsDesignMode { get; set; }
            public override XamlDirective GetXamlDirective(string xamlNamespace, string name)
            {

                if (IsDesignMode && name == DesignContextDirective.Name && xamlNamespace == DesignContextDirective.PreferredXamlNamespace)
                {
                    return DesignContextDirective;
                }
                return base.GetXamlDirective(xamlNamespace, name);
            }

            public override bool TryGetCompatibleXamlNamespace(string xamlNamespace, out string compatibleNamespace)
            {
                //Forces XamlXmlReader to not ignore our namespace if mc:Ignorable is set
                if (
                    IsDesignMode &&
                    xamlNamespace == DesignContextDirective.PreferredXamlNamespace)
                {
                    compatibleNamespace = xamlNamespace;
                    return true;
                }
                return base.TryGetCompatibleXamlNamespace(xamlNamespace, out compatibleNamespace);
            }
        }
        class CustomWriter : XamlObjectWriter
        {
            public bool IsDesignMode { get; set; }
            private static PropertyInfo DataContextProperty = typeof(Foo).GetProperty("DataContext");
            public CustomWriter(XamlSchemaContext schemaContext) : base(schemaContext)
            {
            }

            public override void WriteStartMember(XamlMember property)
            {
                if (IsDesignMode && property == DesignContextDirective)
                    base.WriteStartMember(new XamlMember(DataContextProperty, SchemaContext));
                else
                    base.WriteStartMember(property);
            }
        }

        static void Main(string[] args)
        {
            foreach (var designMode in new[] {true, false})
            {

                XamlXmlReaderSettings settings = new XamlXmlReaderSettings {LocalAssembly = typeof(Foo).Assembly};

                var rdr = new XamlXmlReader(new StringReader(@"
<Foo xmlns='clr-namespace:Sandbox'
xmlns:d='http://schemas.microsoft.com/expression/blend/2008'
xmlns:mc='http://schemas.openxmlformats.org/markup-compatibility/2006'
mc:Ignorable='d'
d:DataContext='wat'>
</Foo>"), new CustomContext(){IsDesignMode = designMode}, settings);

                var writer = new CustomWriter(rdr.SchemaContext){IsDesignMode = designMode};
                XamlServices.Transform(rdr, writer);

                var obj = (Foo) writer.Result;
                Console.WriteLine($"DesignMode: {designMode}, DataContext: {obj.DataContext}");
            }

        }




    }
}