Wpf: XamlXmlWriter is including the namespace for each element

[ryalanms]

• .NET Core Version: (e.g. 3.0 Preview1, or daily build number, use dotnet --info)

Preview6. Regression from Preview5.

• Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes/No

No.

Problem description:

XamlXmlWriter is including the namespace where it wasn't before.

Minimal repro:

            XamlSchemaContext xsc = new XamlSchemaContext();
            var generated = new StringBuilder();
            var xmlSettings = new XmlWriterSettings { Indent = true, OmitXmlDeclaration = true };

            var hashSetAssemblyName = typeof(HashSet<List<BigContainer>>).GetAssemblyName();
            var listAssemblyName = typeof(List<Dictionary<int, HashSet<List<BigContainer>>>>).GetAssemblyName();

            using (XamlXmlWriter writer = new XamlXmlWriter(XmlWriter.Create(new StringWriter(generated), xmlSettings), xsc))
            {
                XamlType list = xsc.GetXamlType(typeof(List<Dictionary<int, HashSet<List<BigContainer>>>>));
                var ns1 = $"clr-namespace:System.Collections.Generic;assembly={hashSetAssemblyName}";
                var ns2 = "clr-namespace:Test.Elements;assembly=XamlTestClasses";
                var ns3 = "http://schemas.microsoft.com/winfx/2006/xaml";
                writer.WriteNamespace(new NamespaceDeclaration(ns1, "a"));
                writer.WriteNamespace(new NamespaceDeclaration(ns2, "b"));
                writer.WriteNamespace(new NamespaceDeclaration(ns3, "c"));
                writer.WriteStartObject(list);
                writer.WriteEndObject();
            }


            var expected = @"<List c:TypeArguments=""Dictionary(c:Int32, a:HashSet(List(b:BigContainer)))"" xmlns=""clr-namespace:System.Collections.Generic;assembly={0}"" xmlns:a=""clr-namespace:System.Collections.Generic;assembly={1}"" xmlns:b=""clr-namespace:Test.Elements;assembly=XamlTestClasses"" xmlns:c=""http://schemas.microsoft.com/winfx/2006/xaml"" />";
            expected = string.Format(expected, listAssemblyName, hashSetAssemblyName);

            Assert.AreEqual(expected, generated.ToString());

--
  |   | - |   | "<List c:TypeArguments=\"Dictionary(c:Int32, a:HashSet(List(b:BigContainer)))\" xmlns=\"clr-namespace:System.Collections.Generic;assembly=System.Private.CoreLib\" xmlns:a=\"clr-namespace:System.Collections.Generic;assembly=System.Private.CoreLib\" xmlns:b=\"clr-namespace:Test.Elements;assembly=XamlTestClasses\" xmlns:c=\"http://schemas.microsoft.com/winfx/2006/xaml\" />"\r\n
1 |   | + |   | {<a:List c:TypeArguments="a:Dictionary(c:Int32, a:HashSet(a:List(b:BigContainer)))" xmlns:a="clr-namespace:System.Collections.Generic;assembly=System.Private.CoreLib" xmlns:b="clr-namespace:Test.Elements;assembly=XamlTestClasses" xmlns:c="http://schemas.microsoft.com/winfx/2006/xaml" />}
COMMENTS:

[vatsan-madhavan]

Is this a true bug, or a change in formatting that’s both syntactically legal and semantically correct but perhaps happens to break a test etc?

[ryalanms]

The output is syntactically legal and semantically correct, and it actually looks like an improvement to me. Prior to Preview6, the first namespace is written out twice: once without an name prefix, as the default namespace, and once with a name prefix.

xmlns=\"clr-namespace:System.Collections.Generic;assembly=System.Private.CoreLib\" xmlns:a=\"clr-namespace:System.Collections.Generic;assembly=System.Private.CoreLib\" 

In Preview6, there is no default namespace, and the first namespace is only written once. The absence of the default namespace requires that each element also get a name prefix.

xmlns:a="clr-namespace:System.Collections.Generic;assembly=System.Private.CoreLib" 

<a:List c:TypeArguments=...>

This is what I would expect the line below, rather than having the writer give a default namespace _and_ a name prefix for the same namespace.

writer.WriteNamespace(new NamespaceDeclaration(ns1, "a"));

So to answer your question, yes, this is syntactically legal and semantically correct and actually looks better than before, but the writer output is changed from previous releases.

And this is not a bug unless WPF provides a guarantee about the consistency of the output of the XamlXmlWriter, release to release.

[vatsan-madhavan]

We don’t provide preview-to-preview compat.

Serialization compatibility is something you should take into consideration between netcoreapp3.1 & net48, net5.0 & netcoreapp3.1, net5.0 & net4.8.

Serialization compat doesn’t mean that the output of XamlXmlWriter remains static; it just means that it remains compatible within reason, and in the context of safe (ie secure) [de]serialization actions.

I strongly suspect that a valid XML would remain compatible.

[ryalanms]

In that case, it sounds like the DRT should be updated. This is definitely not blocking Preview6.

[fabiant3]

Sounds like tests need to be updated here.