Add xml.xsd to lexicon schema set

[klightspeed]

.NET 5.0 and above disable remote XML schema downloads by default, resulting in lexicon schema validation failing due to the XML namespace schema (http://www.w3.org/XML/1998/namespace) not being present in the available schemas.

Add a cached copy of https://www.w3.org/2009/01/xml.xsd (the current immutable version of the schema at http://www.w3.org/2001/xml.xsd) to the lexicon schema set.

Reported as #2814

Error reported in EDCD#eddi Discord:

Hi, my lexicon file, which used to work, is now getting renamed to malformed.
I get the following error in the log file:

2026-05-15T10:07:46.2462358Z [16] [Warning] SpeechFormatter:IsValidXML Could not load lexicon file '%APPDATA%\EDDI\lexicons\en.pls', please review.: {"ClassName":"System.Xml.Schema.XmlSchemaValidationException","Message":"The 'http://www.w3.org/XML/1998/namespace:base' attribute is not declared.","Data":null,"InnerException":null,"HelpURL":null,"StackTraceString":"   at System.Xml.Schema.XmlSchemaValidator.SendValidationEvent(XmlSchemaValidationException e, XmlSeverityType severity)\r\n   at System.Xml.Schema.XmlSchemaValidator.RecompileSchemaSet()\r\n   at System.Xml.Schema.XmlSchemaValidator.Init()\r\n   at System.Xml.Schema.XNodeValidator.Validate(XObject source, XmlSchemaObject partialValidationType, Boolean addSchemaInfo)\r\n   at EddiSpeechService.SpeechPreparation.SpeechFormatter.IsValidXML(String filename) in EddiSpeechService\SpeechPreparation\SpeechFormatter.cs:line 277","RemoteStackTraceString":null,"RemoteStackIndex":0,"ExceptionMethod":null,"HResult":-2146231999,"Source":"System.Private.Xml","WatsonBuckets":null,"res":"The '{0}' attribute is not declared.","args":["http://www.w3.org/XML/1998/namespace:base"],"sourceUri":"","lineNumber":69,"linePosition":14,"version":"2.0"}

I get what looks like the same error if I use the example apple to orange lexicon from here https://github.com/EDCD/EDDI/wiki/Lexicons
I think this is probably easy to fix by editing my lexicon file, but I don't understand the problem well enough to fix it. I thought I'd metion here as it feels like the example file will need the same fix.
I thiiink this started happening when I updated EDDI to 5.0, but I'm not certain.

Example program showing underlying issue:

using System.Diagnostics;
using System.Security.Cryptography;
using System.Xml.Linq;
using System.Xml.Schema;

var pls = """
<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-gb">
  <lexeme>
    <grapheme>apple</grapheme>
    <phoneme>ˈɔɹɪnd͡ʒ</phoneme>
  </lexeme>
</lexicon>
""";

static async Task AddRemoteSchema(HttpClient client, XmlSchemaSet xmlschemas, string url, string targetns, string sha256sum)
{
    if (!xmlschemas.Contains(targetns))
    {
        var schemaData = await client.GetByteArrayAsync(url);
        Debug.Assert(Convert.ToHexStringLower(SHA256.HashData(schemaData)) == sha256sum);
        using var memstream = new MemoryStream(schemaData);
        var schema = XmlSchema.Read(memstream, null);
        Debug.Assert(schema != null);
        Debug.Assert(schema.TargetNamespace == targetns);
        xmlschemas.Add(schema);
    }
}

var xmlschemas = new XmlSchemaSet();

using var client = new HttpClient();

await AddRemoteSchema(
    client,
    xmlschemas,
    "https://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd",
    "http://www.w3.org/2005/01/pronunciation-lexicon",
    "5c15b4a8862e5232512bc26ca6359f0c679f0059515dc66af8ae8be125a74061"
);

if (args.Contains("--import-xml-schema"))
{
    await AddRemoteSchema(
        client,
        xmlschemas,
        "https://www.w3.org/2009/01/xml.xsd",
        "http://www.w3.org/XML/1998/namespace",
        "cc701736c42cc64126fad063bb95f94484b5de3b5f808a86ea098b0957aff829"
    );
}

var xml = XDocument.Parse(pls);

xml.Validate(xmlschemas, (o, e) =>
{
    if (e.Severity is XmlSeverityType.Error or XmlSeverityType.Warning)
    {
        throw new XmlSchemaValidationException(e.Message, e.Exception);
    }
});

Console.WriteLine("OK");

klightspeed@Capella MINGW64 ~/source/repos/ConsoleApp1/app
$ dotnet run Program.cs
Unhandled exception. System.Xml.Schema.XmlSchemaValidationException: The 'http://www.w3.org/XML/1998/namespace:base' attribute is not declared.
   at System.Xml.Schema.XmlSchemaValidator.SendValidationEvent(XmlSchemaValidationException e, XmlSeverityType severity)
   at System.Xml.Schema.XmlSchemaValidator.RecompileSchemaSet()
   at System.Xml.Schema.XmlSchemaValidator.Init()
   at System.Xml.Schema.XNodeValidator.Validate(XObject source, XmlSchemaObject partialValidationType, Boolean addSchemaInfo)
   at Program.<Main>$(String[] args)
   at Program.<Main>(String[] args)

klightspeed@Capella MINGW64 ~/source/repos/ConsoleApp1/app
$ dotnet run Program.cs --import-xml-schema
OK

xml.xsd should have blob hash bd291f3d4be818edcb1498697ffd03f0226a9cf8:

$ IFS='' read -r -d '' xml_xsd < <(curl -s https://www.w3.org/2009/01/xml.xsd); printf "%s" "$xml_xsd" | sha256sum; printf "blob %d\0%s" "$(printf "%s" "$xml_xsd" | wc -c)" "$xml_xsd" | sha1sum
cc701736c42cc64126fad063bb95f94484b5de3b5f808a86ea098b0957aff829  -
bd291f3d4be818edcb1498697ffd03f0226a9cf8  -
$ curl -s -L -H "Accept: application/vnd.github.object" -H "X-GitHub-Api-Version: 2026-03-10" https://api.github.com/repos/klightspeed/EDDI/contents/SpeechService/Properties/xml.xsd?ref=add-xml-xsd | jq '.sha'
"bd291f3d4be818edcb1498697ffd03f0226a9cf8"

[klightspeed]

The original pull commit had a typo - Eddn instead of Eddi. This has been fixed.

Testing:

• Add a en.pls mapping Adder to ˈɔɹɪnd͡ʒ
• Set the startup project to Eddi
• Place a breakpoint on xml.Validate(...
• Comment out FetchSchemasFromResource( "EddiSpeechService.Properties.xml.xsd" );
• Debug the project
• On the Text-to-Speech tab, select Microsoft David, ship Adder, and click Test voice
• Verify that I get the validation error
• Speech continues with "This is how I will sound in your Adder"
• Exit EDDI
• Uncomment FetchSchemasFromResource( "EddiSpeechService.Properties.xml.xsd" );
• Debug the project
• On the Text-to-Speech tab, select Microsoft David, ship Adder, and click Test voice
• Verify that validation succeeds
• Verify that I hear "This is how I will sound in your Orange"