Why is my XML passing XSD validation but is expected failure?

Question

I have been trying to figure out how to accurately validate this XML, it looks like it shouldn't be passing validation but it is. I have tried a C# implementation and a powershell implementation, both pass but should not. For reference, my XSD imports 4 other XSDs but I have attached the relevant parts for brevity;

Basic C# validation

    class XmlSchemaSetExample
    {
        static void Main()
        {
            XmlReaderSettings xmlSettings = new XmlReaderSettings();
            xmlSettings.Schemas.Add("http://url.org/Contract", @"C:\XML\Contract.xsd");
            xmlSettings.ValidationType = ValidationType.Schema;
            xmlSettings.ValidationEventHandler += new ValidationEventHandler(xmlSettingsValidator);


            XmlReader xml = XmlReader.Create(@"C:\XML\response2.xml", xmlSettings);

            while (xml.Read()) { }
        }

        static void xmlSettingsValidator(object sender, ValidationEventArgs e)
        {
            if (e.Severity == XmlSeverityType.Warning)
            {
                Console.Write("WARNING: ");
                Console.WriteLine(e.Message);
            }
            else if (e.Severity == XmlSeverityType.Error)
            {
                Console.Write("ERROR: ");
                Console.WriteLine(e.Message);
            }
        }
    }

I have tried other XSD/XML combinations and I can trigger a failure but I cannot trigger a failure with this specific XSD/XML payload -- where the ErrorDetail tag should trigger a validation error with ErrorDetailFault in the XML instead of the defined tag. However, I cannot find a way to trigger this validation error. I am defining the ErrorDetail complex type and then referencing the type in the XSD

XSD

<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:ser="http://schemas.microsoft.com/2003/10/Serialization/" xmlns:tns="http://url.org/Contract" xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="http://url.org/Contract" elementFormDefault="qualified">
    <xs:import namespace="http://schemas.microsoft.com/2003/10/Serialization/" schemaLocation="Serialization.xsd"/>
    <xs:complexType name="ErrorDetail">
        <xs:sequence>
            <xs:element name="Time" type="xs:dateTime" nillable="true" minOccurs="0"/>
            <xs:element name="ID" nillable="true" minOccurs="0">
                <xs:simpleType>
                    <xs:restriction base="xs:string">
                        <xs:minLength value="0" fixed="false"/>
                        <xs:maxLength value="64" fixed="false"/>
                        <xs:whiteSpace value="preserve"/>
                        <xs:pattern value=".{0,64}"/>
                    </xs:restriction>
                </xs:simpleType>
            </xs:element>
            <xs:element name="CallSite" nillable="true" minOccurs="0">
                <xs:simpleType>
                    <xs:restriction base="xs:string">
                        <xs:minLength value="0" fixed="false"/>
                        <xs:maxLength value="64" fixed="false"/>
                        <xs:whiteSpace value="preserve"/>
                        <xs:pattern value=".{0,64}"/>
                    </xs:restriction>
                </xs:simpleType>
            </xs:element>
            <xs:element name="Message" nillable="true" minOccurs="0">
                <xs:simpleType>
                    <xs:restriction base="xs:string">
                        <xs:minLength value="0" fixed="false"/>
                        <xs:maxLength value="64" fixed="false"/>
                        <xs:whiteSpace value="preserve"/>
                        <xs:pattern value=".{0,64}"/>
                    </xs:restriction>
                </xs:simpleType>
            </xs:element>
            <xs:element name="Exceptions" nillable="true" minOccurs="0">
                <xs:simpleType>
                    <xs:restriction base="xs:string">
                        <xs:minLength value="0" fixed="false"/>
                        <xs:maxLength value="64" fixed="false"/>
                        <xs:whiteSpace value="preserve"/>
                        <xs:pattern value=".{0,64}"/>
                    </xs:restriction>
                </xs:simpleType>
            </xs:element>
            <xs:element name="Tag1" nillable="true" minOccurs="0">
                <xs:simpleType>
                    <xs:restriction base="xs:string">
                        <xs:minLength value="0" fixed="false"/>
                        <xs:maxLength value="64" fixed="false"/>
                        <xs:whiteSpace value="preserve"/>
                        <xs:pattern value=".{0,64}"/>
                    </xs:restriction>
                </xs:simpleType>
            </xs:element>
            <xs:element name="Tag2" nillable="true" minOccurs="0">
                <xs:simpleType>
                    <xs:restriction base="xs:string">
                        <xs:minLength value="0" fixed="false"/>
                        <xs:maxLength value="64" fixed="false"/>
                        <xs:whiteSpace value="preserve"/>
                        <xs:pattern value=".{0,64}"/>
                    </xs:restriction>
                </xs:simpleType>
            </xs:element>
        </xs:sequence>
    </xs:complexType>
    <xs:element name="ErrorDetail" type="tns:ErrorDetail" nillable="true"/>
</xs:schema>

XML

        <ErrorDetailFault xmlns:a="http://url.org/Contract" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
            <a:Time>2022-01-31T19:15:50.324427Z</a:Time>
            <a:ID>b9201014-2b11-4523-a30c-55e3112d29d1</a:ID>
            <a:CallSite>data</a:CallSite>
            <a:Message/>
            <a:Exceptions>Exception</a:Exceptions>
            <a:Tag1>"data"</a:Tag1>
            <a:Tag2/>
        </ErrorDetailFault>

Any insights into what I've done incorrectly here?

Powershell validation attempt for additional context to troubleshooting this -- also passes on an expected failure

PS

function Test-XmlBySchema
{
    [CmdletBinding()]
    [OutputType([bool])]
    param
    (
        [Parameter(Mandatory)]
        [ValidateScript({ Test-Path -Path $_ })]
        [ValidatePattern('\.xml')]
        [string]$XmlFile,
        [Parameter(Mandatory)]
        [ValidateScript({ Test-Path -Path $_ })]
        [ValidatePattern('\.xsd')]
        [string]$SchemaPath,
        [string]$NSUri
    )

    try
    {
        [xml]$xml = Get-Content $XmlFile
        $xml.Schemas.Add($NSUri, $SchemaPath) | Out-Null
        $xml.Validate($null)
        Write-Verbose "Successfully validated $XmlFile against schema ($SchemaPath)"
        $result = $true
    }
    catch
    {
        $err = $_.Exception.Message
        $err
        Write-Verbose "Failed to validate $XmlFile against schema ($SchemaPath)`nDetails: $err"
        $result = $false
    }
    finally
    {
        $result
    }
}

Answer 1

In your XML, ErrorDetailFault,

    <ErrorDetailFault xmlns:a="http://url.org/Contract" 
                      xmlns:i="http://www.w3.org/2001/XMLSchema-instance">

is not in the target namespace of the XSD, http://url.org/Contract.

To place ErrorDetailFault in the http://url.org/Contract namespace, add the a namespace prefix to the ErrorDetailFault:

    <a:ErrorDetailFault xmlns:a="http://url.org/Contract" 
                        xmlns:i="http://www.w3.org/2001/XMLSchema-instance">

or have it be the default namespace (after removing the a prefix from the other elements in the coument):

    <ErrorDetailFault xmlns="http://url.org/Contract" 
                      xmlns:i="http://www.w3.org/2001/XMLSchema-instance">