Exception thrown when streaming certain XML values via XmlReader

Describe the bug

When streaming with CommandBehavior.SequentialAccess, if an xml value is a fragment beginning with any character other than <, then SqlDataReader.GetXmlReader(…) returns a reader doomed to throw an exception.

System.Xml.XmlException
  HResult=0x80131940
  Message='.', hexadecimal value 0x00, is an invalid character. Line 1, position 2.
  Source=System.Private.Xml
  StackTrace:
   at System.Xml.XmlTextReaderImpl.Throw(Exception e)
   at System.Xml.XmlTextReaderImpl.Throw(String res, String[] args)
   at System.Xml.XmlTextReaderImpl.ParseText(Int32& startPos, Int32& endPos, Int32& outOrChars)
   at System.Xml.XmlTextReaderImpl.ParseText()
   at System.Xml.XmlTextReaderImpl.ParseDocumentContent()
   at Program.<Main>$(String[] args) in D:\Code\Self\XmlReaderEncodingBug\XmlReaderEncodingBug\Program.cs:line 14

This issue is fixed by #4757.

To reproduce

using Microsoft.Data.SqlClient;
using System.Data;

using var connection = new SqlConnection("Server=.;Integrated Security=true;Encrypt=false");
connection.Open();

using var command = connection.CreateCommand();
command.CommandText = "SELECT CONVERT(xml, 'a<b/>');";

using var results = command.ExecuteReader(CommandBehavior.SequentialAccess);
results.Read(); // => true

using var xml = results.GetXmlReader(0);
xml.Read(); // throws

Expected behavior

Program exits cleanly.

Further technical details

Microsoft.Data.SqlClient version: 7.1.0
.NET target: .NET 10.0
SQL Server version: SQL Server 2025
Operating system: Windows 11 25H2

🔍 Triage Summary

Check	Result
Issue type	Bug
Environment	All required environment details provided for investigation
Area	No exact match in the standard area label list (this affects XML value streaming via `SqlDataReader.GetXmlReader`/`SqlStream` encoding detection, not pooling/AKV/JSON/SNI/bulk copy/vector/async); closest general fit would be `Area\Netcore`, though the root cause is in shared encoding-detection logic likely affecting both netfx and netcore code paths
Duplicates	None found (exact match); related but distinct: #1877 (O(N^2) perf reading XML with SequentialAccess), #1903 (NullReferenceException streaming XML async, closed), #3035 (ARM test flakiness in XmlReader test)
Regression	Not indicated — no prior-working-version boundary mentioned

Analysis

This bug affects SqlDataReader.GetXmlReader() when streaming an xml value under CommandBehavior.SequentialAccess. SQL Server streams xml values as BOM-less UTF-16LE, but some code paths that construct the resulting XmlReader (unlike others that wrap with a BOM-synthesizing SqlStream) don't specify an explicit encoding, so .NET's XML encoding-detection logic can misidentify BOM-less UTF-16LE content as UTF-8 whenever the first character isn't <. This produces a corrupted read and an XmlException on the very first Read() call. The report includes a clear, minimal repro, full stack trace, and complete environment details. Severity: P2 (correctness bug with a clear, narrow root cause and workaround already identified, but no data loss/security impact and a fix is already proposed).

Next Steps

A fix is already proposed in Fix exception thrown when streaming certain XML values via XmlReader #4757 (same author), which sets the XmlReader encoding explicitly to Encoding.Unicode to bypass the faulty detection logic, and updates DataStreamTest manual tests (GetFieldValueAsync_OfXmlReader, GetFieldValue_OfXmlReader, GetXmlReader) to cover the pathological case. Maintainers should prioritize reviewing/merging Fix exception thrown when streaming certain XML values via XmlReader #4757 rather than assigning this to Copilot coding agent for independent investigation, since the fix and tests are already in hand.
Recommend reviewing O(N^2) performance when reading XML with SequentialAccess #1877 and NullReferenceException when streaming XML asynchronously from DB #1903 for context, as they involve related (but not identical) XML-over-SequentialAccess streaming pitfalls in the same code area; worth confirming Fix exception thrown when streaming certain XML values via XmlReader #4757 doesn't regress those scenarios.
Once Fix exception thrown when streaming certain XML values via XmlReader #4757 merges, this issue should auto-close via the "Closes Exception thrown when streaming certain XML values via XmlReader #4756" reference.

Note: This triage summary is auto-generated by an AI agent. The analysis and suggestions above have not been verified by a human maintainer. Please treat as preliminary guidance only.

Generated by SqlClient Issue Auto-Triage for #4756 · copilot · auto · 35.8 AIC · ⌖ 12.7 AIC · ⊞ 11.8K · ◷

Suggested label: Repro Available ✔️

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Exception thrown when streaming certain XML values via XmlReader #4756

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Exception thrown when streaming certain XML values via XmlReader #4756

Description

Describe the bug

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github-actions commented on Sep 26, 2026

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sharpjs commented on Sep 26, 2026

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