Fix JUnit reporter crash on invalid XML characters in test names and exceptions #4262

Copilot · 2026-01-08T12:05:03Z

The JUnit reporter crashes with ArgumentException when test arguments or exception messages contain invalid XML 1.0 characters (e.g., \x04), terminating the test run.

Changes

Added SanitizeForXml method to validate characters against XML 1.0 spec and replace invalid characters with hex notation [0xNN]
Applied sanitization to all string outputs: test names, class names, exception messages, stack traces, and metadata (assembly names, hostnames, filters, etc.)
Preserved valid control characters: tab (0x9), newline (0xA), carriage return (0xD) pass through as XML entities

Example

Before (crashes):

[Test]
[Arguments("A string with an invalid \x04 character")]
public async Task TestSomething(string parameter) { }

After (generates valid XML):

<testcase name="TestSomething(A string with an invalid [0x4] character)" .../>

Exception messages are similarly sanitized:

<error message="Error with invalid [0x4] character">...</error>

Original prompt

This section details on the original issue you should resolve

<issue_title>[Bug]: JUnit reporter crashes if test arguments contains an "invalid character"</issue_title>
<issue_description>### Description

When running a test of the form

[Test]
[Arguments("Some valid string")]
[Arguments("A string with an invalid \x04 character")]
public async Task TestSomething(string parameter)
{
   await Assert.That(parameter).IsNotNull();  
}

The test runner will crash with the following exception: Error output: Unhandled exception in AppDomain: System.ArgumentException: '�', hexadecimal value 0x04, is an invalid character.

Expected Behavior

The invalid character should be escaped in the resulting name / the runner should not crash.

Actual Behavior

The runner crashed when running with the JUnit reporter active. The tests behave fine when running from an IDE or without reporting.

Steps to Reproduce

Create a test class with the following test:

[Test]
[Arguments("Some valid string")]
[Arguments("A string with an invalid \x04 character")]
public async Task TestSomething(string parameter)
{
   await Assert.That(parameter).IsNotNull();  
}

Build the associated test project.
Run the tests using a command of the form dotnet test --test-modules <Path to test> --report-trx --results-directory <Path to results>
Observe that the test runner crashes

TUnit Version

1.9.26

.NET Version

.NET 10

Operating System

Linux

IDE / Test Runner

dotnet CLI (dotnet test / dotnet run)

Error Output / Stack Trace

Exit code: -532462766
  Error output: Unhandled exception in AppDomain: System.ArgumentException: '�', hexadecimal value 0x04, is an invalid character.
     at System.Xml.XmlEncodedRawTextWriter.WriteAttributeTextBlock(Char* pSrc, Char* pSrcEnd)
     at System.Xml.XmlEncodedRawTextWriter.WriteString(String text)
     at System.Xml.XmlWellFormedWriter.WriteString(String text)
     at System.Xml.XmlWriter.WriteAttributeString(String localName, String value)
     at TUnit.Engine.Xml.JUnitXmlWriter.WriteTestCase(XmlWriter writer, TestNodeUpdateMessage test)
     at TUnit.Engine.Xml.JUnitXmlWriter.WriteTestSuite(XmlWriter writer, IEnumerable`1 tests, String assemblyName, String targetFramework, TestSummary summary, String filter)
     at TUnit.Engine.Xml.JUnitXmlWriter.GenerateXml(IEnumerable`1 testUpdates, String filter)
     at TUnit.Engine.Reporters.JUnitReporter.AfterRunAsync(Int32 exitCode, CancellationToken cancellation)
     at Microsoft.Testing.Platform.Hosts.CommonHost.RunTestAppAsync(CancellationToken testApplicationCancellationToken) in /_/src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs:line 122
     at Microsoft.Testing.Platform.Hosts.CommonHost.RunAsync() in /_/src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs:line 54
     at Microsoft.Testing.Platform.Hosts.CommonHost.RunAsync() in /_/src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs:line 62
     at Microsoft.Testing.Platform.Hosts.CommonHost.RunAsync() in /_/src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs:line 74
     at Microsoft.Testing.Platform.Builder.TestApplication.RunAsync() in /_/src/Platform/Microsoft.Testing.Platform/Builder/TestApplication.cs:line 227
...
  Unhandled exception. System.ArgumentException: '�', hexadecimal value 0x04, is an invalid character.
     at System.Xml.XmlEncodedRawTextWriter.WriteAttributeTextBlock(Char* pSrc, Char* pSrcEnd)
     at System.Xml.XmlEncodedRawTextWriter.WriteString(String text)
     at System.Xml.XmlWellFormedWriter.WriteString(String text)
     at System.Xml.XmlWriter.WriteAttributeString(String localName, String value)
     at TUnit.Engine.Xml.JUnitXmlWriter.WriteTestCase(XmlWriter writer, TestNodeUpdateMessage test)
     at TUnit.Engine.Xml.JUnitXmlWriter.WriteTestSuite(XmlWriter writer, IEnumerable`1 tests, String assemblyName, String targetFramework, TestSummary summary, String filter)
     at TUnit.Engine.Xml.JUnitXmlWriter.GenerateXml(IEnumerable`1 testUpdates, String filter)
     at TUnit.Engine.Reporters.JUnitReporter.AfterRunAsync(Int32 exitCode, CancellationToken cancellation)
     at Microsoft.Testing.Platform.Hosts.CommonHost.RunTestAppAsync(CancellationToken testApplicationCancellationToken) in /_/src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs:line 122
     at Microsoft.Testing.Platform.Hosts.CommonHost.RunAsync() in /_/src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs:line 54
     at Microsoft.Testing.Platform.Hosts.CommonHost.RunAsync() in /_/src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs:line 62
     at Microsoft.Testing.Platform.Hosts.CommonHost.RunAsync() in /_/src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs:line 74
     at Microsoft.Testing.Platform.Builder.TestApplication.RunAsync() in /_/src/Platform/Microsoft.Testing.Platform/Builder/TestApplication.cs:line 227
...

Add...

Fixes [Bug]: JUnit reporter crashes if test arguments contains an "invalid character" #4261

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

…on method Co-authored-by: thomhurst <[email protected]>

…ssage Co-authored-by: thomhurst <[email protected]>

Copilot

Pull request overview

This PR fixes a critical bug where the JUnit reporter crashes with ArgumentException when test arguments or exception messages contain invalid XML 1.0 characters (such as \x04), which was terminating the entire test run.

Key Changes:

Added a SanitizeForXml method that validates characters against the XML 1.0 specification and replaces invalid characters with hex notation [0xNN]
Applied sanitization to all string outputs in the JUnit XML writer including test names, class names, exception messages, stack traces, and metadata
Added integration tests to verify the fix works for various scenarios including multiple invalid characters and exception messages

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
TUnit.Engine/Xml/JUnitXmlWriter.cs	Implements SanitizeForXml method and applies it to all XML string outputs to prevent crashes from invalid XML characters
TUnit.TestProject/JUnitReporterInvalidXmlCharacterTests.cs	Adds integration tests covering invalid XML characters in test arguments and exception messages

Comments suppressed due to low confidence (1)

TUnit.Engine/Xml/JUnitXmlWriter.cs:350

These 'if' statements can be combined.

                if (timing.GlobalTiming.StartTime is { } startTime)
                {
                    if (earliestStartTime is null || startTime < earliestStartTime)
                    {
                        earliestStartTime = startTime;
                    }
                }

Copilot · 2026-01-08T13:50:03Z

TUnit.Engine/Xml/JUnitXmlWriter.cs

+        foreach (var ch in value!)
+        {
+            // Check if character is valid according to XML 1.0 spec
+            if (ch == 0x9 || ch == 0xA || ch == 0xD ||
+                (ch >= 0x20 && ch <= 0xD7FF) ||
+                (ch >= 0xE000 && ch <= 0xFFFD))
+            {
+                builder.Append(ch);
+            }
+            else
+            {
+                // Replace invalid character with its hex representation
+                builder.Append($"[0x{((int)ch):X}]");
+            }


The SanitizeForXml method does not correctly handle Unicode surrogate pairs. When iterating character-by-character, characters in the range U+10000 to U+10FFFF (which are valid per XML 1.0 spec) are represented as two char values (high and low surrogates). The current implementation will incorrectly process each surrogate individually, potentially replacing valid characters with hex notation. Consider using StringInfo or Rune enumeration to properly handle multi-char Unicode sequences.

Suggested change

foreach (var ch in value!)

{

// Check if character is valid according to XML 1.0 spec

if (ch == 0x9 || ch == 0xA || ch == 0xD ||

(ch >= 0x20 && ch <= 0xD7FF) ||

(ch >= 0xE000 && ch <= 0xFFFD))

{

builder.Append(ch);

}

else

{

// Replace invalid character with its hex representation

builder.Append($"[0x{((int)ch):X}]");

}

var length = value!.Length;

for (var i = 0; i < length; i++)

{

var ch = value[i];

int codePoint;

var charsConsumed = 1;

// Handle surrogate pairs to correctly process characters in the range U+10000 to U+10FFFF

if (char.IsHighSurrogate(ch) && i + 1 < length && char.IsLowSurrogate(value[i + 1]))

{

codePoint = char.ConvertToUtf32(ch, value[i + 1]);

charsConsumed = 2;

}

else

{

codePoint = ch;

}

// Check if character is valid according to XML 1.0 spec

var isValid =

codePoint == 0x9 ||

codePoint == 0xA ||

codePoint == 0xD ||

(codePoint >= 0x20 && codePoint <= 0xD7FF) ||

(codePoint >= 0xE000 && codePoint <= 0xFFFD) ||

(codePoint >= 0x10000 && codePoint <= 0x10FFFF);

if (isValid)

{

if (charsConsumed == 1)

{

builder.Append((char)codePoint);

}

else

{

// Append the original surrogate pair

builder.Append(ch);

builder.Append(value[i + 1]);

}

}

else

{

// Replace invalid character (or unpaired surrogate) with its hex representation

builder.Append($"[0x{codePoint:X}]");

}

if (charsConsumed == 2)

{

i++;

}

Copilot · 2026-01-08T13:50:03Z

TUnit.Engine/Xml/JUnitXmlWriter.cs

+        var builder = new StringBuilder(value!.Length);
+        foreach (var ch in value!)


The redundant null-forgiving operators (!) on lines 26 and 27 are unnecessary since the check on line 20 already confirms the value is not null or empty. The value cannot be null at this point in the execution.

Suggested change

var builder = new StringBuilder(value!.Length);

foreach (var ch in value!)

var builder = new StringBuilder(value.Length);

foreach (var ch in value)

Copilot · 2026-01-08T13:50:03Z

TUnit.TestProject/JUnitReporterInvalidXmlCharacterTests.cs

+
+namespace TUnit.TestProject;
+
+public class JUnitReporterInvalidXmlCharacterTests


The test class is missing Category attributes. According to TUnit's testing approach, tests in TUnit.TestProject should be properly categorized (e.g., "Pass" or "Fail") to enable filtered test execution. The TestFailingWithInvalidCharacterInException test intentionally fails and should have a [Category("Fail")] attribute. The other tests that pass should have [Category("Pass")] attributes. This enables proper filtering when running the test suite.

Copilot · 2026-01-08T13:50:04Z

TUnit.Engine/Xml/JUnitXmlWriter.cs

        {
-            writer.WriteAttributeString("message", exception.Message);
+            writer.WriteAttributeString("message", SanitizeForXml(exception.Message));
            writer.WriteAttributeString("type", exception.GetType().FullName ?? "AssertionException");


Exception type names from exception.GetType().FullName are written to XML without sanitization. While standard .NET type names won't contain invalid XML characters, custom exception types could theoretically have non-ASCII or problematic characters in their namespace or type names. For consistency and defensive programming, these should also be sanitized.

Copilot · 2026-01-08T13:50:04Z

TUnit.Engine/Xml/JUnitXmlWriter.cs

        {
-            writer.WriteAttributeString("message", exception.Message);
+            writer.WriteAttributeString("message", SanitizeForXml(exception.Message));
            writer.WriteAttributeString("type", exception.GetType().FullName ?? "Exception");


Exception type names from exception.GetType().FullName are written to XML without sanitization. While standard .NET type names won't contain invalid XML characters, custom exception types could theoretically have non-ASCII or problematic characters in their namespace or type names. For consistency and defensive programming, these should also be sanitized.

Copilot · 2026-01-08T13:50:04Z

TUnit.Engine/Xml/JUnitXmlWriter.cs

+    private static string SanitizeForXml(string? value)
+    {
+        if (string.IsNullOrEmpty(value))
+        {
+            return string.Empty;
+        }
+
+        // At this point, value is guaranteed to not be null
+        var builder = new StringBuilder(value!.Length);
+        foreach (var ch in value!)
+        {
+            // Check if character is valid according to XML 1.0 spec
+            if (ch == 0x9 || ch == 0xA || ch == 0xD ||
+                (ch >= 0x20 && ch <= 0xD7FF) ||
+                (ch >= 0xE000 && ch <= 0xFFFD))
+            {
+                builder.Append(ch);
+            }
+            else
+            {
+                // Replace invalid character with its hex representation
+                builder.Append($"[0x{((int)ch):X}]");
+            }
+        }
+
+        return builder.ToString();
+    }


The SanitizeForXml method lacks dedicated unit tests. While integration tests exist in TUnit.TestProject, there should be unit tests in TUnit.Engine.Tests that directly test the XML sanitization logic with various edge cases (null, empty, valid control characters, invalid characters, surrogate pairs, boundary conditions). This would ensure the sanitization function works correctly in isolation.

Initial plan

c919bb5

Copilot AI assigned Copilot and thomhurst Jan 8, 2026

Copilot started work on behalf of thomhurst January 8, 2026 12:05 View session

Copilot AI and others added 2 commits January 8, 2026 12:15

Fix JUnit reporter crash with invalid XML characters - add sanitizati…

442dfa4

…on method Co-authored-by: thomhurst <[email protected]>

Add test for failing test with invalid XML characters in exception me…

b69e524

…ssage Co-authored-by: thomhurst <[email protected]>

Copilot AI changed the title ~~[WIP] Fix JUnit reporter crash for invalid characters in test arguments~~ Fix JUnit reporter crash on invalid XML characters in test names and exceptions Jan 8, 2026

Copilot AI requested a review from thomhurst January 8, 2026 12:30

Copilot finished work on behalf of thomhurst January 8, 2026 12:30

Copilot AI temporarily deployed to Pull Requests January 8, 2026 12:34 Inactive

thomhurst marked this pull request as ready for review January 8, 2026 13:42

Copilot AI review requested due to automatic review settings January 8, 2026 13:42

thomhurst merged commit 6941bc1 into main Jan 8, 2026
12 of 13 checks passed

thomhurst deleted the copilot/fix-junit-reporter-crash branch January 8, 2026 13:42

Copilot started reviewing on behalf of thomhurst January 8, 2026 13:42 View session

Copilot AI reviewed Jan 8, 2026

View reviewed changes

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

Uh oh!

Fix JUnit reporter crash on invalid XML characters in test names and exceptions #4262

Fix JUnit reporter crash on invalid XML characters in test names and exceptions #4262

Uh oh!

Copilot AI commented Jan 8, 2026 •

edited

Loading

Uh oh!

Uh oh!

Copilot AI left a comment

Uh oh!

Copilot AI Jan 8, 2026

Uh oh!

Copilot AI Jan 8, 2026

Uh oh!

Copilot AI Jan 8, 2026

Uh oh!

Copilot AI Jan 8, 2026

Uh oh!

Copilot AI Jan 8, 2026

Uh oh!

Copilot AI Jan 8, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

-        foreach (var ch in value!)
-        {
-            // Check if character is valid according to XML 1.0 spec
-            if (ch == 0x9 || ch == 0xA || ch == 0xD ||
-                (ch >= 0x20 && ch <= 0xD7FF) ||
-                (ch >= 0xE000 && ch <= 0xFFFD))
-            {
-                builder.Append(ch);
-            }
-            else
-            {
-                // Replace invalid character with its hex representation
-                builder.Append($"[0x{((int)ch):X}]");
-            }
+        var length = value!.Length;
+        for (var i = 0; i < length; i++)
+        {
+            var ch = value[i];
+            int codePoint;
+            var charsConsumed = 1;
+            // Handle surrogate pairs to correctly process characters in the range U+10000 to U+10FFFF
+            if (char.IsHighSurrogate(ch) && i + 1 < length && char.IsLowSurrogate(value[i + 1]))
+            {
+                codePoint = char.ConvertToUtf32(ch, value[i + 1]);
+                charsConsumed = 2;
+            }
+            else
+            {
+                codePoint = ch;
+            }
+            // Check if character is valid according to XML 1.0 spec
+            var isValid =
+                codePoint == 0x9 ||
+                codePoint == 0xA ||
+                codePoint == 0xD ||
+                (codePoint >= 0x20 && codePoint <= 0xD7FF) ||
+                (codePoint >= 0xE000 && codePoint <= 0xFFFD) ||
+                (codePoint >= 0x10000 && codePoint <= 0x10FFFF);
+            if (isValid)
+            {
+                if (charsConsumed == 1)
+                {
+                    builder.Append((char)codePoint);
+                }
+                else
+                {
+                    // Append the original surrogate pair
+                    builder.Append(ch);
+                    builder.Append(value[i + 1]);
+                }
+            }
+            else
+            {
+                // Replace invalid character (or unpaired surrogate) with its hex representation
+                builder.Append($"[0x{codePoint:X}]");
+            }
+            if (charsConsumed == 2)
+            {
+                i++;
+            }

		var builder = new StringBuilder(value!.Length);
		foreach (var ch in value!)


		namespace TUnit.TestProject;

		public class JUnitReporterInvalidXmlCharacterTests

Uh oh!

Fix JUnit reporter crash on invalid XML characters in test names and exceptions #4262

Fix JUnit reporter crash on invalid XML characters in test names and exceptions #4262

Uh oh!

Conversation

Copilot AI commented Jan 8, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Changes

Example

Expected Behavior

Actual Behavior

Steps to Reproduce

TUnit Version

.NET Version

Operating System

IDE / Test Runner

Error Output / Stack Trace

Add...

Uh oh!

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Reviewed changes

Uh oh!

Copilot AI Jan 8, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 8, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 8, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 8, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 8, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 8, 2026

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

Copilot AI commented Jan 8, 2026 •

edited

Loading