Test run aborts when a .NET Framework test name contains an unpaired surrogate (regression in 18.11) #16512

Description

Since the runner started deserializing test host messages with System.Text.Json, a single test whose display name contains an unpaired UTF-16 surrogate aborts the whole run on .NET Framework:

The active test run was aborted. Reason: The JSON value could not be converted to Microsoft.VisualStudio.TestPlatform.ObjectModel.TestCase. Path: $ | LineNumber: 0 | BytePositionInLine: 1413.

The inner exception is:

System.InvalidOperationException: Cannot read incomplete UTF-16 JSON text as string with missing low surrogate.

testhost.net472.exe serializes with Jsonite (JsonDataSerializer: Using Jsonite serializer in the host diag log) and writes the lone high surrogate as the escape \uD83D. The runner reads the message with System.Text.Json (JsonDataSerializer.Stj.cs, TestRequestSender.OnExecutionMessageReceived), which rejects unpaired surrogates by design. Every result after that point is lost.

Unpaired surrogates are easy to hit without intending to: xUnit v2 truncates long string arguments in theory display names by UTF-16 length, so any emoji in theory data can be cut in half.

Steps to reproduce

Repro.csproj:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFrameworks>net472;net10.0</TargetFrameworks>
    <LangVersion>latest</LangVersion>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.0" />
    <PackageReference Include="xunit" Version="2.9.3" />
    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
  </ItemGroup>
</Project>

Tests.cs:

using Xunit;

public class Tests
{
    // xUnit v2 truncates long string arguments in the display name. The leading "a" makes
    // the cut fall between the two UTF-16 halves of an emoji, leaving a lone high surrogate.
    [Theory]
    [InlineData("a😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀😀")]
    public void LongEmojiArgument(string value) => Assert.NotEmpty(value);

    [Fact]
    public void Other() { }
}

Then dotnet test -f net472.

Expected behavior

Both tests pass, as they do with SDK 10.0.400. At worst, the one affected result is reported with a replaced character; the run should not abort.

Actual behavior

Changing the prefix to "ab", so the truncation falls between two emoji, makes net472 pass on SDK 11 as well.

Microsoft.TestPlatform.CommunicationUtilities.dll references Newtonsoft.Json in 18.7 and System.Text.Json in 18.11, which matches the regression window.

Environment

• Windows 10 (10.0.19045), x64
• .NET SDK 11.0.100-rc.1.26425.128 (vstest 18.11.0-preview-26425-128)
• Microsoft.NET.Test.Sdk 18.0.0, xunit 2.9.3, xunit.runner.visualstudio 3.1.4

🤖 Investigated and drafted with Claude Code