[0.16] --json files are written with UTF-8 BOM

[martincostello]

Trying to run some benchmarks for an application target .NET 11 using the latest CI build (0.16.0-nightly.20260331.487), trying to read the file contents output via --json in a JavaScript GitHub Action parsing the JSON is failing.

Error: Error: Failed to parse '/home/runner/work/costellobot/costellobot/BenchmarkDotNet.Artifacts/results/MartinCostello.Costellobot.Benchmarks.AppBenchmarks-report-full-compressed.json' as BenchmarkDotNet JSON output. Results must be a JSON file generated with the '--exporters json' option: SyntaxError: Unexpected token '', "{"Title":"... is not valid JSON

This is what the code in my action does:

const json = await fs.promises.readFile(fileName, { encoding: 'utf8' });
return JSON.parse(json);

After some debugging and pasting the content into https://repljs.com/ I spotted this:

Looks like the refactoring in #2970 to remove SimpleJson has changed the default encoding to UTF-8 with a Byte Order Mark.

I'm guessing this is an unintentional change caused by the System.Text.Json defaults?

BenchmarkDotNet/src/BenchmarkDotNet/Serialization/BdnSimpleJsonSerializer.cs

Lines 13 to 28 in 2252a3b

	private static readonly JsonSerializerOptions DefaultOptions = new()
	{
	Converters =
	{
	new SimpleJsonFloatConverter(),
	new SimpleJsonDoubleConverter(),
	},
	// Disable escaping non ASCII chars. https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/character-encoding#serialize-all-characters
	Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
	TypeInfoResolverChain =
	{
	// Use Reflection based serialization/deserialization. Because Summary object contains anonymous type.
	new DefaultJsonTypeInfoResolver(),
	},
	RespectNullableAnnotations = true,
	};

[martincostello]

Actually, I think this was broken by #2958.

CancelableStreamWriter writes with Encoding.UTF8, which writes a BOM by default.

BenchmarkDotNet/src/BenchmarkDotNet/Helpers/CancelableStreamWriter.cs

Line 8 in 2252a3b

public CancelableStreamWriter(Stream stream) : this(stream, Encoding.UTF8, 1024) { }

This also explains why Markdown isn't rendering properly for me in GitHub Actions workflow step summaries either:

With 0.15.8

With 0.16.0-nightly.20260331.487

Hex difference shown in BeyondCompare:

Code before:

using (var stream = new StreamWriter(filePath, append: false))
{
    using (var streamLogger = new StreamLogger(stream))
    {
        ExportToLog(summary, streamLogger);
    }
}

Code now:

using var fileStream = new FileStream(filePath, FileMode.Create, FileAccess.Write, FileShare.Read, 4096, useAsync: true);
using var writer = new CancelableStreamWriter(fileStream);
await ExportAsync(summary, writer, cancellationToken).ConfigureAwait(false);

The default StreamWriter constructor uses a UTF8 with no BOM encoding: source