Add typed schema for AllocationSampled (EventID 303, .NET 10+) in ClrTraceEventParser

[Copilot]

Fixes #2385.

AllocationSampled (EventID 303), added in .NET 10 via dotnet/runtime#104955, had no entry in ClrTraceEventParser. Because CLR runtime events carry no embedded manifest in EventPipe .nettrace files, the Dynamic parser cannot synthesise a schema — all PayloadByName/PayloadValue calls silently return null or a sentinel exception string, and source.Clr.All never fires for this event.

Schema definition (from ClrEtwAll.man)

Task AllocationSampling (value=40, GUID {CC82530E-A21C-4EB0-A68A-FA4F7E66498F}), EventID 303, opcode 0 (Info), keyword AllocationSamplingKeyword = 0x80000000000.

Field	Type
AllocationKind	win:UInt32 (0=SOH, 1=LOH, 2=POH)
ClrInstanceID	win:UInt16
TypeID	win:Pointer
TypeName	win:UnicodeString
Address	win:Pointer
ObjectSize	win:UInt64
SampledByteOffset	win:UInt64

Changes

• ClrTraceEventParser.Keywords — added AllocationSampling = 0x80000000000
• ClrTraceEventParser — added public event Action AllocationSampling (EventID 303, Task 40, Opcode 0). The property is named AllocationSampling rather than AllocationSampled because opcode=0 (Info) collapses EventName to just the task name, which must match the internal template/event validator
• AllocationSampledTraceData — new sealed class with all 7 typed payload accessors, PayloadNames, PayloadValue, ToXml, Validate, and GetKeywords. Pointer-relative offsets use the HostOffset(10, 1) convention (10 = 6 fixed bytes + 4-byte minimum for TypeID pointer; HostOffset adds the remaining 4 bytes on 64-bit traces)
• EnumerateTemplates — array size 147→148, new entry wired in
• ClrTraceEventParser.cs.base — mirrored all changes

After this change:

source.Clr.AllocationSampled += data =>
{
    // All fields now decode correctly
    Console.WriteLine(data.TypeName);           // e.g. "System.String"
    Console.WriteLine(data.ObjectSize);         // e.g. 104
    Console.WriteLine(data.AllocationKind);     // GCAllocationKind.Small
    Console.WriteLine(data.PayloadValue(0));    // GCAllocationKind.Small (no exception sentinel)
};

Test

Added AllocationSampledEventRoutesAndDecodesPayload in EventPipeParsing.cs: builds a minimal in-memory V6 nettrace containing one AllocationSampled event with a known binary payload and asserts the event dispatches through source.Clr.AllocationSampling, all 7 typed accessors decode correctly, and PayloadByName/PayloadValue round-trip cleanly.

Original prompt

---

This section details on the original issue you should resolve

AllocationSampled (EventID 303, .NET 10+) has no typed schema in ClrTraceEventParser
## Problem

AllocationSampled (EventID 303) was added to the CLR in .NET 10 via dotnet/runtime#104955. ClrTraceEventParser.cs.base has no entry for it — EventID 303 is entirely absent from the parser, which currently covers events up to approximately EventID 190.

Because CLR runtime events do not embed ETW manifest blocks in EventPipe .nettrace files (unlike EventSource events), the Dynamic parser cannot synthesise a schema at runtime. All field accessors fail — and critically, PayloadValue fails silently: it neither throws nor returns null but returns an opaque sentinel string, making it impossible to distinguish a missing schema from a valid value:

source.Dynamic.All += data =>
{
    if (data.ProviderName != "Microsoft-Windows-DotNETRuntime") return;
    if ((int)data.ID != 303) return;

    // All of these yield nothing useful:
    Console.WriteLine(data.PayloadNames?.Length);     // 0  ← empty array, no fields
    Console.WriteLine(data.PayloadByName("TypeName")); // null
    Console.WriteLine(data.PayloadByName("ObjectSize")); // null
    Console.WriteLine(data.PayloadValue(0));           // "<<>>"
};

source.Clr.All never fires for EventID 303 — the event is not routed through ClrTraceEventParser at all.

The event carries the following payload fields (from ClrEtwAll.man):

#	Field	Type
0	AllocationKind	win:UInt32 (0=SOH, 1=LOH, 2=POH)
1	ClrInstanceID	win:UInt16
2	TypeID	win:Pointer
3	TypeName	win:UnicodeString
4	Address	win:Pointer
5	ObjectSize	win:UInt64
6	SampledByteOffset	win:UInt64

---

Expected behaviour

ClrTraceEventParser should expose a typed event handler (e.g. Clr.AllocationSampled) backed by an AllocationSampledTraceData class, analogous to GCAllocationTickTraceData (EventID 10). PayloadNames should return ["AllocationKind", "ClrInstanceID", "TypeID", "TypeName", "Address", "ObjectSize", "SampledByteOffset"] and all PayloadByName/PayloadValue calls should return correctly typed values.

---

Actual behaviour

PayloadNames returns an empty string[] (length 0, not null). PayloadByName(...) returns null. PayloadValue(0) returns the sentinel string <<>> — TraceEvent catches the internal IndexOutOfRangeException and surfaces it as a string rather than rethrowing. The only way to extract any data is raw byte parsing via TraceEvent.EventData() using the binary layout reverse-engineered from the dotnet/runtime reference consumer.

---

Workaround

Parse the raw payload from data.EventData() using the known binary layout. Pointer size is read from data.PointerSize.

AllocationKind(4) + ClrInstanceID(2) + TypeID(ptr) + TypeName(UTF-16LE+\0\0)
    + Address(ptr) + ObjectSize(8) + SampledByteOffset(8)

Full implementation: TryParseAllocationSampled in the repro project.

---

Environment

Microsoft.Diagnostics.Tracing.TraceEvent	3.1.30 (latest)
.NET runtime	10.0+ — event is never emitted on .NET 9 or earlier
Provider	Microsoft-Windows-DotNETRuntime
Keyword	AllocationSamplingKeyword = 0x80000000000

---

Repro

Self-contained .NET 10 console project: https://github.com/baal2000/AllocationSampledRepro

git clone https://github.com/baal2000/AllocationSampledRepro
cd AllocationSampledRepro
dotnet run

The project instruments itself via DiagnosticsClient with AllocationSamplingKeyword, allocates 200 000 objects to guarantee sampled events, then parses the resulting trace twice — once via the native TraceEvent API (broken), once via EventData() raw bytes (workaround).

Actual console output

CI-verified on GitHub-hosted ubuntu-latest, .NET 10.0.5, TraceEvent 3.1.30 — job log:

══ Native TraceEvent API (broken) ═════════════════════════════════════
  PayloadNames:                
  PayloadByName("TypeName"):   
  PayloadByName("ObjectSize"): 
  PayloadValue(0):             <<>>
  source.Clr.All hits for EventID 303:  0  (0 = not routed through ClrTraceEventParser)
  source.Dynamic.All hits for EventID 303: 88

══ Raw EventData() workaround (works) ══════════════════════════════════
  [1] TypeName=InvokeFunc_RefArgs  ObjectSize=64 bytes
  [2] TypeName=System.Reflectio...

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[Brian Robbins (brianrob)]

Copilot, please address these comments.

[Brian Robbins (brianrob)]

Remove the dynamic test and dynamicHandlerHits.

[Copilot]

Done in 7569f1a — removed the Dynamic.All subscription and dynamicHandlerHits variable.

[Brian Robbins (brianrob)]

Undo the changes in this file.

[Copilot]

Done in 7569f1a — reverted ClrTraceEventParser.cs.base to its pre-PR state.

[Brian Robbins (brianrob)]

Suggested change

	/// Events for randomized allocation sampling (AllocationSampled, EventID 303). Requires .NET 10+.
	/// Events for randomized allocation sampling.

[Copilot]

Done in 7569f1a.

[Brian Robbins (brianrob)]

Suggested change

	/// Data for the AllocationSampled event (EventID 303, .NET 10+).
	/// Data for the AllocationSampled event.

[Copilot]

Done in 7569f1a.

[Brian Robbins (brianrob)]

Suggested change

	/// is enabled on the Microsoft-Windows-DotNETRuntime provider.
	/// is enabled.

[Copilot]

Done in 7569f1a.

[Copilot]

Copilot, please address these comments.

All comments addressed in 7569f1a: reverted .base, simplified both doc comments, and removed the Dynamic.All handler + dynamicHandlerHits from the test.

[Brian Robbins (brianrob)]

Copilot, please move the last two words on this line to the line above.

[Copilot]

Done in 7dcdd30 — merged "is enabled." onto the preceding line.