[jogibear9988]

A lambda with 65,535 explicit parameters plus FEC's hidden closure parameter exceeds the runtime method-signature limit, but compilation can currently return an unusable delegate.

Reject the excessive count before allocating its signature array. The check is confined to the existing unpooled large-signature branch: zero-argument and small pooled signatures retain their current paths. No new traversal or context metadata.

Validation

Covers early rejection of excessive signatures, including the strict/throw flags, and supported 255/256/300-argument delegates with correct argument indices, compared with System.

• The unfixed master regression fails in both APIs; the same fixture passes after the patch on .NET 8, .NET 10 and .NET Framework 4.7.2. Strict FEC compilation is used.
• Full Release solution build passes with zero warnings/errors.
• Legacy suites pass: 1,756 on .NET 10 and 1,755 on .NET 8.
• Framework runner passes 249 TestX + 1,639 legacy cases (en-US culture for existing message assertions).
• Full modern runners retain only the upstream nullable MultiplyChecked parity failure for (1,1)/(2,1), in both APIs. Unmodified master was rebuilt and checked separately. No assertions were skipped or weakened; legacy suites ran separately because that existing TestX failure gates the normal runner.

Performance checks

Compared with master at f8f2a240abd650dda738162957606aac2659873f using already-working expressions, for both Standard and LightExpression. The broken regression establishes correctness and is not used as a before/after speed baseline. All selected working controls retain identical decoded emitted IL and the same measured compilation/invocation allocation counts.

Measurement setup and full results

Windows ARM64, SDK 10.0.301, .NET 8.0.28/10.0.9, Release. Prebuilt trees; Light conversion outside the timed section; strict CompileFast without System fallback; fixed CPU affinity, tiering disabled, 300 compilation warmups and 100,000 invocation warmups. Two fresh processes with opposite load order, nine alternating samples per side, GC before samples and a 256 MiB no-GC region where available. Table values are geometric means of process medians; compilation uses the longer approximately 100 ms paired-calibration run and invocation uses approximately 24 ms. Framework is correctness-tested, not timed.

Runtime / API / workload	Compile ns, master → fix	Change	Compile B/op, both	Invoke ns, master → fix	Invoke B/op, both
net10.0 / Standard / add_control	4292.4 → 2293.6	-46.6%	872	1.88 → 1.91	0
net10.0 / Standard / reference_constant	1671.9 → 1752.6	+4.8%	984	1.87 → 1.90	0
net10.0 / Standard / arguments_16	8489.6 → 7002.1	-17.5%	1168	2.19 → 2.14	0
net10.0 / Light / add_control	1505.4 → 1296.7	-13.9%	872	1.90 → 1.86	0
net10.0 / Light / reference_constant	1671.0 → 1861.0	+11.4%	984	1.87 → 1.87	0
net10.0 / Light / arguments_16	3185.7 → 3369.5	+5.8%	1168	2.14 → 2.13	0
net8.0 / Standard / add_control	1115.5 → 1121.2	+0.5%	872	2.03 → 1.94	0
net8.0 / Standard / reference_constant	949.2 → 905.4	-4.6%	984	1.90 → 1.93	0
net8.0 / Standard / arguments_16	2104.7 → 2126.9	+1.1%	1168	2.25 → 2.25	0
net8.0 / Light / add_control	951.5 → 939.6	-1.2%	872	1.91 → 1.88	0
net8.0 / Light / reference_constant	903.1 → 900.7	-0.3%	984	1.87 → 1.94	0
net8.0 / Light / arguments_16	2033.9 → 2003.5	-1.5%	1168	2.21 → 2.18	0

The full short run covers add, reference constant, root/nested constructors, a 16-argument delegate, flat field assignment and nested field reads. The longer run focuses on this fix's affected working paths plus unchanged controls. Slower observations are included above. Identical-binary master/master compilation variation in the same setup was net10.0: -11.6% to +10.4%; net8.0: -21.3% to +14.0%. These timings do not prove zero overhead for every input/machine, and no speedup claim is made. Runtime timing differences above are observations of identical emitted control IL.

All-process compilation timing ranges

These ranges include the less stable observations, not just the longer-run aggregate in the table. They are individual process-median changes across working controls; they are not speedup claims.

Runtime	Short-run range	Longer-run range	Unchanged master/master longer-run range
net10.0	-26.8% to +31.2%	-73.3% to +14.5%	-13.9% to +10.7%
net8.0	-14.1% to +34.0%	-8.1% to +4.5%	-29.5% to +25.0%