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A major focus of this release was to find the small (and not so small) bugs of the decompiler via a revamp of our testing by adding two new tools that allow mass-testing (for more information see TestTools/README.md):

nugetfuzz: downloads packages from nuget.org and runs them through a DEBUG build of ilspycmd (sanity checks/assertions enabled). It records: crash, assertion fail, warnings in the decompiler output (as comment "IL_<nnnn>: ...") as well as any residue of compiler-generated identifiers in C# code (hints at missing language feature/pattern support).
decompdiff: runs an assembly through two different versions of the decompiler (driven via ilspycmd) and calculates a diff of the generated source code.

You should see better decompilation results as an effect of this push and the general push of fixing previously reported decompiler bugs by users.

New Language Features

PR #4014: Emit C# 7.1 default literals where the context supplies the type
PR #3930: Decompile C# 14 implicit span conversions (first-class span types)
PR #3975: Fix #3624: decompile semi-auto properties with the C# 14 field keyword

User Interface

PR #4039: Fix five Avalonia UI regressions: mouse navigation, derived types, Enter, Delete, Analyze in the Analyzer pane
PR #4056: Fix #4054: Accept drops from sources that do not permit the Move effect
PR #4150: Fix #4146: keep Options section names readable on hover
PR #3998: Fix #3996: edit the display font size in points, not pixels
PR #4154: Fix #3526: show WPF resource entry names unescaped
PR #4139: Scroll a node's new children into view when it is expanded
PR #4129: Reject the name of an existing list in the assembly list prompt
PR #4116: Fix #2093: find a navigation target in a reference assembly too
PR #4117: Empty the panes when the assembly list is cleared
PR #4044: Fix #4040: Compose accessibility and static overlays onto search result icons
PR #4025: Fix package search and navigation follow-ups from #4023
PR #3997: Fix #3993, #3994: main-menu shortcuts and dark-mode hover tooltips
PR #4002: Give dark-theme syntax colors a perceptual contrast floor
PR #3989: Prevent double dark conversion of syntax colors
PR #3988: Keep the editor zoom separate from the configured font size

Contributions

PR #4133: Remove the delegate cache left by a discarded method-group conversion — @dualfroz, thank you!
PR #4134: Recognize a VB exception filter's type test — @dualfroz, thank you!
PR #4053: Handle unresolved value-type call targets — @sonyps5201314, thank you!
PR #4022: Improve floating-point fraction and negative constant reconstruction — @SychicBoy, thank you!
PR #3897: Fix #3894: keep lambda parameter typing consistent — @sailro, thank you!
PR #3888: Fix #3853: infer scoped locals from ref safety — @sailro, thank you!
PR #3887: Model scoped and UnscopedRef lifetimes in the type system — @sailro, thank you!
PR #3980: Keep instance-dependent assignments in constructors — @sailro, thank you!

Performance

PR #4156: Keep the UI responsive while searching and drawing reference nodes
PR #4106: Read the file system once per reference closure, not per reference
PR #4023: Search inside packages without stalling, and navigate to what it finds

Enhancements

PR #4158: Invent a syntax for ldtoken.
PR #4147: Share the lookup and overload-resolution stages
PR #4144: Share one escalation loop across the member-reference builders
PR #4091: Split StackType.O into StackType.Obj+StackType.VT
PR #4113: Remove ITypeReference
PR #4090: Extend InferType() to every ILInstruction
PR #4089: Always pass the type system into InferType().
PR #4085: Pass the compilation to ILInstruction.CheckInvariant
PR #4077: Improvements for type inference
PR #4075: Infer the C# type of ldlen
PR #4064: Expand nested conditional operators back into if-else
PR #4062: Detect local functions renamed by obfuscators
PR #4070: Drop isReferenceType when erasing unknown types
PR #4071: Improve types for variables generated from stack slots
PR #4068: Type inference for switch-expressions
PR #4065: Insert casts in switch expression to ensure a best common type exists
PR #4047: Bound every .resources length against the remaining stream before allocating
PR #4048: Never size bundle entry decompression from the manifest's declared size
PR #4046: Cap LightJson parser nesting depth to prevent stack overflow
PR #4006: Tidy up the resolve-result hierarchy and document the tuple-cast rule
PR #4005: Merge tuple element names during type inference
PR #4008: Make ThisResolveResult an ILVariableResolveResult of the this parameter
PR #3974: Extend UseLambdaSyntax to emit statement lambdas

ILSpyCmd

PR #4101: Fix #3154: accept single-file bundles as ilspycmd input
PR #4063: Accept member IDs without a signature in --navigateto

Documentation

PR #4112: Update Unix/Mac setup guidance
PR #4067: Add code review guidance and require Assisted-by on every agent commit
PR #4051: Require spec references and design sign-off for engine changes
PR #3999: Fix #3664: Move ipynb to Verso and adapt to v11 API surface

Testing / Infrastructure

PR #4161: Rule out non-Framework processes without reading their modules
PR #4131: Make every nugetfuzz finding traceable to the assembly it came from
PR #4128: Report compiler-generated names leaking into decompiled output
PR #4125: September 2026 NuGet Updates
PR #4115: decompdiff: build both sides by default, add --no-build
PR #4104: Fix decompdiff bad diffs
PR #4097: Wait for the resolved dependencies, not for the application to go idle
PR #4088: Widen decompdiff reference probing and prune unfixable misses
PR #4087: Report field naming violations as suggestions
PR #4081: Fix #4079: make stable Roslyn restorable from the solution directory
PR #4083: Synchronize headless UI tests on idle and hit-tested clicks
PR #4078: Run decompdiff against git refs, and build a corpus from nuget's most-downloaded packages
PR #4069: Update NuGet packages; adapt to Dock.Serializer.SystemTextJson 12.1.0.6
PR #4074: Bind each corpus assembly against its own reference-assembly pack
PR #4073: Wait for the click target instead of counting frames
PR #4060: Pin the deconstruction shapes the transform has to keep matching
PR #4050: #2764: Add regression test for duplicate assembly references
PR #4032: Tie the .NET Framework fixture host to the test host's lifetime
PR #4033: Use server GC for the decompiler test suite (workstation GC on the shared Windows CI runner)
PR #4035: Render a frame while the headless UI tests wait
PR #4029: Walk the whole decompiler pipeline in the Debug Steps pane
PR #4013: Add ILAst output to ilspycmd debug builds
PR #4012: Stop ILSpy.Tests from retaining every test's app graph (15 GB -> 0.7 GB)
PR #4020: Add TestTools with the nugetfuzz and decompdiff corpus tools
PR #4009: Aim the tree-gesture pointer tests at a hit-testable row
PR #4007: Fix pre-commit hook failure when package source mapping is active
PR #3991: Cover the field-keyword name collisions with fixtures
PR #3982: Remove UI tests that only restate the code they cover

Bug Fixes

PR #4162: Fix #3316: keep unresolved plain XAML properties plain
PR #4119: Fix #2372: report a field's address loads on their own
PR #4043: Fix #3282: leave out trailing optional arguments in indexer accesses
PR #4132: Fix #3320: detect dynamic await in pre-Roslyn state machines
PR #4130: Look past comment placeholders when matching statement sequences
PR #4127: Ignore nullability when comparing backing field and property type of auto properties
PR #4124: Fix #2823 and add a PDB verification mode to nugetfuzz
PR #4111: Fix #3568: read a record's member order off its generated members
PR #4110: Fix #2930: follow references past the ones a document's assembly names
PR #4109: Fix #2919: keep a settings file that cannot be parsed
PR #4108: Fix the parts of #2253 that still reproduce
PR #4105: Fix #2080: resolve GAC references onto the installed version
PR #4103: #3315, #2253: make exported WPF projects build without hand edits
PR #4102: Fix #3315: make exported WPF projects reproduce their resources
PR #4100: Fix #3008: local function forwarding a display class loses its scope
PR #4098: Fix #2054: repair a chain of type forwarders that cycles
PR #4096: Fix two BAML bugs that make the decompiled XAML unwritable
PR #4057: Fix uncompilable lambda parameter lists
PR #4086: Parenthesize a ref-conditional assignment target
PR #4084: Fix the C# 14 field keyword being printed next to a surviving backing field
PR #4080: Fix #3714: declare a local function in its innermost capture scope
PR #4072: Fix #3704: keep the static type of a dynamic call target
PR #4021: Add an async/await pattern test suite and fix the await operand conversions it found
PR #4024: Avoid using query expressions if there's indications that lambda parameter types cannot be omitted.
PR #3964: Delay exceptions when copy-propagating an address into a StObj target
PR #3920: Fix #1142: recover enum members for out-of-range constant operands
PR #3995: Report a call as unresolvable when nothing matched at all
PR #4000: Fix #3729: recognize value-type ctor call on unresolved declaring type
PR #4001: Print the init-only marker after the set keyword again
PR #3973: Fix the remaining deconstruction issues

For a full list of changes click here.

This release is based on .NET 10.0. Please make sure that you have it installed on your machine beforehand.

Note for Mac users: see https://github.com/icsharpcode/ILSpy/wiki/Build-Artifacts#running-the-macos-artifact because the ILSpy.app is neither signed nor notarized.

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icsharpcode/ILSpy v11.1 ILSpy 11.1 on GitHub