[ds5678]

Problem

Decompiling the class generated by this causes an error in the latest version of ILSpy, but did not in earlier versions.

// ILSpy could not decompile this member: Error decompiling @060000A1 UpdateEngineAutoGenerated.UpdateMain0: Value cannot be null. (Parameter 'source')
// Please report the exception below,
// along with the assembly it came from, at https://github.com/icsharpcode/ILSpy/issues/new
// Error decompiling @060000A1 UpdateEngineAutoGenerated.UpdateMain0
// in assembly "N:\StrideProjects\FirstPersonShooter\Bin\Windows\Debug\FirstPersonShooter.Game.dll"
//  ---> System.ArgumentNullException: Value cannot be null. (Parameter 'source')
//    at System.Linq.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument) in offset 11
//    at System.Linq.Enumerable.Take[TSource](IEnumerable`1 source, Int32 count) in offset 3
//    at ICSharpCode.Decompiler.CSharp.ArgumentList.GetArgumentResolveResultsDirect(Int32 skipCount) in CallBuilder.cs:line 128
//    at ICSharpCode.Decompiler.CSharp.Disambiguator.Probe() in Disambiguator.cs:line 493
//    at ICSharpCode.Decompiler.CSharp.Disambiguator.Run() in Disambiguator.cs:line 438
//    at ICSharpCode.Decompiler.CSharp.Disambiguator.ForMethodReference(ExpressionBuilder expressionBuilder, IMethod method, IType targetType, TranslatedExpression target, Boolean requireTarget, ExpectedTargetDetails expectedTargetDetails, Boolean isExtensionMethodReference, Boolean castTargetUpFront) in Disambiguator.cs:line 131
//    at ICSharpCode.Decompiler.CSharp.CallBuilder.DisambiguateDelegateReference(IMethod method, IMethod invokeMethod, ExpectedTargetDetails expectedTargetDetails, ILInstruction thisArg) in CallBuilder.cs:line 1825
//    at ICSharpCode.Decompiler.CSharp.CallBuilder.BuildDelegateReference(IMethod method, IMethod invokeMethod, ExpectedTargetDetails expectedTargetDetails, ILInstruction thisArg) in CallBuilder.cs:line 1755
//    at ICSharpCode.Decompiler.CSharp.CallBuilder.BuildMethodReference(IMethod method, Boolean isVirtual) in CallBuilder.cs:line 1746
//    at ICSharpCode.Decompiler.CSharp.ExpressionBuilder.VisitLdFtn(LdFtn inst, TranslationContext context) in ExpressionBuilder.cs:line 5084
//    at ICSharpCode.Decompiler.IL.LdFtn.AcceptVisitor[C,T](ILVisitor`2 visitor, C context) in Instructions.cs:line 3198
//    at ICSharpCode.Decompiler.CSharp.ExpressionBuilder.Translate(ILInstruction inst, IType typeHint) in ExpressionBuilder.cs:line 173
//    at ICSharpCode.Decompiler.CSharp.CallBuilder.BuildArgumentList(ExpectedTargetDetails expectedTargetDetails, ResolveResult target, IMethod method, Int32 firstParamIndex, IReadOnlyList`1 callArguments, IReadOnlyList`1 argumentToParameterMap, Boolean writtenAsAssignment) in CallBuilder.cs:line 1084
//    at ICSharpCode.Decompiler.CSharp.CallBuilder.Build(OpCode callOpCode, IMethod method, IReadOnlyList`1 callArguments, IReadOnlyList`1 argumentToParameterMap, IType constrainedTo) in CallBuilder.cs:line 464
//    at ICSharpCode.Decompiler.CSharp.CallBuilder.Build(CallInstruction inst, IType typeHint) in CallBuilder.cs:line 258
//    at ICSharpCode.Decompiler.CSharp.ExpressionBuilder.VisitNewObj(NewObj inst, TranslationContext context) in ExpressionBuilder.cs:line 540
//    at ICSharpCode.Decompiler.IL.NewObj.AcceptVisitor[C,T](ILVisitor`2 visitor, C context) in Instructions.cs:line 4612
//    at ICSharpCode.Decompiler.CSharp.ExpressionBuilder.Translate(ILInstruction inst, IType typeHint) in ExpressionBuilder.cs:line 173
//    at ICSharpCode.Decompiler.CSharp.CallBuilder.BuildArgumentList(ExpectedTargetDetails expectedTargetDetails, ResolveResult target, IMethod method, Int32 firstParamIndex, IReadOnlyList`1 callArguments, IReadOnlyList`1 argumentToParameterMap, Boolean writtenAsAssignment) in CallBuilder.cs:line 1084
//    at ICSharpCode.Decompiler.CSharp.CallBuilder.Build(OpCode callOpCode, IMethod method, IReadOnlyList`1 callArguments, IReadOnlyList`1 argumentToParameterMap, IType constrainedTo) in CallBuilder.cs:line 464
//    at ICSharpCode.Decompiler.CSharp.CallBuilder.Build(CallInstruction inst, IType typeHint) in CallBuilder.cs:line 258
//    at ICSharpCode.Decompiler.CSharp.ExpressionBuilder.VisitCall(Call inst, TranslationContext context) in ExpressionBuilder.cs:line 2548
//    at ICSharpCode.Decompiler.IL.Call.AcceptVisitor[C,T](ILVisitor`2 visitor, C context) in Instructions.cs:line 2213
//    at ICSharpCode.Decompiler.CSharp.ExpressionBuilder.Translate(ILInstruction inst, IType typeHint) in ExpressionBuilder.cs:line 173
//    at ICSharpCode.Decompiler.CSharp.StatementBuilder.Default(ILInstruction inst) in StatementBuilder.cs:line 93
//    at ICSharpCode.Decompiler.IL.ILVisitor`1.VisitCall(Call inst) in Instructions.cs:line 7717
//    at ICSharpCode.Decompiler.IL.Call.AcceptVisitor[T](ILVisitor`1 visitor) in Instructions.cs:line 2209
//    at ICSharpCode.Decompiler.CSharp.StatementBuilder.Convert(ILInstruction inst) in StatementBuilder.cs:line 82
//    at ICSharpCode.Decompiler.CSharp.StatementBuilder.ConvertBlockContainer(BlockStatement blockStatement, BlockContainer container, IEnumerable`1 blocks, Boolean isLoop) in StatementBuilder.cs:line 1581
//    at ICSharpCode.Decompiler.CSharp.StatementBuilder.ConvertBlockContainer(BlockContainer container, Boolean isLoop) in StatementBuilder.cs:line 1468
//    at ICSharpCode.Decompiler.CSharp.StatementBuilder.VisitBlockContainer(BlockContainer container) in StatementBuilder.cs:line 1354
//    at ICSharpCode.Decompiler.IL.BlockContainer.AcceptVisitor[T](ILVisitor`1 visitor) in Instructions.cs:line 863
//    at ICSharpCode.Decompiler.CSharp.StatementBuilder.Convert(ILInstruction inst) in StatementBuilder.cs:line 82
//    at ICSharpCode.Decompiler.CSharp.StatementBuilder.ConvertAsBlock(ILInstruction inst) in StatementBuilder.cs:line 87
//    at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.DecompileBody(IMethod method, EntityDeclaration entityDecl, DecompileRun decompileRun, ITypeResolveContext decompilationContext, ExtensionInfo extensionInfo) in CSharpDecompiler.cs:line 2457
// -- continuing with outer exception (ICSharpCode.Decompiler.DecompilerException) --
// 

Solution

• At least one test covering the code changed

The error occurs when IL references an accessor method directly—for example, using ldftn to create a delegate to a property getter.

Previously, Disambiguator.Probe() treated every accessor method as a property or event access, even when it was being used as a method reference. That path expects an argument list, but method references leave it uninitialized, so LINQ throws ArgumentNullException.

The fix distinguishes accessor syntax from accessor metadata. Actual property, indexer, and event accesses still use accessor resolution; method references use method-reference resolution instead. The regression fixture exercises a Func<int> targeting a property getter.

[siegfriedpammer]

Hey Jeremy, thank you for the bugfix... I removed the extra "accessorSyntax" variable since !invocationSyntax + IMethod.IsAccessor should already be enough information. I updated the documentation as well...