C#: Remove expanded assignments.

[michaelnebel]

This PR has two goals, primarily to avoid making multiple upgrade/downgrade scripts:

• Remove expanded assignments. This aligns our implementation with Java and should make adoption of the shared CFG library easier. It also simplifies expression handling and avoids caching a large predicate. Previously, when the extractor encountered a += b, it would synthesize a = a + b.
• Swap the left- and right-hand-side child indices for assignments. This lets us remove several hacks/rotations currently used in the code.

Review on a commit-by-commit basis is encouraged.

Implementation notes

• The implementation now “correctly” classifies a += b (e.g., when a and b are int) as an operator call, since it implicitly invokes the user-defined static operator on Int32.
• The only compound assignment that does not call an operator is ??= as the null-coalescing operator is a built-in short-circuit like operation.
• Added a new set of classes to represent expressions involving a specific operation (e.g., AddOperation, which can represent either a + b or a += b).

DCA notes

• Performance appears unchanged.
• The missing result for cs/unsynchronized-getter is due to removal of a false positive: the query did not properly account for expanded assignments.
• The missing result for cs/web/xss looks acceptable: one alert is removed, but it is essentially a shorter path to the same underlying alert (which is here)
• The new results for cs/useless-upcast look acceptable.
• Remaining differences appear consistent with normal DCA background noise.

[michaelnebel]

@hvitved : I would really appreciate a review of this PR even though it is still in draft mode. The PR still needs upgrade/downgrade scripts and more DCA runs.

[aschackmull]

Spurious "or"

Suggested change

	* (`AssignArithmeticOperation`), a bitwise assignment operation or
	* (`AssignArithmeticOperation`), a bitwise assignment operation

[michaelnebel]

@aschackmull @hvitved : This PR is now ready for review.

• The review comments have been addressed.
• Upgrade/downgrade scripts have been added. I have decided to only "claim" partial correctness for these scripts. The primary focus has been to ensure that the expressions and parent/child relation are correct - there might be dangling/missing references in other relations. I have done some minor testing locally.
  • The upgrade script contracts the parent/child relation for expressions and deletes the synthetic (expanded assignment related) expressions.
  • The downgrade script re-introduce the synthetic expanded assignment, by adding new expressions entities and adjust the parent/child relation accordingly.
• DCA identified a performance issue in the cs/coalesce-of-identical-expressions, which has been fixed.

[Copilot]

Pull request overview

This PR removes extractor-synthesized expanded forms of compound assignments and rotates assignment child indices, introducing new “operation” classes (e.g., AddOperation, NullCoalescingOperation) to represent both regular and compound assignment operator forms consistently.

Changes:

• Stop synthesizing expanded assignments (e.g., a += b → a = a + b) and update CFG/dataflow expectations accordingly.
• Swap assignment child indices (LHS/RHS) and update extractor + QL libraries/queries to match.
• Add new operation kinds/classes to unify x OP y and x OP= y modeling (plus ??/??= via NullCoalescingOperation).

Reviewed changes

Copilot reviewed 99 out of 99 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
csharp/ql/test/query-tests/WriteOnlyContainer/WriteOnlyContainer.cs	Adds coverage for ??= returning a non-write-only container.
csharp/ql/test/query-tests/Language Abuse/UselessNullCoalescingExpression/UselessNullCoalescingExpression.cs	Adds ??= cases (bad/good) for the query test.
csharp/ql/test/query-tests/Language Abuse/UselessNullCoalescingExpression/UselessNullCoalescingExpression.expected	Updates expected results to include ??= findings.
csharp/ql/test/query-tests/Dead Code/DeadStoreOfLocal/DeadStoreOfLocal.expected	Updates expected output to reflect compound assignment nodes (... += ...).
csharp/ql/test/query-tests/Concurrency/SynchSetUnsynchGet/SynchSetUnsynchGet.cs	Adds locked getter/setter example using ??=.
csharp/ql/test/library-tests/structuralcomparison/structuralComparison.expected	Updates structural comparison expectations for rotated assignment child ordering.
csharp/ql/test/library-tests/dynamic/PrintAst.expected	Updates AST print expectations to represent dynamic operator calls as OperatorCall.
csharp/ql/test/library-tests/dynamic/DynamicOperatorCall.expected	Updates expectations for compound assignment operator calls on dynamic.
csharp/ql/test/library-tests/dispatch/viableCallable.expected	Updates expected callability results; includes ... += ... viable callable.
csharp/ql/test/library-tests/dispatch/ViableCallable.cs	Adds comment clarifying viable callable for x += 42.
csharp/ql/test/library-tests/dispatch/GetADynamicTarget.expected	Adjusts dynamic target expectations due to line/shape changes.
csharp/ql/test/library-tests/dispatch/CallGraph.expected	Updates call graph expectations after CFG/assignment modeling changes.
csharp/ql/test/library-tests/dispatch/CallContext.expected	Updates call-context expectations after node/line shifts.
csharp/ql/test/library-tests/dataflow/ssa/SsaExplicitDef.expected	Updates SSA expectations for compound assignment definitions.
csharp/ql/test/library-tests/dataflow/ssa/SsaDefElement.expected	Updates SSA def element expectations for compound assignments.
csharp/ql/test/library-tests/dataflow/ssa/DefAdjacentRead.expected	Updates adjacent def/read expectations for compound assignments.
csharp/ql/test/library-tests/dataflow/signanalysis/SignAnalysis.expected	Updates sign analysis expectations to use ... += ... nodes.
csharp/ql/test/library-tests/dataflow/nullcoalescing/nullCoalescingFlow.ql	Adds a new path-problem test query for null-coalescing flows.
csharp/ql/test/library-tests/dataflow/nullcoalescing/nullCoalescingFlow.expected	Adds expected path-graph output for null-coalescing flow tests.
csharp/ql/test/library-tests/dataflow/nullcoalescing/NullCoalescing.cs	Introduces test code covering both ?? and ??= flows.
csharp/ql/test/library-tests/dataflow/modulusanalysis/ModulusAnalysis.expected	Updates modulus analysis expectations for compound assignment operations.
csharp/ql/test/library-tests/dataflow/local/TaintTrackingStep.expected	Updates taint-tracking step expectations around compound assignments.
csharp/ql/test/library-tests/dataflow/local/DataFlowStep.expected	Updates local dataflow step expectations around compound assignments.
csharp/ql/test/library-tests/csharp8/NullCoalescingControlFlow.expected	Updates CFG expectations for ??= as a branching expression.
csharp/ql/test/library-tests/csharp8/NullCoalescingAssignment.ql	Removes dependency on expanded assignment API from the test.
csharp/ql/test/library-tests/csharp8/NullCoalescingAssignment.expected	Updates expected output after removing expanded assignment.
csharp/ql/test/library-tests/csharp11/operators.expected	Fixes class name for unsigned right shift assign expr in expectations.
csharp/ql/test/library-tests/csharp11/PrintAst.expected	Updates AST print expectation for AssignUnsignedRightShiftExpr.
csharp/ql/test/library-tests/controlflow/graph/NodeGraph.expected	Updates CFG node graph expectations after assignment modeling changes.
csharp/ql/test/library-tests/controlflow/graph/EntryElement.expected	Updates entry-element expectations after assignment modeling changes.
csharp/ql/test/library-tests/controlflow/graph/EnclosingCallable.expected	Updates enclosing callable expectations after CFG/assignment changes.
csharp/ql/test/library-tests/controlflow/graph/Condition.expected	Updates condition expectations after CFG/assignment changes.
csharp/ql/test/library-tests/controlflow/graph/BasicBlock.expected	Updates basic block expectations after CFG/assignment changes.
csharp/ql/test/library-tests/assignments/AssignOperationExpanded.ql	Removes a test relying on expanded assignments.
csharp/ql/test/library-tests/assignments/AssignOperationExpanded.expected	Removes expected output for expanded assignment test.
csharp/ql/test/library-tests/arguments/parameterGetArguments.expected	Updates expected argument extraction for compound assignments.
csharp/ql/test/library-tests/arguments/argumentType.expected	Updates expected argument typing results after AST changes.
csharp/ql/test/library-tests/arguments/argumentByParameter.expected	Updates expected parameter-to-argument mapping for compound assigns.
csharp/ql/test/library-tests/arguments/argumentByName.expected	Updates expected named-argument mapping for compound assignments.
csharp/ql/src/experimental/Security Features/CWE-759/HashWithoutSalt.ql	Switches string concat step to AddOperation.
csharp/ql/src/experimental/CWE-918/RequestForgery.qll	Switches string concat step to AddOperation.
csharp/ql/src/Security Features/InsecureRandomness.ql	Updates assignment-operation handling after removal of expanded assignments.
csharp/ql/src/Security Features/CWE-119/LocalUnvalidatedArithmetic.ql	Switches pointer arithmetic matching to AddOperation.
csharp/ql/src/Performance/StringConcatenationInLoop.ql	Removes workaround for expanded assignments.
csharp/ql/src/Likely Bugs/PossibleLossOfPrecision.ql	Generalizes arithmetic conversions to new *Operation classes.
csharp/ql/src/Likely Bugs/DangerousNonShortCircuitLogic.ql	Removes expanded-assignment special casing.
csharp/ql/src/Likely Bugs/Collections/WriteOnlyContainer.ql	Broadens initialization recognition to rotated assignment/decl shapes.
csharp/ql/src/Language Abuse/UselessNullCoalescingExpression.ql	Refactors to use NullCoalescingOperation (covers ?? and ??=).
csharp/ql/src/Dead Code/DeadStoreOfLocal.ql	Treats compound assignment definitions as relevant definitions.
csharp/ql/src/Complexity/ComplexCondition.ql	Attempts to include ??= in logical-operation counting.
csharp/ql/src/Bad Practices/Control-Flow/ConstantCondition.ql	Switches null-check/whitelist logic to NullCoalescingOperation.
csharp/ql/lib/upgrades/e73ca2c93df8aae162f1704edc4817a5cb330529/upgrade.properties	Adds upgrade scripts for new operation kinds + assignment rotation.
csharp/ql/lib/upgrades/e73ca2c93df8aae162f1704edc4817a5cb330529/assignments.ql	Upgrade logic to remove expanded assignment expressions + rotate children.
csharp/ql/lib/semmlecode.csharp.dbscheme	Introduces operation kinds and treats assign-op-call expressions as calls.
csharp/ql/lib/semmle/code/csharp/metrics/Complexity.qll	Counts NullCoalescingOperation as a branching expression.
csharp/ql/lib/semmle/code/csharp/frameworks/system/Xml.qll	Switches bitwise-or operand matching to BitwiseOrOperation.
csharp/ql/lib/semmle/code/csharp/exprs/Operation.qll	Adds new operation classes (AddOperation, NullCoalescingOperation, etc.).
csharp/ql/lib/semmle/code/csharp/exprs/LogicalOperation.qll	Makes NullCoalescingExpr also a NullCoalescingOperation.
csharp/ql/lib/semmle/code/csharp/exprs/Expr.qll	Removes expanded-assignment implicitness; updates local decl child indices.
csharp/ql/lib/semmle/code/csharp/exprs/Dynamic.qll	Simplifies DynamicOperatorCall (behavior via OperatorCall.toString).
csharp/ql/lib/semmle/code/csharp/exprs/Call.qll	Updates OperatorCall to cover assignment-operator calls via dbscheme union.
csharp/ql/lib/semmle/code/csharp/exprs/BitwiseOperation.qll	Connects bitwise expr classes to new *Operation superclasses.
csharp/ql/lib/semmle/code/csharp/exprs/Assignment.qll	Deprecates expanded assignment API; models assign-op expressions as calls.
csharp/ql/lib/semmle/code/csharp/exprs/ArithmeticOperation.qll	Connects arithmetic expr classes to new *Operation superclasses.
csharp/ql/lib/semmle/code/csharp/dispatch/Dispatch.qll	Updates dynamic event-call heuristic for new assignment modeling.
csharp/ql/lib/semmle/code/csharp/dataflow/internal/rangeanalysis/SsaUtils.qll	Switches SSA delta logic to AddOperation/SubOperation.
csharp/ql/lib/semmle/code/csharp/dataflow/internal/rangeanalysis/SignAnalysisSpecific.qll	Aligns expression-node types + supports assign-operation definitions.
csharp/ql/lib/semmle/code/csharp/dataflow/internal/rangeanalysis/RangeUtils.qll	Updates SSA update steps and renames ExprNode operation classes.
csharp/ql/lib/semmle/code/csharp/dataflow/internal/rangeanalysis/ModulusAnalysisSpecific.qll	Switches operation-node classes to new *Operation names.
csharp/ql/lib/semmle/code/csharp/dataflow/internal/TaintTrackingPrivate.qll	Taint propagation now treats AddOperation as the concat step.
csharp/ql/lib/semmle/code/csharp/dataflow/internal/DataFlowPrivate.qll	Uses NullCoalescingOperation for local flow steps / barriers.
csharp/ql/lib/semmle/code/csharp/dataflow/Nullness.qll	Uses NullCoalescingOperation for maybe-null expression modeling.
csharp/ql/lib/semmle/code/csharp/controlflow/internal/Splitting.qll	Uses NullCoalescingOperation in CFG completion splitting.
csharp/ql/lib/semmle/code/csharp/controlflow/internal/ControlFlowGraphImpl.qll	Removes expanded-assignment CFG hack; adapts to new assignment ordering.
csharp/ql/lib/semmle/code/csharp/controlflow/internal/Completion.qll	Switches exception completion triggers to *Operation classes.
csharp/ql/lib/semmle/code/csharp/controlflow/Guards.qll	Updates guard logic to account for *Operation (incl. compound assigns).
csharp/ql/lib/semmle/code/csharp/commons/Strings.qll	Adds implicit-to-string handling for AssignAddExpr string concatenation.
csharp/ql/lib/semmle/code/csharp/Variable.qll	Updates field initializer indexing due to assignment child rotation.
csharp/ql/lib/semmle/code/csharp/Property.qll	Updates property initializer indexing due to assignment child rotation.
csharp/ql/lib/semmle/code/csharp/ExprOrStmtParent.qll	Removes expanded-assignment adjusted parenting; uses rotated expr_parent.
csharp/ql/lib/semmle/code/csharp/Assignable.qll	Adds assignment-operation definitions for correct def/use modeling.
csharp/ql/lib/experimental/code/csharp/Cryptography/NonCryptographicHashes.qll	Uses AddOperation for hash-shape detection logic.
csharp/ql/lib/change-notes/2026-03-26-expanded-assignments.md	Adds change note about removal of expanded compound assignments.
csharp/ql/campaigns/Solorigate/src/ModifiedFnvFunctionDetection.ql	Simplifies XOR detection using BitwiseXorOperation.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Property.cs	Rotates property initializer assignment children to match new ordering.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Field.cs	Rotates field initializer assignment children to match new ordering.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/VariableDeclaration.cs	Rotates local variable decl/access/initializer child indices.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Query.cs	Rotates query range-variable decl child indices.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/ObjectCreation/AnonymousObjectCreation.cs	Rotates anonymous object init assignment children.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Initializer.cs	Rotates initializer assignment children; reorders RHS extraction accordingly.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Assignment.cs	Removes expanded assignment extraction and rotates assignment children.
csharp/downgrades/19b8cc3e2dc768d4cbc03d6e3773b709bbebd036/upgrade.properties	Adds downgrade scripts to re-introduce expanded assignments + rotate back.
csharp/downgrades/19b8cc3e2dc768d4cbc03d6e3773b709bbebd036/assignments.ql	Downgrade logic to synthesize expanded nodes and restore old parenting.

Comments suppressed due to low confidence (7)

csharp/ql/lib/semmlecode.csharp.dbscheme:1

• The dbscheme definition for @assign_op_call_expr is missing a terminating semicolon, which will make the dbscheme invalid. Add ; at the end of the @assign_op_call_expr = ... line (and ensure formatting matches surrounding productions).
  csharp/ql/src/Complexity/ComplexCondition.ql:1
• logicalParent is typed to RelevantOperations, but the transitive closure is invoked with RelevantBinaryOperations op. Since RelevantBinaryOperations is not a subtype of RelevantOperations, this will not type-check (or will fail to traverse as intended). Consider making RelevantBinaryOperations extend RelevantOperations, or change logicalParent/the count to use a single consistent type hierarchy for traversal and counting.
  csharp/ql/src/Complexity/ComplexCondition.ql:1
• logicalParent is typed to RelevantOperations, but the transitive closure is invoked with RelevantBinaryOperations op. Since RelevantBinaryOperations is not a subtype of RelevantOperations, this will not type-check (or will fail to traverse as intended). Consider making RelevantBinaryOperations extend RelevantOperations, or change logicalParent/the count to use a single consistent type hierarchy for traversal and counting.
  csharp/ql/src/Complexity/ComplexCondition.ql:1
• logicalParent is typed to RelevantOperations, but the transitive closure is invoked with RelevantBinaryOperations op. Since RelevantBinaryOperations is not a subtype of RelevantOperations, this will not type-check (or will fail to traverse as intended). Consider making RelevantBinaryOperations extend RelevantOperations, or change logicalParent/the count to use a single consistent type hierarchy for traversal and counting.
  csharp/ql/src/Complexity/ComplexCondition.ql:1
• logicalParent is typed to RelevantOperations, but the transitive closure is invoked with RelevantBinaryOperations op. Since RelevantBinaryOperations is not a subtype of RelevantOperations, this will not type-check (or will fail to traverse as intended). Consider making RelevantBinaryOperations extend RelevantOperations, or change logicalParent/the count to use a single consistent type hierarchy for traversal and counting.
  csharp/ql/src/Security Features/InsecureRandomness.ql:1
• The exists clause uses multiple | separators, which is easy to misread and makes operator precedence less obvious. Refactor into a single-condition form (for example using and plus a parenthesized or block) to reduce the risk of subtle logic mistakes during future edits.
  csharp/ql/lib/upgrades/e73ca2c93df8aae162f1704edc4817a5cb330529/assignments.ql:1
• Correct spelling of 'seperatly' to 'separately'.

[aschackmull]

These could be CompoundAssignmentExpr instead of Expr, right? I don't know if it matters - it probably doesn't.

[michaelnebel]

Yes! I will change it.

[aschackmull]

LGTM