C# 14: User defined compound assignment operators. by michaelnebel · Pull Request #21372 · github/codeql

michaelnebel · 2026-02-25T12:52:20Z

In this PR we introduce support for user-defined compound assignment operators.

class A {
    // Compound operator declaration.
    public void operator +=(A x) {
       ...
    }
}

class B {
    public void M(A a1, A a2) {
        a1 += a2; // Compound operator invocation.
    }
}

Compound assignment operator calls are not implicitly assignments, but instead the left hand side of the compound assignment is updated. That is, user-defined compound operator calls behave more like instance method calls than static operator calls.
Had we instead declared

class A {
    public static A operator +(A x1, A x2) {
        ...
    }
}

then a1 += a2 is also valid, but instead it means a1 + a2 and assign the result to a1.

Notes on the implementation

Even though it is claimed that there is some support for dynamic operator calls, I could not produce a running example (and thus haven’t introduced dispatch logic for this case).
There will be a separate PR for the increment/decrement operator.

michaelnebel · 2026-03-10T08:25:19Z

DCA looks good

Performance doesn't appear to be affected.
Fewer extraction errors.

Copilot

Pull request overview

This PR adds C# 14 support for user-defined compound assignment operators (for example a += b where operator += is declared) by extracting them as operator invocations rather than desugaring them into a = a + b, and updates QL dispatch/dataflow plus tests accordingly.

Changes:

Add new extractor handling for compound assignment expressions to emit direct operator-call extraction for user-defined (instance) compound assignment operators.
Extend the C# QL library to represent and dispatch CompoundAssignmentOperatorCall distinctly from ordinary OperatorCall.
Update/extend AST and dataflow library tests and expected outputs for compound assignment operator declarations and invocations (including extension-based operators).

Reviewed changes

Copilot reviewed 19 out of 20 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
csharp/ql/test/library-tests/operators/operators.cs	Adds user-defined compound assignment operator declarations and invocations for AST coverage.
csharp/ql/test/library-tests/operators/PrintAst.expected	Updates AST expectations to include compound assignment operators/calls.
csharp/ql/test/library-tests/operators/Operators1.expected	Updates operator location expectations after AST shifts.
csharp/ql/test/library-tests/operators/Operators2.expected	Updates operator location expectations after AST shifts.
csharp/ql/test/library-tests/operators/Operators3.expected	Updates conversion-operator location expectations after AST shifts.
csharp/ql/test/library-tests/operators/Operators4.expected	Updates conversion-operator location expectations after AST shifts.
csharp/ql/test/library-tests/dataflow/operators/Operator.cs	Adds a dataflow test for user-defined compound assignment operator calls.
csharp/ql/test/library-tests/dataflow/operators/operatorFlow.expected	Updates expected dataflow graph to reflect compound assignment operator modeling.
csharp/ql/test/library-tests/dataflow/extensions/extensions.cs	Adds an extension-based compound assignment operator and a flow test covering it.
csharp/ql/test/library-tests/dataflow/extensions/ExtensionFlow.expected	Updates expected dataflow output for the new extension compound assignment test.
csharp/ql/lib/semmle/code/csharp/exprs/Call.qll	Introduces `CompoundAssignmentOperatorCall` in the QL expression hierarchy.
csharp/ql/lib/semmle/code/csharp/dispatch/Dispatch.qll	Adjusts dispatch to treat compound assignment operator calls as dispatchable calls with a qualifier.
csharp/ql/lib/semmle/code/csharp/Callable.qll	Adds QL operator classes for compound assignment operator declarations.
csharp/ql/lib/change-notes/2026-03-06-compound-assignment-operations.md	Adds changelog entry for the new C# 14 support.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/UserCompoundAssignmentInvocation.cs	New extractor expression node for user-defined compound assignment operator invocations.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Invocation.cs	Refactors call target resolution to use a shared helper.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Factory.cs	Routes compound assignment syntax kinds through new compound-assignment extraction logic.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/CompoundAssignment.cs	New helper to decide between desugaring vs direct operator-call extraction for compound assignments.
csharp/extractor/Semmle.Extraction.CSharp/CodeAnalysisExtensions/SymbolExtensions.cs	Adds `ExpressionNodeInfo.GetTargetSymbol` helper for consistent target resolution.
csharp/extractor/Semmle.Extraction.CSharp.Util/SymbolExtensions.cs	Extends operator-symbol parsing to recognize checked and assignment operator method names.

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Copilot · 2026-03-10T08:29:53Z

 * Either a unary operator (`UnaryOperator`), a binary operator
- * (`BinaryOperator`), or a conversion operator (`ConversionOperator`).
+ * (`BinaryOperator`), a conversion operator (`ConversionOperator`), or
+ * a (`CompoundAssignmentOperator`).


Minor wording/grammar: “or a (CompoundAssignmentOperator).” reads oddly and is missing the noun. Consider changing to “or a compound assignment operator (CompoundAssignmentOperator).”

Suggested change

* a (`CompoundAssignmentOperator`).

* a compound assignment operator (`CompoundAssignmentOperator`).

Copilot · 2026-03-10T08:29:53Z

+ * class A {
+ *   public void operator+=(A other) {
+ *     ...
+ *   }
+ *
+ *   public A Add(A other) {
+ *     return this += other;
+ *   }


The example in this doc comment appears to be invalid C#: the operator is declared to return void, but the sample then uses return this += other;. Consider adjusting the example to either not return the compound assignment, or to show a (valid) return pattern that matches the operator’s return type.

Copilot · 2026-03-10T08:29:53Z

+            IntVector iv3 = new IntVector(4); // vector of 4 x 0
+            iv3 += iv2;   // iv3 contains 4 x 2
+
+            // The following operations doesn't do anything.


Grammar nit: “The following operations doesn't do anything.” → “The following operations don't do anything.”

Suggested change

// The following operations doesn't do anything.

// The following operations don't do anything.

hvitved

LGTM, some trivial comments.

hvitved · 2026-03-17T12:38:22Z

            return symbol.CanBeReferencedByName ? name : name.Substring(symbol.Name.LastIndexOf('.') + 1);
        }

+        private static readonly Dictionary<string, string> methodToOperator = new Dictionary<string, string>


Nit: Can be a ReadOnlyDictionary.

hvitved · 2026-03-17T12:43:15Z

+ * A user-defined compound assignment operator.
+ *
+ * Either an addition operator (`AddCompoundAssignmentOperator`), a checked addition operator
+ * (`CheckedAddCompoundAssignmentOperator`) a subtraction operator (`SubCompoundAssignmentOperator`), a checked


missing comma

hvitved · 2026-03-17T12:47:11Z

+
+  override Expr getArgument(int i) {
+    exists(int j | result = this.getChildExpr(j) |
+      if this.isOrdinaryStaticCall() then (j = i and j >= 0) else (j = i - 1 and j >= -1)


Alternatively i >=0 and if this.isOrdinaryStaticCall() then j = i else j = i - 1

michaelnebel · 2026-04-15T08:31:14Z

Putting this back in Draft as there has been significant changes in main (removal of expanded assignments).

…orSymbol method.

…ht operator symbol (operators can also be declared in extensions).

…ssignable definition).

github-actions Bot added the C# label Feb 25, 2026

michaelnebel force-pushed the csharp14/usercompoundassignment branch 3 times, most recently from 07345fe to a9a8936 Compare March 6, 2026 08:35

github-actions Bot added the documentation label Mar 6, 2026

michaelnebel force-pushed the csharp14/usercompoundassignment branch 3 times, most recently from 76f65d5 to fafe360 Compare March 9, 2026 15:34

michaelnebel marked this pull request as ready for review March 10, 2026 08:25

michaelnebel requested a review from a team as a code owner March 10, 2026 08:25

Copilot AI review requested due to automatic review settings March 10, 2026 08:25

Copilot started reviewing on behalf of michaelnebel March 10, 2026 08:26 View session

Copilot AI reviewed Mar 10, 2026

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hvitved reviewed Mar 17, 2026

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michaelnebel marked this pull request as draft April 15, 2026 08:31

michaelnebel added 7 commits April 22, 2026 13:03

C#: Add support for compound assignment operators in the TryGetOperat…

ee2ab37

…orSymbol method.

C#: Re-use the GetTargetSymbol logic from invocations to find the rig…

a8f2d3b

…ht operator symbol (operators can also be declared in extensions).

C#: Add change-note.

eca0fed

C#: Add compound assignment operator QL classes.

231336c

C#: Add compound assignment operator call classes.

f030794

C#: Handle compound assignment operators in the dispatch logic (and a…

37e4db5

…ssignable definition).

C#: Add tests and update expected test output.

4018db1

michaelnebel force-pushed the csharp14/usercompoundassignment branch from fafe360 to 4018db1 Compare April 22, 2026 14:03

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C# 14: User defined compound assignment operators.#21372

C# 14: User defined compound assignment operators.#21372
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michaelnebel commented Feb 25, 2026 •

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	* a (`CompoundAssignmentOperator`).
	* a compound assignment operator (`CompoundAssignmentOperator`).

	// The following operations doesn't do anything.
	// The following operations don't do anything.

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michaelnebel commented Feb 25, 2026 •

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