C# 14: User defined compound assignment operators.

csharp/extractor/Semmle.Extraction.CSharp.Util/SymbolExtensions.cs

@@ -1,3 +1,5 @@
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Text.RegularExpressions;
 using Microsoft.CodeAnalysis;
 
@@ -18,114 +20,68 @@ public static string GetName(this ISymbol symbol, bool useMetadataName = false)
             return symbol.CanBeReferencedByName ? name : name.Substring(symbol.Name.LastIndexOf('.') + 1);
         }
 
+        private static readonly ReadOnlyDictionary<string, string> methodToOperator = new(new Dictionary<string, string>
+        {
+            { "op_LogicalNot", "!" },
+            { "op_BitwiseAnd", "&" },
+            { "op_Equality", "==" },
+            { "op_Inequality", "!=" },
+            { "op_UnaryPlus", "+" },
+            { "op_Addition", "+" },
+            { "op_UnaryNegation", "-" },
+            { "op_Subtraction", "-" },
+            { "op_Multiply", "*" },
+            { "op_Multiplication", "*" },
+            { "op_Division", "/" },
+            { "op_Modulus", "%" },
+            { "op_GreaterThan", ">" },
+            { "op_GreaterThanOrEqual", ">=" },
+            { "op_LessThan", "<" },
+            { "op_LessThanOrEqual", "<=" },
+            { "op_Decrement", "--" },
+            { "op_Increment", "++" },
+            { "op_Implicit", "implicit conversion" },
+            { "op_Explicit", "explicit conversion" },
+            { "op_OnesComplement", "~" },
+            { "op_RightShift", ">>" },
+            { "op_UnsignedRightShift", ">>>" },
+            { "op_LeftShift", "<<" },
+            { "op_BitwiseOr", "|" },
+            { "op_ExclusiveOr", "^" },
+            { "op_True", "true" },
+            { "op_False", "false" }
+        });
+
         /// <summary>
         /// Convert an operator method name in to a symbolic name.
         /// A return value indicates whether the conversion succeeded.
         /// </summary>
         public static bool TryGetOperatorSymbol(this ISymbol symbol, out string operatorName)
         {
-            static bool TryGetOperatorSymbolFromName(string methodName, out string operatorName)
+            var methodName = symbol.GetName(useMetadataName: false);
+
+            // Most common use-case.
+            if (methodToOperator.TryGetValue(methodName, out var opName))
             {
-                var success = true;
-                switch (methodName)
-                {
-                    case "op_LogicalNot":
-                        operatorName = "!";
-                        break;
-                    case "op_BitwiseAnd":
-                        operatorName = "&";
-                        break;
-                    case "op_Equality":
-                        operatorName = "==";
-                        break;
-                    case "op_Inequality":
-                        operatorName = "!=";
-                        break;
-                    case "op_UnaryPlus":
-                    case "op_Addition":
-                        operatorName = "+";
-                        break;
-                    case "op_UnaryNegation":
-                    case "op_Subtraction":
-                        operatorName = "-";
-                        break;
-                    case "op_Multiply":
-                        operatorName = "*";
-                        break;
-                    case "op_Division":
-                        operatorName = "/";
-                        break;
-                    case "op_Modulus":
-                        operatorName = "%";
-                        break;
-                    case "op_GreaterThan":
-                        operatorName = ">";
-                        break;
-                    case "op_GreaterThanOrEqual":
-                        operatorName = ">=";
-                        break;
-                    case "op_LessThan":
-                        operatorName = "<";
-                        break;
-                    case "op_LessThanOrEqual":
-                        operatorName = "<=";
-                        break;
-                    case "op_Decrement":
-                        operatorName = "--";
-                        break;
-                    case "op_Increment":
-                        operatorName = "++";
-                        break;
-                    case "op_Implicit":
-                        operatorName = "implicit conversion";
-                        break;
-                    case "op_Explicit":
-                        operatorName = "explicit conversion";
-                        break;
-                    case "op_OnesComplement":
-                        operatorName = "~";
-                        break;
-                    case "op_RightShift":
-                        operatorName = ">>";
-                        break;
-                    case "op_UnsignedRightShift":
-                        operatorName = ">>>";
-                        break;
-                    case "op_LeftShift":
-                        operatorName = "<<";
-                        break;
-                    case "op_BitwiseOr":
-                        operatorName = "|";
-                        break;
-                    case "op_ExclusiveOr":
-                        operatorName = "^";
-                        break;
-                    case "op_True":
-                        operatorName = "true";
-                        break;
-                    case "op_False":
-                        operatorName = "false";
-                        break;
-                    default:
-                        var match = CheckedRegex().Match(methodName);
-                        if (match.Success)
-                        {
-                            TryGetOperatorSymbolFromName($"op_{match.Groups[1]}", out var uncheckedName);
-                            operatorName = $"checked {uncheckedName}";
-                            break;
-                        }
-                        operatorName = methodName;
-                        success = false;
-                        break;
-                }
-                return success;
+                operatorName = opName;
+                return true;
             }
 
-            var methodName = symbol.GetName(useMetadataName: false);
-            return TryGetOperatorSymbolFromName(methodName, out operatorName);
+            // Attempt to parse using a regexp.
+            var match = OperatorRegex().Match(methodName);
+            if (match.Success && methodToOperator.TryGetValue($"op_{match.Groups[2]}", out var rawOperatorName))
+            {
+                var prefix = match.Groups[1].Success ? "checked " : "";
+                var postfix = match.Groups[3].Success ? "=" : "";
+                operatorName = $"{prefix}{rawOperatorName}{postfix}";
+                return true;
+            }
+
+            operatorName = methodName;
+            return false;
         }
 
-        [GeneratedRegex("^op_Checked(.*)$")]
-        private static partial Regex CheckedRegex();
+        [GeneratedRegex("^op_(Checked)?(.*?)(Assignment)?$")]
+        private static partial Regex OperatorRegex();
     }
 }

csharp/extractor/Semmle.Extraction.CSharp/Entities/Expression.cs

@@ -228,6 +228,41 @@ type.SpecialType is SpecialType.System_IntPtr ||
             return Literal.CreateGenerated(cx, parent, childIndex, type, defaultValue, location);
         }
 
+        /// <summary>
+        /// Given an expression syntax node, attempt to resolve the target method symbol for it.
+        /// The operation takes extension methods into account.
+        /// </summary>
+        /// <param name="node">The expression syntax node.</param>
+        /// <returns>Returns the target method symbol, or null if it cannot be resolved.</returns>
+        protected IMethodSymbol? GetTargetSymbol(ExpressionSyntax node)
+        {
+            var si = Context.GetSymbolInfo(node);
+            if (si.Symbol is ISymbol symbol)
+            {
+                var method = symbol as IMethodSymbol;
+                // Case for compiler-generated extension methods.
+                return method?.TryGetExtensionMethod() ?? method;
+            }
+
+            if (si.CandidateReason == CandidateReason.OverloadResolutionFailure && node is InvocationExpressionSyntax syntax)
+            {
+                // This seems to be a bug in Roslyn
+                // For some reason, typeof(X).InvokeMember(...) fails to resolve the correct
+                // InvokeMember() method, even though the number of parameters clearly identifies the correct method
+
+                var candidates = si.CandidateSymbols
+                    .OfType<IMethodSymbol>()
+                    .Where(method => method.Parameters.Length >= syntax.ArgumentList.Arguments.Count)
+                    .Where(method => method.Parameters.Count(p => !p.HasExplicitDefaultValue) <= syntax.ArgumentList.Arguments.Count);
+
+                return Context.ExtractionContext.IsStandalone ?
+                    candidates.FirstOrDefault() :
+                    candidates.SingleOrDefault();
+            }
+
+            return si.Symbol as IMethodSymbol;
+        }
+
         /// <summary>
         /// Adapt the operator kind depending on whether it's a dynamic call or a user-operator call.
         /// </summary>
@@ -244,10 +279,10 @@ public static ExprKind UnaryOperatorKind(Context cx, ExprKind originalKind, Expr
         /// name if available.
         /// </summary>
         /// <param name="node">The expression.</param>
-        public void OperatorCall(TextWriter trapFile, ExpressionSyntax node)
+        public void AddOperatorCall(TextWriter trapFile, ExpressionSyntax node)
         {
-            var @operator = Context.GetSymbolInfo(node);
-            if (@operator.Symbol is IMethodSymbol method)
+            var @operator = GetTargetSymbol(node);
+            if (@operator is IMethodSymbol method)
             {
                 var callType = GetCallType(Context, node);
                 if (callType == CallType.Dynamic)

csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Assignment.cs

@@ -24,10 +24,9 @@ protected override void PopulateExpression(TextWriter trapFile)
         {
             Create(Context, Syntax.Left, this, 0);
             Create(Context, Syntax.Right, this, 1);
-
             if (Kind != ExprKind.SIMPLE_ASSIGN && Kind != ExprKind.ASSIGN_COALESCE)
             {
-                OperatorCall(trapFile, Syntax);
+                AddOperatorCall(trapFile, Syntax);
             }
         }
 

csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Binary.cs

@@ -40,7 +40,7 @@ private void CreateDeferred(Context cx, ExpressionSyntax node, int child)
 
         protected override void PopulateExpression(TextWriter trapFile)
         {
-            OperatorCall(trapFile, Syntax);
+            AddOperatorCall(trapFile, Syntax);
             CreateDeferred(Context, Syntax.Left, 0);
             CreateDeferred(Context, Syntax.Right, 1);
         }

csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Cast.cs

@@ -25,7 +25,7 @@ protected override void PopulateExpression(TextWriter trapFile)
             else
             {
                 // Type conversion
-                OperatorCall(trapFile, Syntax);
+                AddOperatorCall(trapFile, Syntax);
                 TypeMention.Create(Context, Syntax.Type, this, Type);
             }
         }

csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Invocation.cs

@@ -44,7 +44,7 @@ protected override void PopulateExpression(TextWriter trapFile)
 
             var child = -1;
             string? memberName = null;
-            var target = TargetSymbol;
+            var target = GetTargetSymbol(Syntax);
             switch (Syntax.Expression)
             {
                 case MemberAccessExpressionSyntax memberAccess when IsValidMemberAccessKind():
@@ -129,39 +129,6 @@ private static bool IsOperatorLikeCall(ExpressionNodeInfo info)
                 method.TryGetExtensionMethod()?.MethodKind == MethodKind.UserDefinedOperator;
         }
 
-        public IMethodSymbol? TargetSymbol
-        {
-            get
-            {
-                var si = SymbolInfo;
-
-                if (si.Symbol is ISymbol symbol)
-                {
-                    var method = symbol as IMethodSymbol;
-                    // Case for compiler-generated extension methods.
-                    return method?.TryGetExtensionMethod() ?? method;
-                }
-
-                if (si.CandidateReason == CandidateReason.OverloadResolutionFailure)
-                {
-                    // This seems to be a bug in Roslyn
-                    // For some reason, typeof(X).InvokeMember(...) fails to resolve the correct
-                    // InvokeMember() method, even though the number of parameters clearly identifies the correct method
-
-                    var candidates = si.CandidateSymbols
-                        .OfType<IMethodSymbol>()
-                        .Where(method => method.Parameters.Length >= Syntax.ArgumentList.Arguments.Count)
-                        .Where(method => method.Parameters.Count(p => !p.HasExplicitDefaultValue) <= Syntax.ArgumentList.Arguments.Count);
-
-                    return Context.ExtractionContext.IsStandalone ?
-                        candidates.FirstOrDefault() :
-                        candidates.SingleOrDefault();
-                }
-
-                return si.Symbol as IMethodSymbol;
-            }
-        }
-
         private static bool IsDelegateLikeCall(ExpressionNodeInfo info)
         {
             return IsDelegateLikeCall(info, symbol => IsFunctionPointer(symbol) || IsDelegateInvoke(symbol));

csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/PostfixUnary.cs

@@ -25,7 +25,7 @@ protected override void PopulateExpression(TextWriter trapFile)
             if ((operatorKind == ExprKind.POST_INCR || operatorKind == ExprKind.POST_DECR) &&
                 Kind == ExprKind.OPERATOR_INVOCATION)
             {
-                OperatorCall(trapFile, Syntax);
+                AddOperatorCall(trapFile, Syntax);
                 trapFile.mutator_invocation_mode(this, 2);
             }
         }

csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Unary.cs

@@ -24,7 +24,7 @@ public static Unary Create(ExpressionNodeInfo info)
         protected override void PopulateExpression(TextWriter trapFile)
         {
             Create(Context, Syntax.Operand, this, 0);
-            OperatorCall(trapFile, Syntax);
+            AddOperatorCall(trapFile, Syntax);
 
             if ((operatorKind == ExprKind.PRE_INCR || operatorKind == ExprKind.PRE_DECR) &&
                 Kind == ExprKind.OPERATOR_INVOCATION)

csharp/ql/lib/change-notes/2026-03-06-compound-assignment-operations.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* C# 14: Added support for user-defined compound assignment operators.

csharp/ql/lib/semmle/code/csharp/Assignable.qll

@@ -315,7 +315,8 @@ module AssignableInternal {
       TAddressOfDefinition(AddressOfExpr aoe) or
       TPatternDefinition(TopLevelPatternDecl tlpd) or
       TAssignOperationDefinition(AssignOperation ao) {
-        ao instanceof AssignCallOperation or
+        ao instanceof AssignCallOperation and not ao instanceof CompoundAssignmentOperatorCall
+        or
         ao instanceof AssignCoalesceExpr
       }