C# 14: User defined compound assignment operators.

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

@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using System.Text.RegularExpressions;
 using Microsoft.CodeAnalysis;
 
@@ -18,114 +19,68 @@ public static string GetName(this ISymbol symbol, bool useMetadataName = false)
             return symbol.CanBeReferencedByName ? name : name.Substring(symbol.Name.LastIndexOf('.') + 1);
         }
 
+        private static readonly Dictionary<string, string> methodToOperator = 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/CodeAnalysisExtensions/SymbolExtensions.cs

@@ -4,6 +4,7 @@
 using System.IO;
 using System.Linq;
 using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Semmle.Util;
 using Semmle.Extraction.CSharp.Entities;
 
@@ -856,5 +857,34 @@ public static Parameter.Kind GetParameterKind(this IParameterSymbol parameter)
                     return Parameter.Kind.None;
             }
         }
+
+        public static IMethodSymbol? GetTargetSymbol(this ExpressionNodeInfo info, Context cx)
+        {
+            var si = info.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 && info.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 cx.ExtractionContext.IsStandalone ?
+                    candidates.FirstOrDefault() :
+                    candidates.SingleOrDefault();
+            }
+
+            return si.Symbol as IMethodSymbol;
+        }
     }
 }

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

@@ -0,0 +1,20 @@
+using Microsoft.CodeAnalysis;
+
+namespace Semmle.Extraction.CSharp.Entities.Expressions
+{
+    internal static class CompoundAssignment
+    {
+        public static Expression Create(ExpressionNodeInfo info)
+        {
+            if (info.SymbolInfo.Symbol is IMethodSymbol op &&
+                op.MethodKind == MethodKind.UserDefinedOperator &&
+                !op.IsStatic)
+            {
+                // This is a user-defined instance operator such as `a += b` where `a` is of a type that defines an `operator +=`.
+                // In this case, we want to extract the operator call rather than desugar it into `a = a + b`.
+                return UserCompoundAssignmentInvocation.Create(info);
+            }
+            return Assignment.Create(info);
+        }
+    }
+}

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

@@ -70,6 +70,9 @@ internal static Expression Create(ExpressionNodeInfo info)
                         return NormalElementAccess.Create(info);
 
                     case SyntaxKind.SimpleAssignmentExpression:
+                    case SyntaxKind.CoalesceAssignmentExpression:
+                        return Assignment.Create(info);
+
                     case SyntaxKind.OrAssignmentExpression:
                     case SyntaxKind.AndAssignmentExpression:
                     case SyntaxKind.SubtractAssignmentExpression:
@@ -81,8 +84,7 @@ internal static Expression Create(ExpressionNodeInfo info)
                     case SyntaxKind.UnsignedRightShiftAssignmentExpression:
                     case SyntaxKind.DivideAssignmentExpression:
                     case SyntaxKind.ModuloAssignmentExpression:
-                    case SyntaxKind.CoalesceAssignmentExpression:
-                        return Assignment.Create(info);
+                        return CompoundAssignment.Create(info);
 
                     case SyntaxKind.ObjectCreationExpression:
                         return ExplicitObjectCreation.Create(info);

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 = info.GetTargetSymbol(Context);
             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/UserCompoundAssignmentInvocation.cs

@@ -0,0 +1,40 @@
+using System.IO;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Semmle.Extraction.Kinds;
+
+namespace Semmle.Extraction.CSharp.Entities.Expressions
+{
+    /// <summary>
+    /// Represents a user-defined compound assignment operator such as `a += b` where `a` is of a type that defines an `operator +=`.
+    /// In this case, we don't want to desugar it into `a = a + b`, but instead extract the operator call directly as it should
+    /// be considered an instance method call on `a` with `b` as an argument.
+    /// </summary>
+    internal class UserCompoundAssignmentInvocation : Expression<AssignmentExpressionSyntax>
+    {
+        private readonly ExpressionNodeInfo info;
+
+        protected UserCompoundAssignmentInvocation(ExpressionNodeInfo info)
+            : base(info.SetKind(ExprKind.OPERATOR_INVOCATION))
+        {
+            this.info = info;
+        }
+
+        public static Expression Create(ExpressionNodeInfo info) => new UserCompoundAssignmentInvocation(info).TryPopulate();
+
+        protected override void PopulateExpression(TextWriter trapFile)
+        {
+            Create(Context, Syntax.Left, this, -1);
+            Create(Context, Syntax.Right, this, 0);
+
+            var target = info.GetTargetSymbol(Context);
+            if (target is null)
+            {
+                Context.ModelError(Syntax, "Unable to resolve target method for user-defined compound assignment operator");
+                return;
+            }
+
+            var targetKey = Method.Create(Context, target);
+            trapFile.expr_call(this, targetKey);
+        }
+    }
+}

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.