Implementation

analyzers/rspec/cs/S2445_c#.html

@@ -1,30 +1,35 @@
-<p>Synchronizing on a class field synchronizes not on the field itself, but on the object assigned to it. So synchronizing on a non-<code>final</code>
-field makes it possible for the field’s value to change while a thread is in a block synchronized on the old value. That would allow a second thread,
-synchronized on the new value, to enter the block at the same time.</p>
-<p>The story is very similar for synchronizing on parameters; two different threads running the method in parallel could pass two different object
-instances in to the method as parameters, completely undermining the synchronization.</p>
+<p>Locking on a class field synchronizes not on the field itself, but on the object assigned to it. So locking on a non-<code>readonly</code> field
+makes it possible for the field’s value to change while a thread is in a block locked on the old value. That would allow a second thread, locked on
+the new value, to enter the block at the same time.</p>
+<p>The story is very similar for locking on a local variable or on a new instance; two different threads running the method in parallel would lock on
+two different object instances, completely undermining the synchronization.</p>
 <h2>Noncompliant Code Example</h2>
 <pre>
-private String color = "red";
+private string color = "red";
 
-private void doSomething(){
-  synchronized(color) {  // Noncompliant; lock is actually on object instance "red" referred to by the color variable
+private void DoSomething()
+{
+  lock (color) // Noncompliant; lock is actually on object instance "red" referred to by the color variable
+  {
     //...
     color = "green"; // other threads now allowed into this block
     // ...
   }
-  synchronized(new Object()) { // Noncompliant this is a no-op.
+  lock (new object()) // Noncompliant this is a no-op.
+  {
      // ...
   }
 }
 </pre>
 <h2>Compliant Solution</h2>
 <pre>
-private String color = "red";
-private final Object lockObj = new Object();
+private string color = "red";
+private readonly object lockObj = new object();
 
-private void doSomething(){
-  synchronized(lockObj) {
+private void DoSomething()
+{
+  lock (lockObj)
+  {
     //...
     color = "green";
     // ...

analyzers/rspec/cs/S2445_c#.json

@@ -1,5 +1,5 @@
 {
-  "title": "Blocks should be synchronized on \"private final\" fields",
+  "title": "Blocks should be synchronized on \"private readonly\" fields",
   "type": "BUG",
   "status": "ready",
   "remediation": {

analyzers/src/SonarAnalyzer.CSharp/Rules/SynchronizedFieldAssignment.cs

@@ -24,20 +24,38 @@ namespace SonarAnalyzer.Rules.CSharp;
 public sealed class SynchronizedFieldAssignment : SonarDiagnosticAnalyzer
 {
     private const string DiagnosticId = "S2445";
-    private const string MessageFormat = "FIXME";
 
-    private static readonly DiagnosticDescriptor Rule = DescriptorFactory.Create(DiagnosticId, MessageFormat);
+    private static readonly DiagnosticDescriptor Rule = DescriptorFactory.Create(DiagnosticId, "{0}");
 
     public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => ImmutableArray.Create(Rule);
 
     protected override void Initialize(SonarAnalysisContext context) =>
         context.RegisterNodeAction(c =>
             {
-                var node = c.Node;
-                if (true)
+                var expression = ((LockStatementSyntax)c.Node).Expression;
+                if (expression is ObjectCreationExpressionSyntax
+                    or AnonymousObjectCreationExpressionSyntax
+                    or ArrayCreationExpressionSyntax
+                    or ImplicitArrayCreationExpressionSyntax
+                    or QueryExpressionSyntax)
                 {
-                    c.ReportIssue(Diagnostic.Create(Rule, node.GetLocation()));
+                    c.ReportIssue(Diagnostic.Create(Rule, expression.GetLocation(), "Locking on a new instance is a no-op."));
+                }
+                else if (expression.IsAnyKind(SyntaxKind.StringLiteralExpression, SyntaxKind.InterpolatedStringExpression))
+                {
+                    c.ReportIssue(Diagnostic.Create(Rule, expression.GetLocation(), "Strings can be interned, and should not be used for locking."));
+                }
+                else if (expression is IdentifierNameSyntax
+                    && c.SemanticModel.GetSymbolInfo(expression).Symbol is ILocalSymbol lockedSymbol)
+                {
+                    c.ReportIssue(Diagnostic.Create(Rule, expression.GetLocation(), $"'{lockedSymbol.Name}' is a local variable, and should not be used for locking."));
+                }
+                else if (expression is (IdentifierNameSyntax or MemberAccessExpressionSyntax)
+                    && c.SemanticModel.GetSymbolInfo(expression).Symbol is IFieldSymbol lockedField
+                    && !lockedField.IsReadOnly)
+                {
+                    c.ReportIssue(Diagnostic.Create(Rule, expression.GetLocation(), $"'{lockedField.Name}' is not 'private readonly', and should not be used for locking."));
                 }
             },
-            SyntaxKind.InvocationExpression);
+            SyntaxKind.LockStatement);
 }

analyzers/tests/SonarAnalyzer.UnitTest/Rules/SynchronizedFieldAssignmentTest.cs

@@ -30,4 +30,18 @@ public class SynchronizedFieldAssignmentTest
     [TestMethod]
     public void SynchronizedFieldAssignment_CS() =>
         builder.AddPaths("SynchronizedFieldAssignment.cs").Verify();
+
+    [TestMethod]
+    public void SynchronizedFieldAssignment_CSharp8() =>
+        builder
+            .AddPaths("SynchronizedFieldAssignment.CSharp8.cs")
+            .WithOptions(ParseOptionsHelper.FromCSharp8)
+            .Verify();
+
+    [TestMethod]
+    public void SynchronizedFieldAssignment_CSharp11() =>
+        builder
+            .AddPaths("SynchronizedFieldAssignment.CSharp11.cs")
+            .WithOptions(ParseOptionsHelper.FromCSharp11)
+            .Verify();
 }

analyzers/tests/SonarAnalyzer.UnitTest/TestCases/SynchronizedFieldAssignment.CSharp11.cs

@@ -0,0 +1,21 @@
+using System;
+
+class Test
+{
+    static readonly object staticReadonlyField = null;
+    static object staticReadWriteField = null;
+
+    readonly object readonlyField = null;
+    object readWriteField = null;
+
+    void OnANewInstance()
+    {
+        lock ("""a raw string literal""") { }                    // Noncompliant
+        lock ($"""an interpolated {"raw string literal"}""") { } // Noncompliant
+    }
+
+    void TargetTypedObjectCreation()
+    {
+        lock (new() as Tuple<int>) { }                           // Error [CS8754]
+    }
+}

analyzers/tests/SonarAnalyzer.UnitTest/TestCases/SynchronizedFieldAssignment.CSharp8.cs

@@ -0,0 +1,43 @@
+using System;
+
+class Test
+{
+    static readonly object staticReadonlyField = null;
+    static object staticReadWriteField = null;
+
+    readonly object readonlyField = null;
+    object readWriteField = null;
+
+    Test()
+    {
+        ref object refToReadonlyField = ref readonlyField;
+        lock (refToReadonlyField) { }  // Noncompliant, while the reference is to a readonly field, the reference itself is a local variable and as of C# 7.3 can be ref reassigned
+
+        ref object refToReadWriteField = ref readWriteField;
+        lock (refToReadWriteField) { } // Noncompliant
+    }
+
+    void ReadonlyReferences()
+    {
+        lock (RefReturnReadonlyField(this)) { }
+        lock (RefReturnStaticReadonlyField()) { }
+        lock (StaticRefReturnReadonlyField(this)) { }
+        lock (StaticRefReturnStaticReadonlyField()) { }
+
+        ref readonly object RefReturnReadonlyField(Test instance) => ref instance.readonlyField;
+        ref readonly object RefReturnStaticReadonlyField() => ref Test.staticReadonlyField;
+        static ref readonly object StaticRefReturnReadonlyField(Test instance) => ref instance.readonlyField;
+        static ref readonly object StaticRefReturnStaticReadonlyField() => ref Test.staticReadonlyField;
+    }
+
+    void OnANewInstanceOnStack()
+    {
+        lock (stackalloc int[] { }) { }   // Error [CS0185]
+        lock (stackalloc [] { 1 }) { }    // Error [CS0185]
+    }
+
+    void CoalescingAssignment(object oPar)
+    {
+        lock (oPar ??= readonlyField) { } // FN, null conditional assignment not supported
+    }
+}

analyzers/tests/SonarAnalyzer.UnitTest/TestCases/SynchronizedFieldAssignment.cs

@@ -1,5 +1,151 @@
 using System;
+using System.Security.Cryptography.X509Certificates;
+using System.Linq;
 
-public class Program
+class Test
 {
+    static readonly object staticReadonlyField = null;
+    static object staticReadWriteField = null;
+
+    readonly object readonlyField = null;
+    readonly string readonlyStringField = "a string";
+    object readWriteField = null;
+
+    static object StaticReadonlyProperty => null;
+    object ReadonlyProperty => null;
+
+    static object StaticReadWriteProperty { get; set; }
+    object ReadWriteProperty { get; set; }
+
+    void OnAStaticField()
+    {
+        lock (staticReadonlyField) { }
+        lock (staticReadWriteField) { }                  // Noncompliant {{'staticReadWriteField' is not 'readonly', and should not be used for synchronization.}}
+        //    ^^^^^^^^^^^^^^^^^^^^
+        lock (Test.staticReadonlyField) { }
+        lock (Test.staticReadWriteField) { }             // Noncompliant {{'staticReadWriteField' is not 'readonly', and should not be used for synchronization.}}
+        //    ^^^^^^^^^^^^^^^^^^^^^^^^^
+        lock (AnotherClass.staticReadonlyField) { }
+        lock (AnotherClass.staticReadWriteField) { }     // Noncompliant {{'staticReadWriteField' is not 'readonly', and should not be used for synchronization.}}
+        //    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    }
+
+    void OnAFieldOfSameInstance()
+    {
+        lock (readonlyField) { }
+        lock (readonlyStringField) { }
+        lock (readWriteField) { }                        // Noncompliant {{'readWriteField' is not 'readonly', and should not be used for synchronization.}}
+        lock (this.readonlyField) { }
+        lock (this.readWriteField) { }                   // Noncompliant {{'readWriteField' is not 'readonly', and should not be used for synchronization.}}
+    }
+
+    void OnAFieldOfDifferentInstance()
+    {
+        var anotherInstance = new Test();
+        lock (anotherInstance.readonlyField) { }
+        lock (anotherInstance.readWriteField) { }        // Noncompliant {{'readWriteField' is not 'readonly', and should not be used for synchronization.}}
+        lock (anotherInstance.readonlyField) { }
+    }
+
+    void OnALocalVariable()
+    {
+        object localVarNull = null;
+        lock (localVarNull) { }                          // Noncompliant {{'localVarNull' is a local variable, and should not be used for synchronization.}}
+        object localVarReadonlyField = readonlyField;
+        lock (localVarReadonlyField) { }                 // Noncompliant, while the local variable references a readonly field, the local variable itself can mutate
+        object localVarReadWriteField = readWriteField;
+        lock (localVarReadWriteField) { }                // Noncompliant
+    }
+
+    void OnANewInstance()
+    {
+        lock (new object()) { }                          // Noncompliant {{Synchronizing on a new instance is a no-op.}}
+        lock (new ANamespace.AClass()) { }               // Noncompliant
+        lock (new Test[] { })                            // Noncompliant
+        lock (new[] { readonlyField}) { }                // Noncompliant
+        lock (new Tuple<object>(readonlyField)) { }      // Noncompliant
+        lock (new { })                                   // Noncompliant
+
+        lock ("a string") { }                            // Noncompliant
+        lock ($"an interpolated {"string"}") { }         // Noncompliant
+        lock (1) { }                                     // Error [CS0185]
+        lock ((a: readonlyField, b: readonlyField)) { }  // Error [CS0185]
+
+        lock (new ADelegate(x => x)) { }                 // Noncompliant
+        lock (new Func<int, int>(x => x)) { }            // Noncompliant
+        lock (x => x) { }                                // Error [CS0185]
+        lock ((int?)1) { }                               // Error [CS0185]
+
+        lock (from x in new object[2] select x) { }      // Noncompliant
+    }
+
+    void OnAssignment()
+    {
+        object x;
+        lock (x = readonlyField) { }
+        lock (x = readWriteField) { }                    // FN, assignment not supported
+    }
+
+    void OtherCases(object oPar, bool bPar)
+    {
+        lock (null) { }
+
+        lock (oPar) { }
+
+        lock (this) { }
+
+        lock (SomeMethod()) { }
+        lock (oPar.GetType()) { }
+        lock (typeof(Test)) { }
+        lock (default(Test)) { }
+
+        object SomeMethod() => null;
+
+        lock (StaticReadonlyProperty) { }
+        lock (ReadonlyProperty) { }
+        lock (StaticReadWriteProperty) { }
+        lock (ReadWriteProperty) { }
+
+        lock (bPar ? readWriteField : readonlyField) { }
+
+        lock (oPar ?? readonlyField) { }
+        lock (oPar = readonlyField) { }
+    }
+
+    void ReadWriteReferences()
+    {
+        lock (RefReturnReadWriteField(this)) { }         // FN, the method returns a readwrite ref to a member
+        lock (RefReturnStaticReadonlyField(this)) { }    // FN, the method returns a readwrite ref to a member
+
+        ref object RefReturnReadWriteField(Test instance) => ref instance.readWriteField;
+        ref object RefReturnStaticReadonlyField(Test instance) => ref Test.staticReadWriteField;
+    }
+
+    delegate object ADelegate(object oPar);
+}
+
+class TestExplicitCast
+{
+    readonly object readonlyField = null;
+
+    void Test()
+    {
+        lock ((AnotherClass)readonlyField) { } // Compliant, the cast operator can build
+    }
+}
+
+class AnotherClass
+{
+    public static readonly object staticReadonlyField = null;
+    public static object staticReadWriteField = null;
+
+    public readonly object readonlyField = null;
+    public object readWriteField = null;
+
+    public static explicit operator AnotherClass(Test o) => new AnotherClass();
+}
+
+namespace ANamespace
+{
+    class AClass { }
 }