Set state of enumerator object to "after" during disposal (#76090)

docs/compilers/CSharp/Compiler Breaking Changes - DotNet 10.md

@@ -89,3 +89,32 @@ class C
     }
 }
 ```
+
+## Set state of enumerator object to "after" during disposal
+
+***Introduced in Visual Studio 2022 version 17.13***
+
+The state machine for enumerators incorrectly allowed resuming execution after the enumerator was disposed.  
+Now, `MoveNext()` on a disposed enumerator properly returns `false` without executing any more user code.
+
+```csharp
+var enumerator = C.GetEnumerator();
+
+Console.Write(enumerator.MoveNext()); // prints True
+Console.Write(enumerator.Current); // prints 1
+
+enumerator.Dispose();
+
+Console.Write(enumerator.MoveNext()); // now prints False
+
+class C
+{
+    public static IEnumerator<int> GetEnumerator()
+    {
+        yield return 1;
+        Console.Write("not executed after disposal")
+        yield return 2;
+    }
+}
+```
+

docs/compilers/Visual Basic/Compiler Breaking Changes - DotNet 10.md

@@ -0,0 +1,37 @@
+# Breaking changes in Roslyn after .NET 9.0.100 through .NET 10.0.100
+
+This document lists known breaking changes in Roslyn after .NET 9 general release (.NET SDK version 9.0.100) through .NET 10 general release (.NET SDK version 10.0.100).
+
+## Set state of enumerator object to "after" during disposal
+
+***Introduced in Visual Studio 2022 version 17.13***
+
+The state machine for enumerators incorrectly allowed resuming execution after the enumerator was disposed.  
+Now, `MoveNext()` on a disposed enumerator properly returns `false` without executing any more user code.
+
+```vb
+Imports System
+Imports System.Collections.Generic
+
+Module Program
+    Sub Main()
+        Dim enumerator = C.GetEnumerator()
+
+        Console.Write(enumerator.MoveNext()) ' prints True
+        Console.Write(enumerator.Current)    ' prints 1
+
+        enumerator.Dispose()
+
+        Console.Write(enumerator.MoveNext()) ' now prints False
+    End Sub
+End Module
+
+Class C
+    Public Shared Iterator Function GetEnumerator() As IEnumerator(Of Integer)
+        Yield 1
+        Console.Write("not executed after disposal")
+        Yield 2
+    End Function
+End Class
+```
+

src/Compilers/CSharp/Portable/Lowering/IteratorRewriter/IteratorMethodToStateMachineRewriter.cs

@@ -163,6 +163,7 @@ internal void GenerateMoveNextAndDispose(BoundStatement body, SynthesizedImpleme
                 // nothing to finalize
                 var disposeBody = F.Block(
                                     GenerateAllHoistedLocalsCleanup(),
+                                    F.Assignment(F.Field(F.This(), stateField), F.Literal(StateMachineState.FinishedState)),
                                     F.Return());
 
                 F.CloseMethod(disposeBody);
@@ -177,6 +178,7 @@ internal void GenerateMoveNextAndDispose(BoundStatement body, SynthesizedImpleme
                                     F.Assignment(F.Local(stateLocal), F.Field(F.This(), stateField)),
                                     EmitFinallyFrame(rootFrame, state),
                                     GenerateAllHoistedLocalsCleanup(),
+                                    F.Assignment(F.Field(F.This(), stateField), F.Literal(StateMachineState.FinishedState)),
                                     F.Return());
 
                 F.CloseMethod(disposeBody);

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncIteratorTests.cs

@@ -10531,5 +10531,31 @@ public static async System.Collections.Generic.IAsyncEnumerable<S2> M(S2 p)
             comp = CreateCompilation(source, options: TestOptions.UnsafeDebugDll, references: [libS2, missingLibS1], targetFramework: TargetFramework.Net80);
             comp.VerifyEmitDiagnostics();
         }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/76078")]
+        public void StateAfterMoveNext_YieldReturn()
+        {
+            string src = """
+var enumerator = C.Produce();
+System.Console.Write(await enumerator.MoveNextAsync());
+System.Console.Write(enumerator.Current);
+
+await enumerator.DisposeAsync();
+
+System.Console.Write(await enumerator.MoveNextAsync());
+System.Console.Write(enumerator.Current is null ? " null" : throw null);
+
+class C
+{
+    public static async System.Collections.Generic.IAsyncEnumerator<string> Produce()
+    {
+        await System.Threading.Tasks.Task.CompletedTask;
+        yield return " one ";
+        yield return " two ";
+    }
+}
+""";
+            CompileAndVerify(src, expectedOutput: ExpectedOutput("True one False null"), verify: Verification.Skipped, targetFramework: TargetFramework.Net80).VerifyDiagnostics();
+        }
     }
 }