Fix: Detect illegal ContinueWith usage in orchestrators using AsyncLocal

src/WebJobs.Extensions.DurableTask/ContextImplementations/DurableOrchestrationContext.cs

@@ -30,6 +30,20 @@ internal class DurableOrchestrationContext : DurableCommonContext, IDurableOrche
         DurableOrchestrationContextBase // for v1 legacy compatibility.
 #pragma warning restore 618
     {
+        /// <summary>
+        /// Error message for multithreaded execution detection.
+        /// </summary>
+        private const string MultithreadedAccessErrorMessage =
+            "Multithreaded execution was detected. This can happen if the orchestrator function code awaits on a task that was not created by a DurableOrchestrationContext method. More details can be found in this article https://learn.microsoft.com/azure/azure-functions/durable/durable-functions-code-constraints.";
+
+        /// <summary>
+        /// AsyncLocal to track the current orchestration context. This is used in addition to
+        /// OrchestrationContext.IsOrchestratorThread to detect illegal async usage patterns
+        /// like calling activities from within ContinueWith blocks. AsyncLocal properly flows
+        /// with async context, unlike [ThreadStatic] which doesn't.
+        /// </summary>
+        private static readonly AsyncLocal<DurableOrchestrationContext> CurrentContext = new AsyncLocal<DurableOrchestrationContext>();
+
         private readonly Dictionary<string, IEventTaskCompletionSource> pendingExternalEvents =
             new Dictionary<string, IEventTaskCompletionSource>(StringComparer.OrdinalIgnoreCase);
 
@@ -1201,8 +1215,43 @@ internal void ThrowIfInvalidAccess()
 
             if (!OrchestrationContext.IsOrchestratorThread)
             {
-                throw new InvalidOperationException(
-                    "Multithreaded execution was detected. This can happen if the orchestrator function code awaits on a task that was not created by a DurableOrchestrationContext method. More details can be found in this article https://learn.microsoft.com/azure/azure-functions/durable/durable-functions-code-constraints.");
+                throw new InvalidOperationException(MultithreadedAccessErrorMessage);
+            }
+
+            // Additional check using AsyncLocal to detect illegal access from ContinueWith blocks.
+            // The AsyncLocal value properly flows with async context, unlike the [ThreadStatic]
+            // IsOrchestratorThread which can be incorrectly true if thread pool reuses the same thread.
+            if (CurrentContext.Value != this)
+            {
+                throw new InvalidOperationException(MultithreadedAccessErrorMessage);
+            }
+        }
+
+        /// <summary>
+        /// Sets this context as the current context for the async flow.
+        /// This should be called at the start of orchestration execution.
+        /// </summary>
+        internal void SetAsCurrentContext()
+        {
+            CurrentContext.Value = this;
+        }
+
+        /// <summary>
+        /// Clears the current context from the async flow.
+        /// This should be called at the end of orchestration execution.
+        /// </summary>
+        internal void ClearCurrentContext()
+        {
+            // Only clear the context if it matches this instance.
+            // If it doesn't match, it could indicate a programming error or nested orchestration scenario.
+            // We use Debug.Assert to help detect such issues during development.
+            System.Diagnostics.Debug.Assert(
+                CurrentContext.Value == null || CurrentContext.Value == this,
+                "ClearCurrentContext called when the current context does not match this instance. This may indicate a programming error.");
+
+            if (CurrentContext.Value == this)
+            {
+                CurrentContext.Value = null;
             }
         }
 

src/WebJobs.Extensions.DurableTask/Listener/TaskOrchestrationShim.cs

@@ -138,6 +138,10 @@ private async Task InvokeUserCodeAndHandleResults(
             RegisteredFunctionInfo orchestratorInfo,
             OrchestrationContext innerContext)
         {
+            // Set the current context for the async flow to enable detection of illegal
+            // ContinueWith usage. This uses AsyncLocal which properly flows with async context.
+            this.context.SetAsCurrentContext();
+
             try
             {
                 Task invokeTask = this.FunctionInvocationCallback();
@@ -205,6 +209,9 @@ private async Task InvokeUserCodeAndHandleResults(
             finally
             {
                 this.context.IsCompleted = true;
+
+                // Clear the current context from the async flow
+                this.context.ClearCurrentContext();
             }
         }