Add synchronization context #40

breyed · 2025-11-02T04:16:17Z

Supports posting methods to the synchronization context.
Events by default are forwarded directly to the handler on the same thread. However, they can be configured to be posted (useful for events raised on a background thread or to avoid reentrant access).
When a task method completes, the status is posted on the main thread.
C# async/await uses the synchronization context by default, causing continuations to run on the main thread. .NET code that can run safely on a background thread can opt to do so with ConfigureAwait(false).

This PR eliminates background thread calls into FoxPro and C# continuations, avoiding race conditions and data tearing, and providing a deterministic runtime environment.

* Supports posting methods to the synchronization context. * Events by default are forwarded directly to the handler on the same thread. However, they can be configured to be posted (useful for events raised on a background thread or to avoid reentrant access). * When a task method completes, the status is posted on the main thread. * C# async/await uses the synchronization context by default, causing continuations to run on the main thread. .NET code that can run safely on a background thread can opt to do so with `ConfigureAwait(false)`. This commit eliminates background thread calls into FoxPro and C# continuations, avoiding race conditions and data tearing, and providing a deterministic runtime environment.

Copilot

Pull Request Overview

This PR adds support for synchronizing .NET method calls and events with the FoxPro main thread using a custom synchronization context. This enables posting .NET method invocations to the FoxPro message queue to avoid reentrancy issues in event handlers and handle callbacks from background threads.

Key changes:

Introduced FoxProSynchronizationContext to marshal .NET callbacks to the FoxPro main thread via Windows messages
Added PostMethod and PostStaticMethod functions to queue method invocations instead of executing them immediately
Refactored EventSubscriber to support posting events to the synchronization context and simplified the event subscription model

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 16 comments.

Show a summary per file

File	Description
DotnetBridge/wwDotNetBridge.csproj	Enabled latest C# language version to support modern syntax features
DotnetBridge/wwDotNetBridge.cs	Added synchronization context setup, posting support for method invocations, and reorganized using statements
DotnetBridge/Utilities/FoxProSynchronizationContext.cs	New class implementing SynchronizationContext for FoxPro thread marshaling
DotnetBridge/Utilities/EventSubscriber.cs	Refactored event subscription to support synchronization context posting and simplified event handling
Distribution/wwDotnetBridge.PRG	Added FoxPro-side synchronization context setup, new PostMethod/PostStaticMethod functions, and updated event subscription API

Comments suppressed due to low confidence (4)

DotnetBridge/Utilities/FoxProSynchronizationContext.cs:39

Poor error handling: empty catch block.

                try { postedEvent.handler(postedEvent.state); } catch { }

DotnetBridge/wwDotNetBridge.cs:770

Variable args may be null at this access as suggested by this null check.

                if (args[i] is ComValue)

DotnetBridge/wwDotNetBridge.cs:1038

Variable args may be null at this access as suggested by this null check.

                if (args[i] is ComValue)

DotnetBridge/Utilities/FoxProSynchronizationContext.cs:39

Generic catch clause.

                try { postedEvent.handler(postedEvent.state); } catch { }

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Copilot · 2025-11-02T04:19:28Z

DotnetBridge/wwDotNetBridge.cs

        }

+        /// <summary>
+        /// Sets the current synchronization content to use the FoxPro main thread.


Corrected spelling of 'content' to 'context' in the documentation comment.

Suggested change

/// Sets the current synchronization content to use the FoxPro main thread.

/// Sets the current synchronization context to use the FoxPro main thread.

Copilot · 2025-11-02T04:19:28Z

DotnetBridge/wwDotNetBridge.cs

+            return _synchronizationContext.PostMessageId;
+        }
+
+        public void Dispatch() => _synchronizationContext.Dispatch();


Potential null reference exception. The _synchronizationContext field can be null if SetSynchronizationContext is never called, but Dispatch() dereferences it unconditionally. Add a null check or document that SetSynchronizationContext must be called first.

Suggested change

public void Dispatch() => _synchronizationContext.Dispatch();

public void Dispatch()

{

if (_synchronizationContext == null)

throw new InvalidOperationException("SynchronizationContext has not been set. Call SetSynchronizationContext() before Dispatch().");

_synchronizationContext.Dispatch();

}

Copilot · 2025-11-02T04:19:28Z

DotnetBridge/wwDotNetBridge.cs

+        internal object InvokeStaticMethod_Internal(string TypeName, string method, params object[] args)
+        {
+            if (PostInvokedMethods)
+            {


Potential null reference exception. When PostInvokedMethods is true, _synchronizationContext is accessed but may be null if SetSynchronizationContext was not called. Add a null check or validation.

Suggested change

{

{

if (_synchronizationContext == null)

throw new InvalidOperationException("SynchronizationContext is not set. Call SetSynchronizationContext before using PostInvokedMethods.");

Copilot · 2025-11-02T04:19:29Z

DotnetBridge/wwDotNetBridge.cs

+            if (PostInvokedMethods)
+            {
+                PostMethod_Internal(instance, method, args);
+                return null;


Potential null reference exception in PostMethod_Internal. When PostInvokedMethods is true, the method calls _synchronizationContext.Post which will fail with a null reference if SetSynchronizationContext was not called.

Copilot · 2025-11-02T04:19:29Z

DotnetBridge/Utilities/FoxProSynchronizationContext.cs

+    sealed class FoxProSynchronizationContext(int hwnd, object taskWaiter) : SynchronizationContext
+    {
+        private readonly IntPtr _hwnd = (IntPtr)hwnd;
+        private readonly ConcurrentQueue<(SendOrPostCallback handler, object? state)> _postQueue = [];
+        private readonly dynamic _taskWaiter = taskWaiter;


The _taskWaiter field is declared but never used in the class. Consider removing it or documenting its intended future use.

Suggested change

sealed class FoxProSynchronizationContext(int hwnd, object taskWaiter) : SynchronizationContext

{

private readonly IntPtr _hwnd = (IntPtr)hwnd;

private readonly ConcurrentQueue<(SendOrPostCallback handler, object? state)> _postQueue = [];

private readonly dynamic _taskWaiter = taskWaiter;

sealed class FoxProSynchronizationContext(int hwnd) : SynchronizationContext

{

private readonly IntPtr _hwnd = (IntPtr)hwnd;

private readonly ConcurrentQueue<(SendOrPostCallback handler, object? state)> _postQueue = [];

Copilot · 2025-11-02T04:19:31Z

Distribution/wwDotnetBridge.PRG

-***  loHandler: An object with a method OnEvent(loEventName, loParams).
+***  loHandler: An object with handler methods corresponding to each event to subscribe to.
 ***  lcPrefix: The initial part of the event handler function for each event. Defaults to "On".
+***  llPost: When true, events are posted to the synchronization context. Useful for events raised on a background thread or to avoid reentrency.


Corrected spelling of 'reentrency' to 'reentrancy'.

Suggested change

*** llPost: When true, events are posted to the synchronization context. Useful for events raised on a background thread or to avoid reentrency.

*** llPost: When true, events are posted to the synchronization context. Useful for events raised on a background thread or to avoid reentrancy.

Copilot · 2025-11-02T04:19:31Z

DotnetBridge/Utilities/FoxProSynchronizationContext.cs

+        /// <summary>
+        /// Gets the ID of the Windows message posted when a task is ready to run.
+        /// </summary>
+        public int PostMessageId { get; private set; } = RegisterWindowMessage("FoxProSynchronizationContextDispatch");


Replace this call with a call to managed code if possible.

Copilot · 2025-11-02T04:19:31Z

DotnetBridge/Utilities/FoxProSynchronizationContext.cs

+        public override void Post(SendOrPostCallback d, object? state)
+        {
+            _postQueue.Enqueue((d, state));
+            PostMessage(_hwnd, PostMessageId, IntPtr.Zero, IntPtr.Zero);


Replace this call with a call to managed code if possible.

Copilot · 2025-11-02T04:19:31Z

DotnetBridge/Utilities/FoxProSynchronizationContext.cs

+
+        [DefaultDllImportSearchPaths(DllImportSearchPath.System32)]
+        [DllImport("user32.dll", CharSet = CharSet.Unicode, BestFitMapping = false, ThrowOnUnmappableChar = true)]
+        private static extern int RegisterWindowMessage(string lpString);


Minimise the use of unmanaged code.

Copilot · 2025-11-02T04:19:32Z

DotnetBridge/Utilities/FoxProSynchronizationContext.cs

+
+        [DefaultDllImportSearchPaths(DllImportSearchPath.System32)]
+        [DllImport("user32.dll", SetLastError = true)]
+        private static extern bool PostMessage(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam);


Minimise the use of unmanaged code.

breyed · 2025-11-02T11:53:05Z

Superseded by #42. (The branch name was spelled wrong, and Github closed the PR when it got renamed.}

Copilot AI review requested due to automatic review settings November 2, 2025 04:16

This was referenced Nov 2, 2025

Fix event subscription memory leak #39

Closed

Fix event subscriptions for certain parameter types on .NET 9 #35

Closed

Copilot AI reviewed Nov 2, 2025

View reviewed changes

breyed closed this Nov 2, 2025

breyed deleted the synchronization-contenxt branch November 2, 2025 04:34

	/// Sets the current synchronization content to use the FoxPro main thread.
	/// Sets the current synchronization context to use the FoxPro main thread.

-            {
+            {
+                if (_synchronizationContext == null)
+                    throw new InvalidOperationException("SynchronizationContext is not set. Call SetSynchronizationContext before using PostInvokedMethods.");

	*** llPost: When true, events are posted to the synchronization context. Useful for events raised on a background thread or to avoid reentrency.
	*** llPost: When true, events are posted to the synchronization context. Useful for events raised on a background thread or to avoid reentrancy.

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Add synchronization context #40

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breyed commented Nov 2, 2025

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breyed commented Nov 2, 2025

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