UiThreadSynchronizationContext.cs

using System.Collections.Concurrent;
using Windows.Win32;
using Windows.Win32.Foundation;

namespace MinimalWebView;

// based on this very good Stephen Toub article: https://devblogs.microsoft.com/pfxteam/await-synchronizationcontext-and-console-apps/
internal sealed class UiThreadSynchronizationContext : SynchronizationContext
{
    private readonly BlockingCollection<KeyValuePair<SendOrPostCallback, object>> m_queue = new();
    private readonly HWND hwnd;

    public UiThreadSynchronizationContext(HWND hwnd) : base()
    {
        this.hwnd = hwnd;
    }

    public override void Post(SendOrPostCallback d, object state)
    {
        m_queue.Add(new KeyValuePair<SendOrPostCallback, object>(d, state));
        PInvoke.PostMessage(hwnd, Program.WM_SYNCHRONIZATIONCONTEXT_WORK_AVAILABLE, 0, 0);
    }

    public override void Send(SendOrPostCallback d, object state)
    {
        m_queue.Add(new KeyValuePair<SendOrPostCallback, object>(d, state));
        PInvoke.SendMessage(hwnd, Program.WM_SYNCHRONIZATIONCONTEXT_WORK_AVAILABLE, 0, 0);
    }

    public void RunAvailableWorkOnCurrentThread()
    {
        while (m_queue.TryTake(out KeyValuePair<SendOrPostCallback, object> workItem))
            workItem.Key(workItem.Value);
    }

    public void Complete() { m_queue.CompleteAdding(); }
}