Raise event from second thread to main thread, without forms

Question

I'm writing a library that is firing events. This library starts a second thread that is connecting to a server and listens for messages (blocking calls, reason for the second thread).

public virtual event LogEventHandler EntryReceived;

protected virtual void ReceiveEntry(ILogEntry entry)
{
    if (EntryReceived != null)
        EntryReceived(this, new LogEventArgs() { Entry = entry });
}

When a message is received from the server, it is firing an event :

ReceiveEntry(entry);

I'd like the end developper not to have to think about the InvokeRequired/Invoke snippet in his event handler. How could I ensure to fire my event on the "parent" thread (which I know is the same than the thread that did instantiate my class) ?

Answer 1

For that purpose have some winforms elements a property called SynchronizingObject. This property is of type ISynchronizeInvoke which has the needed methods to execute the call within the UI thread.

In your code you check this property for nullity and if it is set you use it:

var sync = this.SynchronizingObject;

if (sync != null && sync.InvokeRequired)
    sync.BeginInvoke(new Action(()=> ReceiveEntry(entry), null);
else                        
   ReceiveEntry(entry); 

The user of the library just has to put in a Control or a Form into that property:

private MyLibraryClass _MyLibraryClass;

public Form1()
{
    InitializeComponent();

    _MyLibraryClass = new MyLibraryClass();
    _MyLibraryClass.SynchronizingObject = this;
    _MyLibraryClass.EntryReceived += OnEntryReceived;
}

private void OnEntryReceived(object sender, LogEventArgs e)
{
    myTextBox.Text += e.Entry.Message;
}

Answer 2

If during object Construction you capture the SynchronizationContext you will be able to Send the events on that context if there is one, if there is no context then your class was constructed on a thread where it does not care which thread will be used to raise the event. This is better than ISynchronizeInvoke because SynchronizationContext will work with WinForms, ASP.NET, and WPF where ISynchronizeInvoke only works for WinForms.

C# 6 version

public class Example
{
    private SynchronizationContext _context;

    public Example()
    {
        var existingContext = SynchronizationContext.Current;
        _context = existingContext?.CreateCopy() ?? new SynchronizationContext();
    }


    public virtual event LogEventHandler EntryReceived;

    protected virtual void ReceiveEntry(ILogEntry entry)
    {
        _context.Send(ContextCallback, entry);
    }

    private void ContextCallback(object entry)
    {
        EntryReceived?.Invoke(this, new LogEventArgs() { Entry = (ILogEntry)entry });
    }
}

C# 5 and lower version

public class Example
{
    private SynchronizationContext _context;

    public Example()
    {
        var existingContext = SynchronizationContext.Current;
        _context = existingContext != null ? existingContext.CreateCopy() : new SynchronizationContext();
    }


    public virtual event LogEventHandler EntryReceived;

    protected virtual void ReceiveEntry(ILogEntry entry)
    {
        _context.Send(ContextCallback, entry);
    }

    private void ContextCallback(object entry)
    {
        var temp = EntryReceived;
        if (temp != null)
        {
            temp(this, new LogEventArgs() {Entry = (ILogEntry)entry});
        }
    }
}