lock (ClientLocker)
{
    Trace.WriteLine("#WriteAll: " + sm.Header);
    foreach (Client c in Clients)
    {
        if (c.LoggedIn)
        {
            Trace.WriteLine("#TryWriteTo[" + c.Id + "](" + sm.Header + ")");
            LazyAsync.Invoke(() => c.WriteMessage(sm));
        }
    }
}

public static class LazyAsync
{
    public static void Invoke(Action a)
    {
        a.BeginInvoke(a.EndInvoke, null);
    }
}

lock (ClientLocker)
{
    Trace.WriteLine("#WriteAll: " + sm.Header);
    foreach (Client c in Clients)
    {
        if (c.LoggedIn)
        {
            Client localC = c;
            Trace.WriteLine("#TryWriteTo[" + c.Id + "](" + sm.Header + ")");
            LazyAsync.Invoke(() => localC.WriteMessage(sm));
        }
    }
}

lock (ClientLocker)
{
    Trace.WriteLine("#WriteAll: " + sm.Header);
    foreach (Client c in Clients)
    {
        if (c.LoggedIn)
        {
            Trace.WriteLine("#TryWriteTo[" + c.Id + "](" + sm.Header + ")");

            Client copyOfC = c;
            LazyAsync.Invoke(() => copyOfC.WriteMessage(sm));
        }
    }
}

foreach (Client c in Clients)
{
    if (c.LoggedIn)
    {
        Trace.WriteLine("#TryWriteTo[" + c.Id + "](" + sm.Header + ")");

        Client client = c;
        LazyAsync.Invoke(() => client.WriteMessage(sm));
    }
}