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I have the following interface:

public interface ILogger
    void Log(string message, string title, StatusType type, DateTime timestamp);
    IAsyncResult LogAsync(string message, string title, StatusType type, DateTime timestamp, AsyncCallback callback);
    void EndLogAsync(IAsyncResult result);

Implemented as a file-based logger...


    public FileInfo LogFileInfo { get; internal set; }

    public void Log(string message, string title, StatusType type, DateTime timestamp)
        lock (syncObj)
            CheckLogFileSize(); //rotates LogFileInfo through numbered log files

            using (var stream = LogFileInfo.AppendText())
                stream.WriteLine(GetFormattedString(message, title, type, timestamp));

    public IAsyncResult LogAsync(string message, string title, StatusType type, DateTime timestamp, AsyncCallback callback)
        var action = new Action<string, string, StatusType, DateTime>(Log);
        return action.BeginInvoke(message, title, type, timestamp, callback, action);

    public void EndLogAsync(IAsyncResult result)

Notice the AsyncState parameter of BeginInvoke; I pass the delegate to its own invocation, so that I can reference it again on the flip side to end the invocation using the same delegate reference I called BeginInvoke on.

The questions:

  • Is it even necessary to keep the actual invoked delegate reference around, or can I simply new up another reference to that method in the EndLogAsync method?
  • Is this an acceptable use of the AsyncState, given that I have no other use for it?
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Is there any chance you're writing on .NET 4.5? And why not use existing logging solutions (NLog, log4net)? – almaz Dec 11 '12 at 5:54
I think the implementation as it stands is the "most correct" using delegates/BeginInvoke/EndInvoke. Let it ride. – Jesse C. Slicer Jan 10 '13 at 14:37
I really think that LogAsync should return a Task and therefore not accept a callback delegate. The APIs that consist Begin*/End* method pairs are ancient. By using TPL you can also let the implementers of ILogger interface use the new async features in .NET 4.5 more easily. – Şafak Gür Jan 11 '13 at 14:29

To answer your question:

  • you can access the delegate instance by reading ((AsyncResult)asyncResult).AsyncDelegate property (see Executing a Callback Method When an Asynchronous Call Completes section of Calling Synchronous Methods Asynchronously). But it will be easier to cache the instance of the delegate in a readonly field and use it in both methods.
  • yes, you can use AsyncState however you want, and sometimes people use it for passing the delegate being invoked

Note that you're using a separate thread (on a threadpool) each time you write something to log, so there will be contention between multiple threads if you write to log quickly enough. I would recommend using existing logging frameworks like NLog or log4net to avoid inventing a wheel.

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