Question

I need a custom event that should raise if a new/existing file in a specific directory was found/created. To check if a new File was created I use the SystemFileWatcher, which works fine. To check if some files exist at program start I wrote some lines and that works to.

I use this class for event args:

public class FileDetectEventArgs : EventArgs
{
    public String Source { get; set; }
    public String Destination { get; set; }
    public String FullName { get; set; }

    public FileDetectEventArgs(String source, String destination, String fullName)
    {
        this.Source = source;
        this.Destination = destination;
        this.FullName = fullName;
    }
}

If the SystemFileWatcher raise a FileCreated Event I use these lines of code:

public void onFileCreated(object sender, FileSystemEventArgs e)
{
    // check if file exist
    if (File.Exists(e.FullPath))
    {
        OnNewFileDetect(new FileDetectEventArgs(source, destination, e.FullPath));  
    }
}

And if a file exist I try to raise the event in that way:

public void checkExistingFiles(String source, String filter)
    {
        DirectoryInfo di = new DirectoryInfo(source);
        FileInfo[] fileInfos = di.GetFiles();
        String fileFilter = filter.Substring(filter.LastIndexOf('.'));       

        foreach (FileInfo fi in fileInfos)
        {
            if (fi.Extension.Equals(fileFilter))
            {
                OnNewFileDetect(new FileDetectEventArgs(source, destination, fi.FullName));                   
            }
        }
    }

Here is the OnNewFileDetect-Event:

protected void OnNewFileDetect(FileDetectEventArgs e)
    {
        if (OnNewFileDetectEvent != null)
        {
            OnNewFileDetectEvent(this, e);
        }
    }

The problem is, if the onFileCreated-Event raises my OnNewFileDetect-Event all works fine. But if the checkExistingFiles found some existing files and try to raise OnNewFileDetect-Event nothing happen. I found out that the OnNewFileDetectEvent-Object is null and so nothing happen. But why it’s not null if the onFileCreated-Event is fired?

Answer 1

But why its not null if the onFileCreated-Event is fired?

This event will be null until something subscribes to the event.

---

On a side note, I’d consider switching to a better pattern for raising the event, as well as using standard C#/.NET naming here. This would be more like:

// The event...
public EventHandler<FileDetectedEventArgs> NewFileDetected;

// Note the naming
protected void OnNewFileDetected(FileDetectedEventArgs e)
{
    // Note this pattern for thread safety...
    EventHandler<FileDetectedEventArgs> handler = this.NewFileDetected; 
    if (handler != null)
    {
        handler(this, e);
    }
}