Use
FileSystemWatcher
Here is a simple code example watching for created file:
public void Watcher()
{
    //Create a new FileSystemWatcher.
    FileSystemWatcher watcher = new FileSystemWatcher();
    //Set the filter to only catch TXT files.
    watcher.Filter = "*.txt";
    //Subscribe to the Created event.
    //Created Occurs when a file or directory in the specified Path is created.
    //You can change this based on what you are trying to do.
    watcher.Created += new FileSystemEventHandler(YOUR_EVENT_HANDLER);
    //Set the path to you want to monitor
    watcher.Path = @"C:\PATH\";
    //Enable the FileSystemWatcher events.
    watcher.EnableRaisingEvents = true;
}
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solved How to track changes in a specified folder on c# [closed]