FileSystemWatcher检测到更改后，读取文本文件的最后一行

Question

I am new to c# so please forgive my ignorance, I am running a fileSystemWatcher on a text file. And it is working fine, I can do some simple tasks after the file has changed. All but what I want to do. I am trying to read the last line of the text file that has changed with this code

public void File_Changed( object source, FileSystemEventArgs e )
{
    string MACH1 = File.ReadText(@"C:\MACHINE_1.txt").Last();
    if (MACH1=="SETUP")
    {
        MACHINE1IND.BackColor = Color.Green; 
    }
    else
    {
        MACHINE1IND.BackColor = Color.Red; 
    }
}

It works fine inside a button but not after file watcher. Says it cannot find file?

Answer 1

One thing to be aware of is that the FSW can issue multiple change notifications during a save operation. You have no way of knowing when the save is complete. As a result, you need to always wrap your code in a try..catch block and support retry after a timeout to allow the file write to be completed. Typically, I will try to move the file to a temp location where I will do my processing. If the move fails, wait a couple seconds and try again.

Answer 2

You'll have to check if the file exists before accessing it.

public void File_Changed(object source, FileSystemEventArgs e)
{
    string filePath = @"C:\MACHINE_1.txt";
    if(!File.Exists(filePath)) //Checks if file exists
        return;
    string MACH1 = File.ReadText(filePath).Last();
    if (MACH1=="SETUP")
    {
        MACHINE1IND.BackColor = Color.Green; 
    }
    else
    {
        MACHINE1IND.BackColor = Color.Red; 
    }
}

Answer 3

As Jim Wooley explains in his answer, the file operation might still be in progress, when FSW fires a Created or Changed event. If FSW is used to communicate between two applications and you are in control of the "sending" application as well, you can solve the problem as follows:

Write the information to a temporary file. Close the file. Rename the temporary file and give it a definitive name.

In the other application (the receiver) watch for the Renamed event using the FileSystemWatcher . The renamed file is guaranteed be complete.