Using C# to check if a file is in use

Question

I saw both of these questions:

Is there a way to check if a file is in use?

And neither of them provided me all of the info that I needed, and I needed more clarification on some of the answers, but the questions were several years old, so I wasn't sure if I should try to get responses from there at this point.

So I posted a new question. Something like

public string myFile;
myFile = @"C:\somepath\somefile.pdf";

if (myFile isinuseorwhatever)
{
     MessageBox.Show("File is in use!! Close it and try again");
     return;
}
else
{
     MessageBox.Show("That worked. Good job!")
     //Do Stuff and lots of lines of stuff.
}

I can do this using exception handling, but the issue is that I need to do the check before I run the many lines of code.

I have a feeling that I need to make a class to check it, then run that class. To be honest, I'm pretty new to coding so I'm not 100% clear on how classes work.

I know about using try and catch, but that won't work here because the exception is happening in the last few lines of code in the try block, so all of that stuff will happen before it hits the exception. Like, this program copies a file, renames it, moves it into a different directory, and then deletes the original, which is the last thing it does. If a user has the file open, it will do all of that but then throw an exception when it tries to delete. I need it to throw the exception BEFORE it copies, renames, moves and so on.

You're fighting too hard. What you should do is try to rename the file. Catch any exception, which means the file is most likely in use. If no exception, move the file (move means there is no original to delete, because it's been moved - you only need to delete if you copy). Doing it in that sequence means that any of the solutions in the posts you've linked will work for you. — Ken White, Jan 11 '19 at 18:41

Code Pope · Accepted Answer · 2019-01-11T20:02:18.177

You can get lock a file for exclusive access by using FileStream with FileShare.None. So if we want to implement the first requests mentioned i.e.

check if the file is in use, if yes, inform the user
if not lock it, so no one opens it
copy the file
rename the file

You can implement the following code:

try
{

    using (Stream stream = new FileStream("1.docx", FileMode.Open, FileAccess.ReadWrite, FileShare.None))
    {
        // Here you can copy your file
        // then rename the copied file
    }
}
catch (Exception ex)
{
    MessageBox.Show("File is in use!! Close it and try again");
    return;
}

I think you know how to copy and rename the file (if not comment it and I will add the code here). The second part of your question is a little bit tricky. Because you cannot use Filestream to delete a file. A soon as you dispose the filestream to call File.Delete("YourFile.doc") it is possible that some accesses it exactly in that moment. I would recommend that you truncate the file when it is locked, so that it unusable for other users. You can also keep your process in a loop until the file is released. The code would look like that:

try
{
    using (Stream stream = new FileStream("1.docx", FileMode.Open, FileAccess.ReadWrite, FileShare.None))
    {
        // Here you can copy your file
        // then rename the copied file
        using (StreamWriter writer = new StreamWriter(stream, Encoding.Unicode))
        {
            writer.Write(""); // truncate the file, making it unusable to others
        }
    }
    while (true)
    {
        try
        {
            File.Delete("1.docx");
        }
        catch 
        {
        }
    }
}
catch (Exception ex)
{
    MessageBox.Show("File is in use!! Close it and try again");
    return;
}

score 2 · Answer 2 · answered Jan 11 '19 at 20:35

Current @code-pope accepted answer includes within a try-catch too much code, it consider every exception happened as "the file is in use", while actually an exception can rise also for other reasons

I would do something like this:

string path = "some path";
FileStream fs = null; 
try
{
    fs = new FileStream("1.docx", FileMode.Open, FileAccess.ReadWrite, FileShare.None);
}
catch(Exception ex)
{
    //ops, cannot open file, better not keep on
    MessageBox.Show("File is in use!! Close it and try again");
    return;
}

// lets go on now...
using (fs)
{
    // do your work
}

In your code the deletion action is not implemented. He also asked for deleting the file. If you want to delete the file, you need another try,catch. — Code Pope, Jan 14 '19 at 05:01

score 1 · Answer 3 · answered Jan 11 '19 at 18:54

1

Use a bool value and set it to false if an exception is thrown when trying to open the file.

Example:

    string path = "Your Path";
    bool available = true;
    try
    {
        using (FileStream fs = File.Open(path, FileMode.Open))
        {

        }
    }
    catch(Exception ex)
    {
        available = false;
    }

answered Jan 11 '19 at 18:54

Dejan

113
1
9

1

This won't work. Once you close the file, some other process could lock, delete, rename, or otherwise make the file unavailable. By the time you get past the `catch` block, there's no guarantee that the value of `available` tells you what the real status of the file is. – Jim Mischel Jan 11 '19 at 19:00

Using C# to check if a file is in use

3 Answers3