How Can I Ensure a File Is Fully Written Before Copying It?

Waiting Until a File Is Fully Written

In certain scenarios, it is crucial to ensure that file copying operations commence only when the source file is entirely written. Unfortunately, this can be challenging when dealing with large files, as premature copying attempts can result in the dreaded "Cannot copy the file, because it's used by another process" error.

A Workaround for the Problem

While there is no complete solution to this issue, a workaround exists that involves periodically checking whether the file is still being modified before initiating the copy process. Here are two methods to accomplish this task:

Method 1

private bool IsFileLocked(FileInfo file)
{
    FileStream stream = null;

    try
    {
        stream = file.Open(FileMode.Open, FileAccess.ReadWrite, FileShare.None);
    }
    catch (IOException)
    {
        return true;
    }
    finally
    {
        if (stream != null)
            stream.Close();
    }

    return false;
}

This method attempts to open the file for exclusive access and returns true if the file is locked (i.e., still being written or processed by another thread).

Method 2

const int ERROR_SHARING_VIOLATION = 32;
const int ERROR_LOCK_VIOLATION = 33;

private bool IsFileLocked(string file)
{
    if (File.Exists(file))
    {
        FileStream stream = null;
        try
        {
            stream = File.Open(file, FileMode.Open, FileAccess.ReadWrite, FileShare.None);
        }
        catch (Exception ex2)
        {
            int errorCode = Marshal.GetHRForException(ex2) & ((1 << 16) - 1);
            if ((ex2 is IOException) && (errorCode == ERROR_SHARING_VIOLATION || errorCode == ERROR_LOCK_VIOLATION))
            {
                return true;
            }
        }
        finally
        {
            if (stream != null)
                stream.Close();
        }
    }

    return false;
}

This method checks for both the existence of the file and its availability for exclusive access, returning true if the file is locked (i.e., inaccessible due to a sharing or lock violation).

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