IOException with File.Copy() inspite of prior File.Exists() check

This is pertaining to a simple file copy operation code. My requirement is that only new files be copied from the source folder to the destination folder, so before I copy the file, I check that:

• it exists in the source folder
• it does not exist in the destination folder

After this I proceed with the copy operation.

However, I randomly get an IOException stating that "The file <filename> already exists."

Now, I have this code running (as part of a win service) on 2 servers so I'm willing to concede that maybe, just maybe, within that short interval where Server1 checked the conditions and proceeded to copy the file, Server2 copied it to destination, resulting in the IOException on Server1.

But, I have several thousands of files being copied and I get this error in thousands. How is this possible? What am I missing? Here's the code:

try
{
    if(File.Exists(String.Format("{0}\\{1}",pstrSourcePath,strFileName)) && !File.Exists(String.Format("{0}\\{1}",pstrDestPath,strFileName)))
            File.Copy(String.Format("{0}\\{1}",pstrSourcePath,strFileName),String.Format("{0}\\{1}",pstrDestPath,strFileName))     
}
catch(IOExcep开发者_如何转开发tion ioEx)
{
    txtDesc.Value=ioEx.Message;
}

I imagine it's a permissions issue. From the docs for File.Exists:

If the caller does not have sufficient permissions to read the specified file, no exception is thrown and the method returns false regardless of the existence of path.

Perhaps the file does exist, but your code doesn't have permission to check it?

Note that your code would be clearer if you used string.Format once for each file and saved the results to temporary variables. It would also be better to use Path.Combine instead of string.Format, like this:

string sourcePath = Path.Combine(pstrSourcePath, strFileName);
string targetPath = Path.Combine(pstrDestPath, strFileName);
if (File.Exists(sourcePath) && !File.Exists(targetPath))
{
    File.Copy(sourcePath, targetPath);
}

(I'd also ditch the str and pstr prefixes, but hey...)

The two server scenario is sufficient to explain the problem. Beware that they'll have a knack for automatically synchronizing to each other's copy operation. Whatever server is behind will quickly catch up because the file is already present in the target machine's file system cache.

You have to give up on the File.Exist test, it just cannot work reliably on a multi-tasking operating system. The race condition is unsolvable, the reason that neither Windows nor .NET has an IsFileLocked() method for example. Just call File.Copy(). You'll get an IOException of course if the file already exists. Filter out the exception messages by using Marshal.GetLastWin32Error(). The ERROR_FILE_EXISTS error code is 80.

The same thing is happening to me and I cannot figure it out. In my case, I am always writing to a new location and yet sometimes I receive the same error. When I look at that location, there is a file with that name zero bytes in size. I can guarantee that file did not exist prior and some other process is not also writing to that location. This is copying across to a network share, not sure if that is significant, but thought I would mention it. It is almost like the File.Copy operation is writing the files, then erring because the file exists (but not always). I am logging my copy operation as I recurse a directory structure and see no duplicate copy operations that might overlap.