I'm being completely confused here folks,

My code throws an exception because File.Exists() returns false

public override sealed TCargo ReadFile(string fileName)
{
    if (!File.Exists(fileName))
    {
        throw new ArgumentException("Provided file name does not exist", "fileName");
    }

Visual studio breaks at the throw statement, and I immediately check the value of File.Exists(fileName) in the immediate window. It returns true. When I drag the breakpoint back up to the if statement and execute it again, it throws again.

fileName is an absolute path to a file. I'm not creating the file, nor writing to it (it's there all along). If I paste the path into the open dialog in Notepad, it reads the file without problems.

The code is executing in a background worker. It's the only complicating factor I can think of. I am positive the file has not been opened already, either in the worker thread or elsewhere.

What's going on here?