Question

In this question I have searched for a simple solution to unblock files.
Thanks to all the comments and answer, I have found a simple solution by PInvoking DeleteFile.

It works, but because I’ve never used file-operations through PInvoke (Win32), I don’t know if there are some pitfalls or if there is another method of calling DeleteFile to delete the alternate stream of a file.

What I also don’t know is if I have to wrap the call in a try/catch or if it is enough just to look the at the boolean result. In my tests, no exceptions were raised, but I don’t know what will happen in the real world.

public class FileUnblocker {

    [DllImport("kernel32", CharSet = CharSet.Unicode, SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool DeleteFile(string name );

    public bool Unblock(string fileName) {
        return DeleteFile(fileName+ ":Zone.Identifier");
    }
}

Does this code look reliable?

Update
I had posted an incomplete method (the unblock method didn’t concatenate the “Zone.Identifier” literal to the file name). I have corrected this now, sorry.

Answer 1

Calling the native method will never raise an exception. If the file deletion fails, for whatever reason, the call to DeleteFile returns false.

Your P/Invoke code is good. You are correctly using Unicode characters, setting SetLastError to true and the parameter marshalling is correct. To check for errors look for the value of the boolean return from DeleteFile. If it is false (i.e. the call failed) then call Marshal.GetLastWin32Error to find out the Win32 error code.

The most obvious causes for the function to fail are:

1. The file does not exist.
2. The alternate stream is not present.
3. The process does not have sufficient rights to delete the alternate stream.

For 1 and 2 an error code of ERROR_FILE_NOT_FOUND will be returned. For 3 you will be given an error code of ERROR_ACCESS_DENIED.