Unblock File from within .net 4 c#

Question

Is there a possibility to unblock a file that is downloaded from the internet from within a c# program. Surfing the internet I have learned, that the information is written in an alternative stream of a (NTFS) file that contains the current zone information (value 3 is from the internet and is interpreted as blocked).

Is there a managed possiblity to either clear or change the zone information (unblock) of a file or is there a managed copy function that copies the files without the zone information? If not, how can I do with PInvoke but without including a foreign assembly (I'm not allowed to do this in a current project).

Answer 1

Based on your input I have done the following code:

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

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

Thanks to Stuart Dunkeld, Alex K(+1) and Sven to show me the direction.

UPDATE I have posted the code here for a feedback if it would work reliable in production environment. If someone want to use it, check out there.