MemoryMappedFile.CreateFromFile always throws Unau

问题:

I realize .NET 4.0 is in Beta, but I'm hoping someone has a resolution for this. I'm trying to create a memory mapped file from a DLL:

FileStream file = File.OpenRead("C:\mydll.dll");
using (MemoryMappedFile mappedFile = MemoryMappedFile.CreateFromFile(file,
    "PEIMAGE", 1024 * 1024, MemoryMappedFileAccess.ReadExecute))
{
    using (MemoryMappedViewStream viewStream = mappedFile.CreateViewStream())
    {
        // read from the view stream
    }
}

Unfortunately, no matter what I do, I always get an UnauthorizedAccessException, for which the MSDN documentation states:

The operating system denied the specified access to the file; for example, access is set to Write or ReadWrite, but the file or directory is read-only.

I've monitored my application with Sysinternals Process Monitor, which shows that the file is indeed being opened successfully. I've also tried memory mapping other non-DLL files, but with the same result.

回答1:

Well, I've got an example based on the above which runs without exceptions. I've made two important changes:

• I'm only specified MemoryMappedFileAccess.Read when creating the MemoryMappedFile. You've opened it for read, so you can only read. I haven't tried fixing it to allow execute as well by changing how the FileStream is opened.
• I've made the CreateViewStream call explicitly use MemoryMappedFileAccess.Read as well. I'm not sure why it doesn't use the existing access rights by itself, but there we go.

Full program:

using System.IO;
using System.IO.MemoryMappedFiles;

class Test
{
    static void Main()
    {
        FileStream file = File.OpenRead("Test.cs");
        using (MemoryMappedFile mappedFile = MemoryMappedFile.CreateFromFile
               (file, "PEIMAGE", file.Length, MemoryMappedFileAccess.Read, null, 0, false))
        {
            using (var viewStream = mappedFile.CreateViewStream
                   (0, file.Length, MemoryMappedFileAccess.Read))
            {
                // read from the view stream
            }
        }
    }
}

回答2:

I had the same behaviour when calling the CreateViewAccessor(...) method.

Turns out, the error was only thrown when the size parameter exceeded the length of the file (it's not the same behaviour as we're used to with streams where size is a maximum value, instead it appears to take the parameter as a literal and the result is an attempt to read past the end of the file).

I fixed my problem by checking that the size doesn't exceed the size of the open file.