Opening a Memory Mapped File causes FileNotFoundEx

问题:

Following the code example from this website, I created a windows console application that creates a mapped memory file:

        using (var file = MemoryMappedFile.CreateNew("myFile", 24))
        {
            var bytes = new byte[24];
            for (var i = 0; i < bytes.Length; i++)
                bytes[i] = (byte)(65 + i);

            using (var writer = file.CreateViewAccessor(0, bytes.Length))
            {
                writer.WriteArray<byte>(0, bytes, 0, bytes.Length);
            }
            Console.WriteLine("Run memory mapped file reader before exit");
            Console.WriteLine("Press any key to exit ...");
            Console.ReadLine();
        }

in a new asp.net web application I read the MMF using the code:

 protected void Page_Load(object sender, EventArgs e)
    {
        string sOutput = "";

        using (var file = MemoryMappedFile.OpenExisting("myFile"))
        {
            using (var reader = file.CreateViewAccessor(0, 24))
            {
                var bytes = new byte[24];
                reader.ReadArray<byte>(0, bytes, 0, bytes.Length);

                for (var i = 0; i < bytes.Length; i++)
                    sOutput += (char)bytes[i] + " ";

            }
        }

        Response.Write(sOutput);
    }

In the development IIS Express the file is read as expected and the page displays the data from the mapped memory file.

However, When deployed to IIS I get the exception System.IO.FileNotFoundException at the locationMemoryMappedFile.OpenExisting("myFile")

I tried changing the app pool identity to the same as the one running the MMF console application but this does didn't work.

回答1:

Add "Global\\" prefix to the mapName. That is the only way it worked for me. When we try to access shared memory created in the first process it runs in a different security context and shared memory objects are not visible unless they are created in the global namespace.

This is the code that worked for me. I used CreateOrOpen in a WinForm application and I used OpenExisting in an IIS process:

mSec = new MemoryMappedFileSecurity();
mSec.AddAccessRule(new AccessRule<MemoryMappedFileRights>(new SecurityIdentifier(WellKnownSidType.WorldSid, null), MemoryMappedFileRights.FullControl, AccessControlType.Allow));

mmf = MemoryMappedFile.CreateOrOpen("Global\\\MmfName", 100, MemoryMappedFileAccess.ReadWrite, MemoryMappedFileOptions.None, mSec, HandleInheritability.Inheritable);

mmf = MemoryMappedFile.OpenExisting("Global\\\MmfName");

回答2:

If Inter Process Communication (IPC) is what you are trying to achieve, you could use WCF with a Named Pipe Binding. You gain a lot of simplicity at the cost of some performance.

回答3:

For non physical file, there are two cases,

1. File is deleted after your process exits.
2. IIS does not run in same user as your console process, IIS runs in little restricted mode, so unless you create File with added security to allow process to access your service.

You will have to use http://msdn.microsoft.com/en-us/library/dd267529(v=vs.110).aspx this constructor with following security.

        MemoryMappedFileSecurity mSec = new MemoryMappedFileSecurity ();
        mSec.AddAccessRule(new AccessRule<MemoryMappedFileRights>(new SecurityIdentifier(WellKnownSidType.WorldSid, null),
            MemoryMappedFileRights .FullControl, AccessControlType.Allow));

And then try to access it from IIS with same security.