MemoryMappedFile: Unable to find the specified file

Question

I'm having trouble getting the MemoryMappedFile pattern to work. I have a directory with 25 excel spreadsheets that I want to load using MemoryMappedFile. The MemoryMappedFile.CreateFromFile works just fine, but when I call MemoryMappedFile.OpenExisting(lender), it reads the first 3 files, and then on the 4th attempt, it fails with exception:

Unable to find the specified file.

public static void Main()
{
    var files = System.IO.Directory.EnumerateFiles(@"lendersheets", "*.xls*").ToList();
    foreach (var file in files)
    {
        var lender = System.IO.Path.GetFileNameWithoutExtension(file);

        MemoryMappedFile.CreateFromFile(file, FileMode.Open, lender);
    }

    foreach (var file in files)
    {
        var lender = System.IO.Path.GetFileNameWithoutExtension(file);

        using (var mmfExisting = MemoryMappedFile.OpenExisting(lender))
        {
            var stream = mmfExisting.CreateViewStream();
            if (stream.CanRead && stream.CanWrite)
            {
                LoadFromStream(stream);
            }
        }                              
    }
}

UPDATE:

I have noticed that when I remove the LoadFromStream method call, the exception doesn't occur anymore. Here is the logic for LoadFromStream:

    private static object LoadFromStream(Stream stream)
    {
        hssfwb = new HSSFWorkbook(stream);
        evaluator = new HSSFFormulaEvaluator(hssfwb);

        return hssfwb;
    }

Any reason you are using memory mappings at all? This particular code does not seem to require them. — usr, Jul 28 '15 at 08:45
For performance I guess. It takes about 5 seconds to load all 25 spreadsheets into memory. Once in memory I call each stream, input values to the excel sheet, and then read values from calculated cells. I don't want to load the 25 excel files each time I need to do calculations. I figured using MemoryMappedFiles was the way to go? — FaNIX, Jul 28 '15 at 23:14
Not sure what you mean. Reading a file twice causes it to be cached. Probably the IO time is like 5% of your total time, all of this is moot. Measure. — usr, Jul 29 '15 at 16:52

score 0 · Accepted Answer · answered Aug 05 '15 at 00:48

I ended up doing the following. I have one singleton instance of a collection of all files in Memory. I then create a new instance of the memory stream when I need to use the specific file in memory for calculations:

private static readonly Dictionary<string, MemoryStream> _lenderPolicyFileStreams = new Dictionary<string, MemoryStream>();

        private static void CreateLenderPolicyFileStreams()
        {
            string appDataPath = HostingEnvironment.MapPath("~/App_Data");
            var files = Directory.EnumerateFiles(appDataPath, "*.xls*").ToList();

            foreach (var file in files)
            {
                var lenderName = Path.GetFileNameWithoutExtension(file);

                // Create a file stream
                var bytes = File.ReadAllBytes(file);
                var stream = new MemoryStream(bytes);

                // Add file stream to singleton collection
                if (lenderName != null) 
                    _lenderPolicyFileStreams.Add(lenderName, stream);
            }
        }

        public static Stream GetMemoryStreamByLenderName(string lender)
        {
            MemoryStream fetchedStream;
            _lenderPolicyFileStreams.TryGetValue(lender, out fetchedStream);

            if (fetchedStream == null)
                throw new Exception("Cannot find specified file stream by lender name");

            var clone = CloneStream(fetchedStream);
            return clone;
        }

        private static MemoryStream CloneStream(MemoryStream fileStream)
        {
            var memoryStream = new MemoryStream();

            fileStream.Position = 0;
            fileStream.CopyTo(memoryStream);
            memoryStream.Position = 0;

            return memoryStream;
        }

MemoryMappedFile: Unable to find the specified file

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