MemoryMappedFiles Could not find a part of the path

Question

I have a few methods which work with MemoryMappedFiles for writing/reading data. They work alright if I use simple string for file name, for example "file.mmf". However if I use full directory path the above mentioned exception is being thrown - Exception has been thrown by the target of an invocation. With inner exception - {"Could not find a part of the path."}. Here is how my method looks like:

public void WriteToFile(string fileName, string value)
{
    string newFileName = CombineDirectory(fileName);
    byte[] newValue = Encoding.UTF8.GetBytes(value);
    long capacity = newValue.Length + INT_MAXVALUE_TO_BYTEARRAY_LENGTH;

    using (var mmf = MemoryMappedFile.CreateFromFile(newFileName, FileMode.Create, newFileName, capacity))
    {
        using (var accesor = mmf.CreateViewAccessor())
        {
            byte[] newValueLength = BitConverter.GetBytes(value.Length);
            accesor.WriteArray(0, newValueLength, 0, newValueLength.Length);
            accesor.WriteArray(INT_MAXVALUE_TO_BYTEARRAY_LENGTH, newValue, 0, newValue.Length);
        }
    }
}

My path looks like this :

"C:\\Users\\MyUser\\Documents\\Visual Studio 2012.mmf"

And I am using

Path.Combine

The exception occurs on the first 'using' line. If I try to create a file using the same file path with

File.Create

the file is being created with no problem.

If anyone has any suggestions, that would be great.

Regards

Answer 1

You need to make sure that the mapName argument (i.e. the third argument in your call to CreateFromFile) is not identical to the file path. It will throw a PathNotFound exception if you do. Not really helpful in figuring out why it is failing, I agree.

So your options for choosing a map name value:

• Generate some unique key, e.g. Guid.NewGuid().ToString()
• Use a constant value, e.g. "MySpecialMapForThings"
• Use some convention, e.g. generate a unique key that you also use for just the file name part of the mapped file.

An example for the last option:

public static Tuple<FileInfo, string> GenerateMapInfo(string mapDirectory, string fileExtension)
{
    var uniqueMapName = Guid.NewGuid().ToString();
    var fileName = Path.Combine(mapDirectory, Path.ChangeExtension(uniqueMapName, fileExtension));
    return Tuple.Create(new FileInfo(fileName), uniqueMapName);
}

public void WriteToFile(Tuple<FileInfo, string> mapInfo, string value)
{
    byte[] newValue = Encoding.UTF8.GetBytes(value);
    long capacity = newValue.Length + INT_MAXVALUE_TO_BYTEARRAY_LENGTH;

    using (var mmf = MemoryMappedFile.CreateFromFile(mapInfo.Item1.FullName, FileMode.Create, mapInfo.Item2, capacity))
    using (var accesor = mmf.CreateViewAccessor())
    {
        byte[] newValueLength = BitConverter.GetBytes(value.Length);
        accesor.WriteArray(0, newValueLength, 0, newValueLength.Length);
        accesor.WriteArray(INT_MAXVALUE_TO_BYTEARRAY_LENGTH, newValue, 0, newValue.Length);
    }
}