How can I quickly read bytes from a memory mapped file in .NET?

Question

In some situations the MemoryMappedViewAccessor class just doesn't cut it for reading bytes efficiently; the best we get is the generic ReadArray<byte> which it the route for all structs and involves several unnecessary steps when you just need bytes.

It's possible to use a MemoryMappedViewStream, but because it's based on a Stream you need to seek to the correct position first, and then the read operation itself has many more unnecessary steps.

Is there a quick, high-performance way to read an array of bytes from a memory-mapped file in .NET, given that it should just be a particular area of the address space to read from?

Answer 1

This solution requires unsafe code (compile with /unsafe switch), but grabs a pointer to the memory directly; then Marshal.Copy can be used. This is much, much faster than the methods provided by the .NET framework.

    // assumes part of a class where _view is a MemoryMappedViewAccessor object

    public unsafe byte[] ReadBytes(int offset, int num)
    {
        byte[] arr = new byte[num];
        byte *ptr = (byte*)0;
        this._view.SafeMemoryMappedViewHandle.AcquirePointer(ref ptr);
        Marshal.Copy(IntPtr.Add(new IntPtr(ptr), offset), arr, 0, num);
        this._view.SafeMemoryMappedViewHandle.ReleasePointer();
        return arr;
    }

    public unsafe void WriteBytes(int offset, byte[] data)
    {
        byte* ptr = (byte*)0;
        this._view.SafeMemoryMappedViewHandle.AcquirePointer(ref ptr);
        Marshal.Copy(data, 0, IntPtr.Add(new IntPtr(ptr), offset), data.Length);
        this._view.SafeMemoryMappedViewHandle.ReleasePointer();
    }