Question

I am experimenting with Marshal.AllocHGlobal and found puzzling that this code would not succeed, instead it throws an OutOfMemory exception:

namespace HAlloc
{
    using System;
    using System.IO;
    using System.Runtime.InteropServices;

    class Program
    {
        static void Main(string[] args)
        {
            // large file ~ 800MB
            string fileName = @"largefile.bin";
            FileInfo fileInfo = new FileInfo(fileName);

            // allocation succeeds
            IntPtr p = Marshal.AllocHGlobal((int)fileInfo.Length);

            // OutOfMemory exception thrown here:
            Marshal.Copy(File.ReadAllBytes(fileName), 0, p, (int)fileInfo.Length);
            Marshal.FreeHGlobal(p);
        }
    }
}

Why would it get an OutOfMemory when the AllocHGlobal call succeeded?

Answer 1

Cause File.ReadAllBytes(fileName) also has to read the file which causes extra ~800 MB