Extensions.cs

using System.IO;

namespace ResourceInDll
{
    internal static class Extensions
    {
        public static byte[] ReadExact(this Stream stream, int size)
        {
            var bytesRead = 0;
            var bs = new byte[size];

            while (bytesRead < size)
            {
                var i = stream.Read(bs, bytesRead, size - bytesRead);
                if (i == 0) throw new EndOfStreamException("EOF");
                bytesRead += i;
            }

            return bs;
        }

        public static int ReadDWord(this Stream stream)
        {
            var bs = stream.ReadExact(4);
            return bs[0] | bs[1] << 8 | bs[2] << 16 | bs[3] << 24;
        }

        public static int ReadWord(this Stream stream)
        {
            var bs = stream.ReadExact(2);
            return bs[0] | bs[1] << 8;
        }
    }
}

ResourceInDll.cs

using System;
using System.IO;

namespace ResourceInDll
{
    public class Class1
    {
        public static Stream GetResourceStream()
        {
            var thisFilePath = typeof(Class1).Assembly.Location;
            var stream = new FileStream(thisFilePath, FileMode.Open, FileAccess.Read);
            
            stream.Seek(60, SeekOrigin.Begin);
            var ntHeaderAddress = stream.ReadDWord();

            stream.Seek(ntHeaderAddress + 6, SeekOrigin.Begin);
            var numberOfSections = stream.ReadWord();

            stream.Seek(12, SeekOrigin.Current);
            var sizeOfOptionalHeader = stream.ReadWord();

            // 最初のセクションヘッダー
            stream.Seek(2 + sizeOfOptionalHeader, SeekOrigin.Current);

            var maxAddress = 0;

            for(var i = 0; i < numberOfSections; i++)
            {
                stream.Seek(16, SeekOrigin.Current);
                var sizeOfRawData = stream.ReadDWord();
                var pointerToRawData = stream.ReadDWord();

                maxAddress = Math.Max(
                    maxAddress,
                    pointerToRawData + sizeOfRawData
                );

                stream.Seek(16, SeekOrigin.Current);
            }

            stream.Seek(maxAddress, SeekOrigin.Begin);
            return stream;
        }
    }
}