如何使用内存映射文件C#读写文件？

Question

我在 D 盘中有一个图像，例如“D:\Image\1.tiff”。我想读取这个文件并将其写入另一个位置，例如路径“D:\Project\”。如何使用内存映射文件来做到这一点？

Answer 1

我现在可以使用以下编码使用内存映射文件来实现读取和写入文件：

FileStream stream = File.OpenRead(@"D:\FFv1\dpx1\1.dpx");
byte[] fileBytes = new byte[stream.Length];
string Output = @"D:\Vanthiya Thevan\FFv1\dpx1\2.dpx";
using (var fileStream = new FileStream(Output, FileMode.Create, FileAccess.ReadWrite, FileShare.ReadWrite))
using (MemoryMappedFile memoryMapped = MemoryMappedFile.CreateFromFile(fileStream, "MapName", fileBytes.Length,
MemoryMappedFileAccess.ReadWrite, new MemoryMappedFileSecurity(), HandleInheritability.Inheritable, true))
{
    var viewStream = memoryMapped.CreateViewStream();
    viewStream.Write(fileBytes, 0, fileBytes.Length); 
}

Answer 2

CreateFromFile 方法从磁盘上的现有文件创建内存映射文件。以下示例创建一个极大文件的一部分的内存映射视图并操作它的一部分。

using System;
using System.IO;
using System.IO.MemoryMappedFiles;
using System.Runtime.InteropServices;

class Program
{
    static void Main(string[] args)
    {
        long offset = 0x10000000; // 256 megabytes 
        long length = 0x20000000; // 512 megabytes 

        // Create the memory-mapped file. 
        using (var mmf = MemoryMappedFile.CreateFromFile(@"c:\ExtremelyLargeImage.data", FileMode.Open,"ImgA"))
        {
            // Create a random access view, from the 256th megabyte (the offset) 
            // to the 768th megabyte (the offset plus length). 
            using (var accessor = mmf.CreateViewAccessor(offset, length))
            {
                int colorSize = Marshal.SizeOf<MyColor>();
                MyColor color;

                // Make changes to the view. 
                for (long i = 0; i < length; i += colorSize)
                {
                    accessor.Read(i, out color);
                    color.Brighten(10);
                    accessor.Write(i, ref color);
                }
            }
        }
    }
}

public struct MyColor
{
    public short Red;
    public short Green;
    public short Blue;
    public short Alpha;

    // Make the view brighter. 
    public void Brighten(short value)
    {
        Red = (short)Math.Min(short.MaxValue, (int)Red + value);
        Green = (short)Math.Min(short.MaxValue, (int)Green + value);
        Blue = (short)Math.Min(short.MaxValue, (int)Blue + value);
        Alpha = (short)Math.Min(short.MaxValue, (int)Alpha + value);
    }
}

以下示例为另一个进程打开相同的内存映射文件。

using System;
using System.IO.MemoryMappedFiles;
using System.Runtime.InteropServices;


class Program
{
    static void Main(string[] args)
    {
        // Assumes another process has created the memory-mapped file. 
        using (var mmf = MemoryMappedFile.OpenExisting("ImgA"))
        {
            using (var accessor = mmf.CreateViewAccessor(4000000, 2000000))
            {
                int colorSize = Marshal.SizeOf<MyColor>();
                MyColor color;

                // Make changes to the view. 
                for (long i = 0; i < 1500000; i += colorSize)
                {
                    accessor.Read(i, out color);
                    color.Brighten(20);
                    accessor.Write(i, ref color);
                }
            }
        }
    }
}

public struct MyColor
{
    public short Red;
    public short Green;
    public short Blue;
    public short Alpha;

    // Make the view brigher. 
    public void Brighten(short value)
    {
        Red = (short)Math.Min(short.MaxValue, (int)Red + value);
        Green = (short)Math.Min(short.MaxValue, (int)Green + value);
        Blue = (short)Math.Min(short.MaxValue, (int)Blue + value);
        Alpha = (short)Math.Min(short.MaxValue, (int)Alpha + value);
    }
}

您还可以阅读更多信息：http://www.codeproject.com/Articles/138290/Programming-Memory-Mapped-Files-with-the-NET-Frame