C# Stream篇（五） -- MemoryStream

public override void Write(byte[] buffer, int offset, int count) {
if (!_isOpen) __Error.StreamIsClosed();
if (!_writable) __Error.WriteNotSupported();
if (buffer==null)
throw new ArgumentNullException("buffer", Environment.GetResourceString("ArgumentNull_Buffer"));
if (offset < 0)
throw new ArgumentOutOfRangeException("offset", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
if (count < 0)
throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
if (buffer.Length - offset < count)
throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen")); int i = _position + count;
// Check for overflow
if (i < 0)
throw new IOException(Environment.GetResourceString("IO.IO_StreamTooLong")); if (i > _length) {
bool mustZero = _position > _length;
if (i > _capacity) {
bool allocatedNewArray = EnsureCapacity(i);
if (allocatedNewArray)
mustZero = false;
}
if (mustZero)
Array.Clear(_buffer, _length, i - _length);
_length = i;
}
if (count <= 8)
{
int byteCount = count;
while (--byteCount >= 0)
_buffer[_position + byteCount] = buffer[offset + byteCount];
}
else
Buffer.InternalBlockCopy(buffer, offset, _buffer, _position, count);
_position = i;
return;
}

//测试byte数组 假设该数组容量是256M
byte[] testBytes=new byte[256*1024*1024];
MemoryStream ms = new MemoryStream();
using (ms)
{
for (int i = 0; i < 1000; i++)
{
try
{
ms.Write(testBytes, 0, testBytes.Length);
}
catch
{
Console.WriteLine("该内存流已经使用了{0}M容量的内存,该内存流最大容量为{1}M,溢出时容量为{2}M",
GC.GetTotalMemory(false) / (1024 * 1024),//MemoryStream已经消耗内存量
ms.Capacity / (1024 * 1024), //MemoryStream最大的可用容量
ms.Length / (1024 * 1024));//MemoryStream当前流的长度（容量）
break;
}
} }
Console.ReadLine();

/// <summary>
/// 演示在xmlWriter中使用MemoryStream
/// </summary>
public static void UseMemoryStreamInXMLWriter()
{
MemoryStream ms = new MemoryStream();
using (ms)
{
//定义一个XMLWriter
using (XmlWriter writer = XmlWriter.Create(ms))
{
//写入xml头
writer.WriteStartDocument(true);
//写入一个元素
writer.WriteStartElement("Content");
//为这个元素新增一个test属性
writer.WriteStartAttribute("test");
//设置test属性的值
writer.WriteValue("逆时针的风");
//释放缓冲，这里可以不用释放，但是在实际项目中可能要考虑部分释放对性能带来的提升
writer.Flush();
Console.WriteLine("此时内存使用量为:{2}KB,该MemoryStream的已经使用的容量为{0}byte,默认容量为{1}byte",
Math.Round((double)ms.Length, 4), ms.Capacity,GC.GetTotalMemory(false)/1024);
Console.WriteLine("重新定位前MemoryStream所在的位置是{0}",ms.Position);
//将流中所在的当前位置往后移动7位，相当于空格
ms.Seek(7, SeekOrigin.Current);
Console.WriteLine("重新定位后MemoryStream所在的位置是{0}", ms.Position);
//如果将流所在的位置设置为如下所示的位置则xml文件会被打乱
//ms.Position = 0;
writer.WriteStartElement("Content2");
writer.WriteStartAttribute("testInner");
writer.WriteValue("逆时针的风Inner");
writer.WriteEndElement();
writer.WriteEndElement();
//再次释放
writer.Flush();
Console.WriteLine("此时内存使用量为:{2}KB,该MemoryStream的已经使用的容量为{0}byte,默认容量为{1}byte",
Math.Round((double)ms.Length, 4), ms.Capacity, GC.GetTotalMemory(false)/1024);
//建立一个FileStream 文件创建目的地是d:\test.xml
FileStream fs = new FileStream(@"d:\test.xml",FileMode.OpenOrCreate);
using (fs)
{
//将内存流注入FileStream
ms.WriteTo(fs);
if(ms.CanWrite)
//释放缓冲区
fs.Flush();
}
}
}
}

public class ImageHandler : IHttpHandler
{
#region IHttpHandler Members public bool IsReusable
{
get { return true; }
} /// <summary>
/// 实现IHTTPHandler后必须实现的方法
/// </summary>
/// <param name="context">HttpContext上下文</param>
public void ProcessRequest(HttpContext context)
{
context.Response.Clear();
//得到图片名
var imageName = context.Request["ImageName"] == null ? "逆时针的风"
: context.Request["ImageName"].ToString();
//得到图片ID,这里只是演示，实际项目中不是这么做的
var id = context.Request["Id"] == null ? "01"
: context.Request["Id"].ToString();
//得到图片地址
var stringFilePath = context.Server.MapPath(string.Format("~/Image/{0}{1}.jpg", imageName, id));
//声明一个FileStream用来将图片暂时放入流中
FileStream stream = new FileStream(stringFilePath, FileMode.Open);
using (stream)
{
//透过GetImageFromStream方法将图片放入byte数组中
byte[] imageBytes = this.GetImageFromStream(stream,context);
//上下文确定写到客户短时的文件类型
context.Response.ContentType = "image/jpeg";
//上下文将imageBytes中的数据写到前段
context.Response.BinaryWrite(imageBytes);
stream.Close();
}
} /// <summary>
/// 将流中的图片信息放入byte数组后返回该数组
/// </summary>
/// <param name="stream">文件流</param>
/// <param name="context">上下文</param>
/// <returns></returns>
private byte[] GetImageFromStream(FileStream stream, HttpContext context)
{
//通过stream得到Image
Image image = Image.FromStream(stream);
//加上水印
image = SetWaterImage(image, context);
//得到一个ms对象
MemoryStream ms = new MemoryStream();
using (ms)
{
//将图片保存至内存流
image.Save(ms, ImageFormat.Jpeg);
byte[] imageBytes = new byte[ms.Length];
ms.Position = 0;
//通过内存流读取到imageBytes
ms.Read(imageBytes, 0, imageBytes.Length);
ms.Close();
//返回imageBytes
return imageBytes;
}
}
/// <summary>
/// 为图片加上水印，这个方法不用在意，只是演示，所以没加透明度
/// 下次再加上吧
/// </summary>
/// <param name="image">需要加水印的图片</param>
/// <param name="context">上下文</param>
/// <returns></returns>
private Image SetWaterImage(Image image,HttpContext context)
{
Graphics graphics = Graphics.FromImage(image);
Image waterImage = Image.FromFile(context.Server.MapPath("~/Image/逆时针的风01.jpg"));
//在大图右下角画上水印图就行
graphics.DrawImage(waterImage,
new Point {
X = image.Size.Width - waterImage.Size.Width,
Y = image.Size.Height - waterImage.Size.Height
});
return image;
} #endregion
}

<httpHandlers>
<add type="ImageHandler.ImageHandler,ImageHandler" verb="*" path="ImageHandler.apsx"/>
</httpHandlers>

<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
<h2>
About
</h2>
<p>
Put content here.
<asp:Image runat="server" ImageUrl="ImageHandler.apsx?ImageName=逆时针的风&Id=02" />
</p>
</asp:Content>