如何：对序列化数据进行分块

项目
08/09/2024

警告

使用 BinaryFormatter 的二进制序列化可能很危险。有关详细信息，请参阅 BinaryFormatter 安全指南和 BinaryFormatter 迁移指南。

在 Web 服务消息中发送大型数据集时，会出现下面两个问题：

因序列化引擎执行缓冲而使工作集（内存）很大。
因使用 Base64 编码后产生 33% 的膨胀而过度占用带宽。

若要解决这两个问题，可以通过实现 IXmlSerializable 接口，来控制序列化和反序列化。具体来讲，就是实现 WriteXml 和 ReadXml 方法，对数据进行分块。

实现服务器端数据分块

在服务器计算机上，必须采用 Web 方法禁用 ASP.NET 缓冲，并返回实现 IXmlSerializable 的类型。
实现 IXmlSerializable 的类型可以采用 WriteXml 方法对数据进行分块。

实现客户端处理

通过更改客户端代理上的 Web 方法，可以返回实现 IXmlSerializable 的类型。可以使用 SchemaImporterExtension 自动执行这项操作，但此处不加以说明。
通过实现 ReadXml 方法，可以读取分块数据流，并将字节写入磁盘。此实现还会引发可供图形控件（如进度栏）使用的进度事件。

示例

下面的代码示例演示客户端上用于禁用 ASP.NET 缓冲的 Web 方法。它还演示了如何在客户端实现 IXmlSerializable 接口，该实现可以采用 WriteXml 方法对数据进行分块。

[WebMethod]
[SoapDocumentMethod(ParameterStyle = SoapParameterStyle.Bare)]
public SongStream DownloadSong(DownloadAuthorization Authorization, string filePath)
{
    // Turn off response buffering.
    System.Web.HttpContext.Current.Response.Buffer = false;
    // Return a song.
    SongStream song = new SongStream(filePath);
    return song;
}

    <WebMethod(), SoapDocumentMethodAttribute(ParameterStyle:=SoapParameterStyle.Bare)>
    Public Function DownloadSong(ByVal Authorization As DownloadAuthorization, ByVal filePath As String) As SongStream

        ' Turn off response buffering.
        System.Web.HttpContext.Current.Response.Buffer = False
        ' Return a song.
        Dim song As New SongStream(filePath)
        Return song

    End Function
End Class

[XmlSchemaProvider("MySchema")]
public class SongStream : IXmlSerializable
{
    private const string ns = "http://demos.Contoso.com/webservices";
    private string filePath;

    public SongStream() { }

    public SongStream(string filePath)
    {
        this.filePath = filePath;
    }

    // This is the method named by the XmlSchemaProviderAttribute applied to the type.
    public static XmlQualifiedName MySchema(XmlSchemaSet xs)
    {
        // This method is called by the framework to get the schema for this type.
        // We return an existing schema from disk.

        XmlSerializer schemaSerializer = new XmlSerializer(typeof(XmlSchema));
        string xsdPath = null;
        // NOTE: replace the string with your own path.
        xsdPath = System.Web.HttpContext.Current.Server.MapPath("SongStream.xsd");
        XmlSchema s = (XmlSchema)schemaSerializer.Deserialize(
            new XmlTextReader(xsdPath), null);
        xs.XmlResolver = new XmlUrlResolver();
        xs.Add(s);

        return new XmlQualifiedName("songStream", ns);
    }

    void IXmlSerializable.WriteXml(System.Xml.XmlWriter writer)
    {
        // This is the chunking code.
        // ASP.NET buffering must be turned off for this to work.

        int bufferSize = 4096;
        char[] songBytes = new char[bufferSize];
        FileStream inFile = File.Open(this.filePath, FileMode.Open, FileAccess.Read);

        long length = inFile.Length;

        // Write the file name.
        writer.WriteElementString("fileName", ns, Path.GetFileNameWithoutExtension(this.filePath));

        // Write the size.
        writer.WriteElementString("size", ns, length.ToString());

        // Write the song bytes.
        writer.WriteStartElement("song", ns);

        StreamReader sr = new StreamReader(inFile, true);
        int readLen = sr.Read(songBytes, 0, bufferSize);

        while (readLen > 0)
        {
            writer.WriteStartElement("chunk", ns);
            writer.WriteChars(songBytes, 0, readLen);
            writer.WriteEndElement();

            writer.Flush();
            readLen = sr.Read(songBytes, 0, bufferSize);
        }

        writer.WriteEndElement();
        inFile.Close();
    }

    XmlSchema IXmlSerializable.GetSchema()
    {
        throw new NotImplementedException();
    }

    void IXmlSerializable.ReadXml(System.Xml.XmlReader reader)
    {
        throw new NotImplementedException();
    }
}

<XmlSchemaProvider("MySchema")>
Public Class SongStream
    Implements IXmlSerializable

    Private Const ns As String = "http://demos.Contoso.com/webservices"
    Private filePath As String

    Public Sub New()

    End Sub

    Public Sub New(ByVal filePath As String)
        Me.filePath = filePath
    End Sub

    ' This is the method named by the XmlSchemaProviderAttribute applied to the type.
    Public Shared Function MySchema(ByVal xs As XmlSchemaSet) As XmlQualifiedName
        ' This method is called by the framework to get the schema for this type.
        ' We return an existing schema from disk.
        Dim schemaSerializer As New XmlSerializer(GetType(XmlSchema))
        Dim xsdPath As String = Nothing
        ' NOTE: replace SongStream.xsd with your own schema file.
        xsdPath = System.Web.HttpContext.Current.Server.MapPath("SongStream.xsd")
        Dim s As XmlSchema = CType(schemaSerializer.Deserialize(New XmlTextReader(xsdPath)), XmlSchema)
        xs.XmlResolver = New XmlUrlResolver()
        xs.Add(s)

        Return New XmlQualifiedName("songStream", ns)

    End Function

    Sub WriteXml(ByVal writer As System.Xml.XmlWriter) Implements IXmlSerializable.WriteXml
        ' This is the chunking code.
        ' ASP.NET buffering must be turned off for this to work.

        Dim bufferSize As Integer = 4096
        Dim songBytes(bufferSize) As Char
        Dim inFile As FileStream = File.Open(Me.filePath, FileMode.Open, FileAccess.Read)

        Dim length As Long = inFile.Length

        ' Write the file name.
        writer.WriteElementString("fileName", ns, Path.GetFileNameWithoutExtension(Me.filePath))

        ' Write the size.
        writer.WriteElementString("size", ns, length.ToString())

        ' Write the song bytes.
        writer.WriteStartElement("song", ns)

        Dim sr As New StreamReader(inFile, True)
        Dim readLen As Integer = sr.Read(songBytes, 0, bufferSize)

        While readLen > 0
            writer.WriteStartElement("chunk", ns)
            writer.WriteChars(songBytes, 0, readLen)
            writer.WriteEndElement()

            writer.Flush()
            readLen = sr.Read(songBytes, 0, bufferSize)
        End While

        writer.WriteEndElement()
        inFile.Close()
    End Sub

    Function GetSchema() As System.Xml.Schema.XmlSchema Implements IXmlSerializable.GetSchema
        Throw New System.NotImplementedException()
    End Function

    Sub ReadXml(ByVal reader As System.Xml.XmlReader) Implements IXmlSerializable.ReadXml
        Throw New System.NotImplementedException()
    End Sub
End Class

public class SongFile : IXmlSerializable
{
    public static event ProgressMade OnProgress;

    public SongFile()
    { }

    private const string ns = "http://demos.teched2004.com/webservices";
    public static string MusicPath;
    private string filePath;
    private double size;

    void IXmlSerializable.ReadXml(System.Xml.XmlReader reader)
    {
        reader.ReadStartElement("DownloadSongResult", ns);
        ReadFileName(reader);
        ReadSongSize(reader);
        ReadAndSaveSong(reader);
        reader.ReadEndElement();
    }

    void ReadFileName(XmlReader reader)
    {
        string fileName = reader.ReadElementString("fileName", ns);
        this.filePath =
            Path.Combine(MusicPath, Path.ChangeExtension(fileName, ".mp3"));
    }

    void ReadSongSize(XmlReader reader)
    {
        this.size = Convert.ToDouble(reader.ReadElementString("size", ns));
    }

    void ReadAndSaveSong(XmlReader reader)
    {
        FileStream outFile = File.Open(
            this.filePath, FileMode.Create, FileAccess.Write);

        string songBase64;
        byte[] songBytes;
        reader.ReadStartElement("song", ns);
        double totalRead = 0;
        while (true)
        {
            if (reader.IsStartElement("chunk", ns))
            {
                songBase64 = reader.ReadElementString();
                totalRead += songBase64.Length;
                songBytes = Convert.FromBase64String(songBase64);
                outFile.Write(songBytes, 0, songBytes.Length);
                outFile.Flush();

                if (OnProgress != null)
                {
                    OnProgress(100 * (totalRead / size));
                }
            }

            else
            {
                break;
            }
        }

        outFile.Close();
        reader.ReadEndElement();
    }

    public void Play()
    {
        System.Diagnostics.Process.Start(this.filePath);
    }

    XmlSchema IXmlSerializable.GetSchema()
    {
        throw new NotImplementedException();
    }

    public void WriteXml(XmlWriter writer)
    {
        throw new NotImplementedException();
    }
}

Public Class SongFile
    Implements IXmlSerializable
    Public Shared Event OnProgress As ProgressMade

    Public Sub New()

    End Sub

    Private Const ns As String = "http://demos.teched2004.com/webservices"
    Public Shared MusicPath As String
    Private filePath As String
    Private size As Double

    Sub ReadXml(ByVal reader As System.Xml.XmlReader) Implements IXmlSerializable.ReadXml
        reader.ReadStartElement("DownloadSongResult", ns)
        ReadFileName(reader)
        ReadSongSize(reader)
        ReadAndSaveSong(reader)
        reader.ReadEndElement()
    End Sub

    Sub ReadFileName(ByVal reader As XmlReader)
        Dim fileName As String = reader.ReadElementString("fileName", ns)
        Me.filePath = Path.Combine(MusicPath, Path.ChangeExtension(fileName, ".mp3"))

    End Sub

    Sub ReadSongSize(ByVal reader As XmlReader)
        Me.size = Convert.ToDouble(reader.ReadElementString("size", ns))

    End Sub

    Sub ReadAndSaveSong(ByVal reader As XmlReader)
        Dim outFile As FileStream = File.Open(Me.filePath, FileMode.Create, FileAccess.Write)

        Dim songBase64 As String
        Dim songBytes() As Byte
        reader.ReadStartElement("song", ns)
        Dim totalRead As Double = 0
        While True
            If reader.IsStartElement("chunk", ns) Then
                songBase64 = reader.ReadElementString()
                totalRead += songBase64.Length
                songBytes = Convert.FromBase64String(songBase64)
                outFile.Write(songBytes, 0, songBytes.Length)
                outFile.Flush()
                RaiseEvent OnProgress((100 * (totalRead / size)))
            Else
                Exit While
            End If
        End While

        outFile.Close()
        reader.ReadEndElement()
    End Sub

    Public Sub Play()
        System.Diagnostics.Process.Start(Me.filePath)
    End Sub

    Function GetSchema() As System.Xml.Schema.XmlSchema Implements IXmlSerializable.GetSchema
        Throw New System.NotImplementedException()
    End Function

    Public Sub WriteXml(ByVal writer As XmlWriter) Implements IXmlSerializable.WriteXml
        Throw New System.NotImplementedException()
    End Sub
End Class

编译代码

代码使用下面的命名空间：System、System.Runtime.Serialization、System.Web.Services、System.Web.Services.Protocols、System.Xml、System.Xml.Serialization 和 System.Xml.Schema。

请参阅

自定义序列化

通过

如何：对序列化数据进行分块

实现服务器端数据分块

实现客户端处理

示例

编译代码

请参阅

其他资源