在 Silverlight 应用中处理文件上传时，直接使用普通的 HTTP 请求可能会受到诸多限制。通过 WCF (Windows Communication Foundation) 服务来实现文件分片上传，不仅可以突破单次传输的大小限制，还能轻松实现上传进度的精确追踪。本文将详细讲解如何利用 WCF 结合流式分块读取技术，在 Silverlight 中实现高效、低内存占用的文件上传功能。

服务端 WCF 接口实现

首先，在 ASP.NET 宿主项目中添加一个"启用 Silverlight 的 WCF 服务"。为了避免大文件传输导致的内存溢出，服务端接口设计为接收文件分块（Chunk），并根据参数决定是创建新文件还是追加到现有文件。

[ServiceContract(Namespace = "")]
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
public class FileTransferService
{
    [OperationContract]
    public void UploadChunk(string targetFileName, byte[] chunkData, bool isAppendMode)
    {
        // 定义上传目录，此处使用 App_Data 避免直接暴露在 Web 根目录
        string uploadDirectory = System.Web.Hosting.HostingEnvironment.MapPath("~/App_Data/Uploads");
        
        if (!System.IO.Directory.Exists(uploadDirectory))
        {
            System.IO.Directory.CreateDirectory(uploadDirectory);
        }

        string fullPath = System.IO.Path.Combine(uploadDirectory, targetFileName);
        System.IO.FileMode fileMode = isAppendMode ? System.IO.FileMode.Append : System.IO.FileMode.Create;

        // 将接收到的数据块写入文件
        using (System.IO.FileStream fs = new System.IO.FileStream(fullPath, fileMode, System.IO.FileAccess.Write))
        {
            fs.Write(chunkData, 0, chunkData.Length);
        }
    }
}

客户端服务引用配置

在 Silverlight 项目中，右键点击项目节点并选择"添加服务引用"。在弹出的对话框中点击"发现"按钮，Visual Studio 会自动查找当前解决方案中的 WCF 服务。选中刚才创建的 FileTransferService，命名空间可保持默认或自定义，点击确定完成引用。注意：如果在发现服务时出现错误，请确保先按 F5 编译并运行一次宿主 Web 项目。

客户端 UI 设计

为了直观展示上传状态，我们在 XAML 中放置一个触发按钮和一个用于显示进度信息的文本块。

<UserControl x:Class="SilverlightUploader.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <StackPanel Orientation="Vertical" Margin="20">
        <Button x:Name="BtnSelectAndUpload" Content="选择并上传文件" Width="160" Height="40" Click="BtnSelectAndUpload_Click"/>
        <TextBlock x:Name="TxtProgress" Margin="0,15,0,0" Text="准备就绪" FontSize="14"/>
    </StackPanel>
</UserControl>

客户端业务逻辑与流式分块读取

原生的实现方式往往会将整个文件读取到内存中的集合里，这在处理大文件时极易引发 OutOfMemoryException。以下重构后的逻辑采用 Stream 游标机制，每次仅从磁盘读取固定大小的字节块（如 4KB）进行异步发送，大幅降低了客户端的内存压力。

public partial class MainPage : UserControl
{
    private const int ChunkSize = 4096; // 每次上传 4KB
    private FileUploadState _uploadState;

    public MainPage()
    {
        InitializeComponent();
    }

    private void BtnSelectAndUpload_Click(object sender, RoutedEventArgs e)
    {
        OpenFileDialog dialog = new OpenFileDialog
        {
            Filter = "所有文件 (*.*)|*.*",
            Multiselect = false
        };

        if (dialog.ShowDialog() == true)
        {
            FileInfo selectedFile = dialog.File;
            _uploadState = new FileUploadState
            {
                FileName = selectedFile.Name,
                TotalBytes = selectedFile.Length,
                UploadedBytes = 0,
                FileStream = selectedFile.OpenRead()
            };

            // 开始上传第一个数据块
            SendNextChunk(false);
        }
    }

    private void SendNextChunk(bool isAppend)
    {
        byte[] buffer = new byte[ChunkSize];
        int bytesRead = _uploadState.FileStream.Read(buffer, 0, ChunkSize);
        _uploadState.LastChunkSize = bytesRead;

        // 如果读取字节数为 0，说明文件已传输完毕
        if (bytesRead == 0)
        {
            _uploadState.FileStream.Close();
            TxtProgress.Text = "上传完成!";
            BtnSelectAndUpload.IsEnabled = true;
            return;
        }

        // 截取实际读取到的字节数组
        byte[] chunkToSend = new byte[bytesRead];
        Array.Copy(buffer, chunkToSend, bytesRead);

        var client = new FileTransferServiceClient();
        client.UploadChunkCompleted += Client_UploadChunkCompleted;
        
        BtnSelectAndUpload.IsEnabled = false;
        client.UploadChunkAsync(_uploadState.FileName, chunkToSend, isAppend, _uploadState);
    }

    private void Client_UploadChunkCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e)
    {
        if (e.Error != null)
        {
            TxtProgress.Text = "上传出错: " + e.Error.Message;
            _uploadState.FileStream.Close();
            BtnSelectAndUpload.IsEnabled = true;
            return;
        }

        _uploadState = e.UserState as FileUploadState;
        _uploadState.UploadedBytes += _uploadState.LastChunkSize;

        // 计算并更新进度显示
        double progress = (_uploadState.UploadedBytes * 100.0) / _uploadState.TotalBytes;
        TxtProgress.Text = $"进度: {progress:F2}% ({_uploadState.UploadedBytes} / {_uploadState.TotalBytes} 字节)";

        // 继续递归上传下一个数据块
        SendNextChunk(true);
    }
}

/// <summary>
/// 维护单次上传任务的状态上下文
/// </summary>
public class FileUploadState
{
    public string FileName { get; set; }
    public long TotalBytes { get; set; }
    public long UploadedBytes { get; set; }
    public System.IO.Stream FileStream { get; set; }
    public int LastChunkSize { get; set; }
}