使用Blazor WASM实现可取消的多文件带校验并发分片上传

前言

上传大文件时，原始HTTP文件上传功能可能会影响使用体验，此时使用分片上传功能可以有效避免原始上传的弊端。由于分片上传不是HTTP标准的一部分，所以只能自行开发相互配合的服务端和客户端。文件分片上传在许多情况时都拥有很多好处，除非已知需要上传的文件一定非常小。分片上传可以对上传的文件进行快速分片校验，避免大文件上传时长时间等待校验，当然完整校验可以在秒传时使用，有这种需求的情况就只能老实等待校验了。

Blazr WASM提供了在 .NET环境中使用浏览器功能的能力，充分利用C#和 .NET能够大幅简化分片上传功能的开发。本次示例使用HTTP标准上传作为分片上传的底层基础，并提供分片校验功能保障上传数据的完整性。

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正文

本示例的Blazor代码位于默认ASP.NET Core托管的Blazor WASM应用模板的Index页面。

在Shared项目添加公共数据模型

/// 
/// 文件分片上传输入模型
/// 
public class FileChunkUploadInput
{
    /// 
    /// 上传任务代码
    /// 
    public string? UploadTaskCode { get; set; }
 
    /// 
    /// 上传请求类型
    /// 
    public string UploadType { get; set; } = null!;
 
    /// 
    /// 文件名
    /// 
    public string FileName { get; set; } = null!;
 
    /// 
    /// 文件大小
    /// 
    public long? FileSize { get; set; }
 
    /// 
    /// 支持的Hash算法，优选算法请靠前
    /// 
    public List<string>? AllowedHashAlgorithm { get; set; }
 
    /// 
    /// 使用的Hash算法
    /// 
    public string? HashAlgorithm { get; set; }
 
    /// 
    /// Hash值
    /// 
    public string? HashValue { get; set; }
 
    /// 
    /// 文件分片数量
    /// 
    public int FileChunkCount { get; set; }
 
    /// 
    /// 文件片段大小
    /// 
    public int? FileChunkSize { get; set; }
 
    /// 
    /// 文件片段偏移量（相对于整个文件）
    /// 
    public long? FileChunkOffset { get; set; }
 
    /// 
    /// 文件片段索引
    /// 
    public int? FileChunkIndex { get; set; }
 
    /// 
    /// 取消上传的原因
    /// 
    public string? CancelReason { get; set; }
}
 
/// 
/// 文件分片上传开始结果
/// 
public class FileChunkUploadStartReault
{
    /// 
    /// 上传任务代码
    /// 
    public string UploadTaskCode { get; set; } = null!;
 
    /// 
    /// 选中的Hash算法
    /// 
    public string SelectedHashAlgorithm { get; set; } = null!;
}
 
/// 
/// Hash助手
/// 
public static class HashHelper
{
    /// 
    /// 把Hash的字节数组转换为16进制字符串表示
    /// 
    /// 原始Hash值
    /// Hash值的16进制文本表示（大写）
    public static string ToHexString(this byte[] bytes)
    {
        StringBuilder sb = new(bytes.Length * 2);
        foreach (var @byte in bytes)
        {
            sb.Append(@byte.ToString("X2"));
        }
        return sb.ToString();
    }
}

服务端控制器

[ApiController]
[Route("[controller]")]
public class UploadController : ControllerBase
{
    /// 
    /// 支持的Hash算法，优选算法请靠前
    /// 
    private static string[] supportedHashAlgorithm = new[] { "MD5", "SHA1", "SHA256" };
 
    /// 
    /// 文件写入锁的线程安全字典，每个上传任务对应一把锁
    /// 
    private static readonly ConcurrentDictionary<string, AsyncLock> fileWriteLockerDict = new();
 
    private readonly ILogger _logger;
    private readonly IWebHostEnvironment _env;
 
    public UploadController(ILogger logger, IWebHostEnvironment env)
    {
        _logger = logger;
        _env = env;
    }
 
    /// 
    /// 分片上传动作
    /// 
    /// 上传表单
    /// 文件片段数据
    /// 请求取消令牌
    /// 片段上传结果
    [HttpPost, RequestSizeLimit(1024 * 1024 * 11)]
    [ProducesResponseType(StatusCodes.Status200OK)]
    [ProducesResponseType(StatusCodes.Status400BadRequest)]
    [ProducesDefaultResponseType]
    public async Task Upload( 
        [FromForm]FileChunkUploadInput input,
        [FromForm]IFormFile? fileChunkData,
        CancellationToken requestAborted)
    {
        switch (input.UploadType)
        {
            // 请求开始一个新的上传任务，协商上传参数
            case "startUpload":
                {
                    //var trustedFileNameForDisplay =
                    //    WebUtility.HtmlEncode(fileChunkData?.FileName ?? input.FileName);
 
                    // 选择双方都支持的优选Hash算法
                    var selectedHashAlgorithm = supportedHashAlgorithm
                        .Intersect(input.AllowedHashAlgorithm ?? Enumerable.Empty<string>())
                        .FirstOrDefault();
 
                    // 验证必要的表单数据
                    if (selectedHashAlgorithm is null or "")
                    {
                        ModelState.AddModelError(x => x.AllowedHashAlgorithm, "can not select hash algorithm");
                    }
 
                    if (input.FileSize is null)
                    {
                        ModelState.AddModelError(x => x.FileSize, "must have value for start、upload and complete");
                    }
 
                    if (ModelState.ErrorCount > 0)
                    {
                        return ValidationProblem(ModelState);
                    }
 
                    // 使用随机文件名提高安全性，并把文件名作为任务代码使用
                    var trustedFileNameForFileStorage = Path.GetRandomFileName();
 
                    var savePath = Path.Combine(
                        _env.ContentRootPath,
                        _env.EnvironmentName,
                        "unsafe_uploads",
                        trustedFileNameForFileStorage);
 
                    var savePathWithFile = Path.Combine(
                        savePath,
                        $"{input.FileName}.tmp");
 
                    if (!Directory.Exists(savePath))
                    {
                        Directory.CreateDirectory(savePath);
                    }
 
                    // 根据表单创建对应大小的文件
                    await using (var fs = new FileStream(savePathWithFile, FileMode.Create))
                    {
                        fs.SetLength(input.FileSize!.Value);
                        await fs.FlushAsync();
                    }
 
                    // 设置锁
                    fileWriteLockerDict.TryAdd(trustedFileNameForFileStorage, new());
 
                    // 返回协商结果
                    return Ok(new FileChunkUploadStartReault
                    {
                        UploadTaskCode = trustedFileNameForFileStorage,
                        SelectedHashAlgorithm = selectedHashAlgorithm!
                    });
                }
 
            // 上传文件片段
            case "uploadChunk":
                // 验证表单
                if (!fileWriteLockerDict.TryGetValue(input.UploadTaskCode!, out var _))
                {
                    ModelState.AddModelError(x => x.UploadTaskCode, $"file upload task with code {input.UploadTaskCode} is not exists");
                    return ValidationProblem(ModelState);
                }
 
                // 使用内存池缓冲数据，注意使用using释放内存
                using (var pooledMemory = MemoryPool<byte>.Shared.Rent((int)fileChunkData!.Length))
                {
                    // 使用切片语法获取精准大小的内存缓冲区装载上传的数据
                    var buffer = pooledMemory.Memory[..(int)fileChunkData!.Length];
                    var readBytes = await fileChunkData.OpenReadStream().ReadAsync(buffer, requestAborted);
                    var readBuffer = buffer[..readBytes];
 
                    Debug.Assert(readBytes == fileChunkData!.Length);
 
                    // 校验Hash
                    var hash = input.HashAlgorithm switch
                    {
                        "SHA1" => SHA1.HashData(readBuffer.Span),
                        "SHA256" => SHA256.HashData(readBuffer.Span),
                        "MD5" => MD5.HashData(readBuffer.Span),
                        _ => Array.Empty<byte>()
                    };
 
                    if (hash.ToHexString() != input.HashValue)
                    {
                        ModelState.AddModelError(x => x.HashValue, "hash does not match");
                        return ValidationProblem(ModelState);
                    }
 
                    var savePath = Path.Combine(
                        _env.ContentRootPath,
                        _env.EnvironmentName,
                        "unsafe_uploads",
                        input.UploadTaskCode!);
 
                    var savePathWithFile = Path.Combine(
                        savePath,
                        $"{input.FileName}.tmp");
 
                    // 使用锁写入数据，文件流不支持写共享，必须串行化
                    if(fileWriteLockerDict.TryGetValue(input.UploadTaskCode!, out var locker))
                    {
                        using (await locker.LockAsync())
                        {
                            await using (var fs = new FileStream(savePathWithFile, FileMode.Open, FileAccess.Write))
                            {
                                // 定位文件流
                                fs.Seek(input.FileChunkOffset!.Value, SeekOrigin.Begin);
 
                                await fs.WriteAsync(readBuffer, requestAborted);
                                await fs.FlushAsync();
                            }
                        }
                    }
                }
 
                return Ok();
 
            // 取消上传
            case "cancelUpload":
                // 验证表单
                if (!fileWriteLockerDict.TryGetValue(input.UploadTaskCode!, out var _))
                {
                    ModelState.AddModelError(x => x.UploadTaskCode, $"file upload task with code {input.UploadTaskCode} is not exists");
                    return ValidationProblem(ModelState);
                }
 
                {
                    var deletePath = Path.Combine(
                        _env.ContentRootPath,
                        _env.EnvironmentName,
                        "unsafe_uploads",
                        input.UploadTaskCode!);
 
                    // 删除文件，清除锁
                    if (fileWriteLockerDict.TryGetValue(input.UploadTaskCode!, out var locker))
                    {
                        using (await locker.LockAsync())
                        {
                            if (Directory.Exists(deletePath))
                            {
                                var dir = new DirectoryInfo(deletePath);
                                dir.Delete(true);
                            }
 
                            fileWriteLockerDict.TryRemove(input.UploadTaskCode!, out _);
                        }
                    }
                }
 
                return Ok();
 
            // 完成上传
            case "completeUpload":
                // 验证表单
                if (!fileWriteLockerDict.TryGetValue(input.UploadTaskCode!, out var _))
                {
                    ModelState.AddModelError(x => x.UploadTaskCode, $"file upload task with code {input.UploadTaskCode} is not exists");
                    return ValidationProblem(ModelState);
                }
 
                {
                    var savePath = Path.Combine(
                        _env.ContentRootPath,
                        _env.EnvironmentName,
                        "unsafe_uploads",
                        input.UploadTaskCode!);
 
                    // 去除文件的临时扩展名，清除锁
                    var savePathWithFile = Path.Combine(savePath, $"{input.FileName}.tmp");
 
                    var fi = new FileInfo(savePathWithFile);
                    fi.MoveTo(Path.Combine(savePath, input.FileName));
 
                    fileWriteLockerDict.TryRemove(input.UploadTaskCode!, out _);
                }
 
                return Ok();
            default:
                return BadRequest();
        }
    }
}

服务端使用三段式上传模式，开始上传，上传数据，完成（取消）上传。开始上传负责协商Hash算法和分配任务代码；上传数据负责具体的传输，并通过表单提供附加信息方便服务端操作。完成上传负责善后和资源清理。其中文件写入的异步锁使用Nito.AsyncEx代替不支持在异步中使用的lock语句。

页面代码（Index.razor）,在结尾追加

支持随时取消的多文件并行分片上传，示例同时上传2个文件，每个文件同时上传2个分片，合计同时上传4个分片
"UploadFile" multiple>
"async (MouseEventArgs e) => uploadCancelSource?.Cancel()">取消上传
 
@code{
    [Inject] private HttpClient _http { get; init; } = null!;
    [Inject] private ILogger _logger { get; init; } = null!;
 
    private CancellationTokenSource? uploadCancelSource;
 
    /// 
    /// 上传文件
    /// 
    /// 上传文件的事件参数
    /// 
    private async Task UploadFile(InputFileChangeEventArgs args)
    {
        // 设置文件并发选项
        var parallelCts = new CancellationTokenSource();
        uploadCancelSource = parallelCts;
        var parallelOption = new ParallelOptions
        {
            MaxDegreeOfParallelism = 2,
            CancellationToken = parallelCts.Token
        };
 
        // 并发上传所有文件
        await Parallel.ForEachAsync(
            args.GetMultipleFiles(int.MaxValue),
            parallelOption,
            async (file, cancellation) =>
            {
                // 这里的取消令牌是并发方法创建的，和并发选项里的令牌不是一个
                if (cancellation.IsCancellationRequested)
                {
                    parallelCts.Cancel();
                    return;
                }
 
                // 使用链接令牌确保外部取消能传递到内部
                var chunkUploadResult = await UploadChunkedFile(
                    file,
                    CancellationTokenSource.CreateLinkedTokenSource(
                        parallelCts.Token,
                        cancellation
                    ).Token
                );
 
                // 如果上传不成功则取消后续上传
                if (chunkUploadResult != FileUploadResult.Success)
                {
                    parallelCts.Cancel();
                    return;
                }
            }
        );
    }
 
    /// 
    /// 分片上传文件
    /// 
    /// 要上传的文件
    /// 取消令牌
    /// 上传结果
    private async Task UploadChunkedFile(IBrowserFile file, CancellationToken cancellation = default)
    {
        if (cancellation.IsCancellationRequested) return FileUploadResult.Canceled;
 
        _logger.LogInformation("开始上传文件：{0}", file.Name);
 
        // 计算分片大小，文件小于10MB分片1MB，大于100MB分片10MB，在其间则使用不超过10片时的所需大小
        var coefficient = file.Size switch
        {
            <= 1024 * 1024 * 10 => 1,
            > 1024 * 1024 * 10 and <= 1024 * 1024 *100 => (int)Math.Ceiling(file.Size / (1024.0 * 1024) / 10),
            _ => 10
        };
 
        // 初始化分片参数，准备字符串格式的数据供表单使用
        var bufferSize = 1024 * 1024 * coefficient; // MB
        var stringBufferSize = bufferSize.ToString();
        var chunkCount = (int)Math.Ceiling(file.Size / (double)bufferSize);
        var stringChunkCount = chunkCount.ToString();
        var stringFileSize = file.Size.ToString();
 
        // 发起分片上传，协商Hash算法，获取任务代码
        var uploadStartContent = new Liststring, string>>
        {
            new("uploadType", "startUpload"),
            new("fileName", file.Name),
            new("fileSize", stringFileSize),
            new("allowedHashAlgorithm", "SHA1"),
            new("allowedHashAlgorithm", "SHA256"),
            new("fileChunkCount", stringChunkCount),
            new("fileChunkSize", stringBufferSize),
        };
 
        var uploadStartForm = new FormUrlEncodedContent(uploadStartContent);
 
        HttpResponseMessage? uploadStartResponse = null;
        try
        {
            uploadStartResponse = await _http.PostAsync("/upload", uploadStartForm, cancellation);
        }
        catch(TaskCanceledException e)
        {
            _logger.LogWarning(e, "外部取消上传，已停止文件：{0} 的上传", file.Name);
            return FileUploadResult.Canceled;
        }
        catch(Exception e)
        {
            _logger.LogError(e, "文件：{0} 的上传参数协商失败", file.Name);
            return FileUploadResult.Fail;
        }
 
        // 如果服务器响应失败，结束上传
        if (uploadStartResponse?.IsSuccessStatusCode is null or false)
        {
            _logger.LogError("文件：{0} 的上传参数协商失败", file.Name);
            return FileUploadResult.Fail;
        }
 
        // 解析协商的参数
        var uploadStartReault = await uploadStartResponse.Content.ReadFromJsonAsync();
        var uploadTaskCode = uploadStartReault!.UploadTaskCode;
        var selectedHashAlgorithm = uploadStartReault!.SelectedHashAlgorithm;
 
        _logger.LogInformation("文件：{0} 的上传参数协商成功", file.Name);
 
        // 设置分片并发选项
        var parallelOption = new ParallelOptions
        {
            MaxDegreeOfParallelism = 2,
        };
 
        var fileUploadCancelSource = new CancellationTokenSource();
        var sliceEnumeratorCancelSource = CancellationTokenSource
            .CreateLinkedTokenSource(
                cancellation,
                fileUploadCancelSource.Token
            );
        // 各个分片的上传结果
        var sliceUploadResults = new FileUploadResult?[chunkCount];
        // 并发上传各个分片，并发循环本身不能用并发选项的取消令牌取消，可能会导致内存泄漏，应该通过切片循环的取消使并发循环因没有可用元素自然结束
        await Parallel.ForEachAsync(
            SliceFileAsync(
                file,
                bufferSize,
                sliceEnumeratorCancelSource.Token
            ),
            parallelOption,
            async (fileSlice, sliceUploadCancel) =>
            {
                // 解构参数
                var (memory, sliceIndex, readBytes, fileOffset) = fileSlice;
 
                // 使用using确保结束后把租用的内存归还给内存池
                using (memory)
                {
                    var stringSliceIndex = sliceIndex.ToString();
 
                    // 主动取消上传，发送取消请求，通知服务端清理资源
                    if (sliceUploadCancel.IsCancellationRequested)
                    {
                        _logger.LogWarning("外部取消上传，已停止文件：{0} 的上传", file.Name);
 
                        fileUploadCancelSource.Cancel();
                        sliceUploadResults[sliceIndex] = FileUploadResult.Canceled;
 
                        var uploadCancelContent = new Dictionary<string, string>()
                        {
                            {"uploadType", "cancelUpload"},
                            {"uploadTaskCode", uploadTaskCode!},
                            {"fileName", file.Name},
                            {"hashAlgorithm", selectedHashAlgorithm},
                            {"fileChunkCount", stringChunkCount},
                            {"fileChunkIndex", stringSliceIndex},
                            {"cancelReason", "调用方要求取消上传。"},
                        };
                        var uploadCancelForm = new FormUrlEncodedContent(uploadCancelContent);
                        var uploadCancelResponse = await _http.PostAsync("/upload", uploadCancelForm);
 
                        return;
                    }
 
                    // 当前上传分片索引应当小于预计的分片数
                    Debug.Assert(sliceIndex < chunkCount);
 
                    // 获取准确大小的缓冲区，从内存池租用时得到的容量可能大于申请的大小，使用C#的新集合切片语法
                    var readBuffer = memory.Memory[..readBytes];
 
                    var sw = Stopwatch.StartNew();
                    // 根据协商的算法计算Hash，wasm环境不支持MD5和全部非对称加密算法
                    var hash = selectedHashAlgorithm switch
                    {
                        "SHA1" => SHA1.HashData(readBuffer.Span),
                        "SHA256" => SHA256.HashData(readBuffer.Span),
                        _ => Array.Empty<byte>()
                    };
                    sw.Stop();
 
                    _logger.LogInformation("文件：{0} 的片段 {1}({2} Bytes) 计算Hash用时 {3}", file.Name, sliceIndex, readBytes, sw.Elapsed);
 
                    var stringReadBytes = readBytes.ToString();
                    var stringFileOffset = fileOffset.ToString();
 
                    // 上传当前分片
                    MultipartFormDataContent uploadFileForm = new();
                    uploadFileForm.Add(new StringContent(uploadTaskCode!), "uploadTaskCode");
                    uploadFileForm.Add(new StringContent("uploadChunk"), "uploadType");
                    uploadFileForm.Add(new StringContent(file.Name), "fileName");
                    uploadFileForm.Add(new StringContent(stringFileSize), "fileSize");
                    uploadFileForm.Add(new StringContent(selectedHashAlgorithm!), "hashAlgorithm");
                    uploadFileForm.Add(new StringContent(hash.ToHexString()), "hashValue");
                    uploadFileForm.Add(new StringContent(stringChunkCount), "fileChunkCount");
                    uploadFileForm.Add(new StringContent(stringReadBytes), "fileChunkSize");
                    uploadFileForm.Add(new StringContent(stringFileOffset), "fileChunkOffset");
                    uploadFileForm.Add(new StringContent(stringSliceIndex), "fileChunkIndex");
 
                    // 如果是未知的文件类型，设置为普通二进制流的MIME类型
                    var fileChunk = new ReadOnlyMemoryContent(readBuffer);
                    fileChunk.Headers.ContentType = new MediaTypeHeaderValue(string.IsNullOrEmpty(file.ContentType) ? "application/octet-stream" : file.ContentType);
                    uploadFileForm.Add(fileChunk, "fileChunkData", file.Name);
 
                    HttpResponseMessage? uploadResponse = null;
                    try
                    {
                        var uploadTaskCancel = CancellationTokenSource
                            .CreateLinkedTokenSource(
                                sliceUploadCancel,
                                sliceEnumeratorCancelSource.Token
                            );
 
                        _logger.LogInformation("文件：{0} 的片段 {1}({2} Bytes) 开始上传", file.Name, sliceIndex, readBytes);
 
                        sw.Restart();
                        uploadResponse = await _http.PostAsync("/upload", uploadFileForm, uploadTaskCancel.Token);
                    }
                    catch (TaskCanceledException e)
                    {
                        _logger.LogWarning(e, "外部取消上传，已停止文件：{0} 的上传", file.Name);
 
                        fileUploadCancelSource.Cancel();
                        sliceUploadResults[sliceIndex] = FileUploadResult.Canceled;
 
                        var uploadCancelContent = new Dictionary<string, string>()
                        {
                            {"uploadType", "cancelUpload"},
                            {"uploadTaskCode", uploadTaskCode!},
                            {"fileName", file.Name},
                            {"hashAlgorithm", selectedHashAlgorithm},
                            {"fileChunkCount", stringChunkCount},
                            {"fileChunkIndex", stringSliceIndex},
                            {"cancelReason", "调用方要求取消上传。"},
                        };
                        var uploadCancelForm = new FormUrlEncodedContent(uploadCancelContent);
                        var uploadCancelResponse = await _http.PostAsync("/upload", uploadCancelForm);
 
                        return;
                    }
                    catch (Exception e)
                    {
                        _logger.LogError(e, "上传发生错误，已停止文件：{0} 的上传", file.Name);
 
                        fileUploadCancelSource.Cancel();
                        sliceUploadResults[sliceIndex] = FileUploadResult.Fail;
 
                        var uploadCancelContent = new Dictionary<string, string>()
                        {
                            {"uploadType", "cancelUpload"},
                            {"uploadTaskCode", uploadTaskCode!},
                            {"fileName", file.Name},
                            {"hashAlgorithm", selectedHashAlgorithm},
                            {"fileChunkCount", stringChunkCount},
                            {"fileChunkSize", stringReadBytes},
                            {"fileChunkOffset", stringFileOffset},
                            {"fileChunkIndex", stringSliceIndex},
                            {"cancelReason", "上传过程中发生错误。"},
                        };
                        var uploadCancelForm = new FormUrlEncodedContent(uploadCancelContent);
                        var uploadCancelResponse = await _http.PostAsync("/upload", uploadCancelForm);
 
                        return;
                    }
                    finally
                    {
                        sw.Stop();
                    }
 
                    // 上传发生错误，发送取消请求，通知服务端清理资源
                    if (uploadResponse?.IsSuccessStatusCode is null or false)
                    {
                        _logger.LogError("上传发生错误，已停止文件：{0} 的上传", file.Name);
 
                        fileUploadCancelSource.Cancel();
                        sliceUploadResults[sliceIndex] = FileUploadResult.Fail;
 
                        var uploadCancelContent = new Dictionary<string, string>()
                        {
                            {"uploadType", "cancelUpload"},
                            {"uploadTaskCode", uploadTaskCode!},
                            {"fileName", file.Name},
                            {"hashAlgorithm", selectedHashAlgorithm},
                            {"fileChunkCount", stringChunkCount},
                            {"fileChunkSize", stringReadBytes},
                            {"fileChunkOffset", stringFileOffset},
                            {"fileChunkIndex", stringSliceIndex},
                            {"cancelReason", "上传过程中发生错误。"},
                        };
                        var uploadCancelForm = new FormUrlEncodedContent(uploadCancelContent);
                        var uploadCancelResponse = await _http.PostAsync("/upload", uploadCancelForm);
 
                        return;
                    }
 
                    _logger.LogInformation("文件：{0} 的片段 {1}({2} Bytes) 上传成功，用时 {3}", file.Name, sliceIndex, readBytes, sw.Elapsed);
 
                    sliceUploadResults[sliceIndex] = FileUploadResult.Success;
                }
            }
        );
 
        // 如果所有分片都上传成功，则发送完成请求完成上传
        if (sliceUploadResults.All(success => success is FileUploadResult.Success))
        {
            var uploadCompleteContent = new Dictionary<string, string>()
            {
                {"uploadType", "completeUpload"},
                {"uploadTaskCode", uploadTaskCode!},
                {"fileName", file.Name},
                {"fileSize", stringFileSize},
                {"hashAlgorithm", selectedHashAlgorithm},
                {"fileChunkCount", stringChunkCount},
                {"fileChunkSize", stringBufferSize},
            };
            var uploadCompleteForm = new FormUrlEncodedContent(uploadCompleteContent);
            var uploadCompleteResponse = await _http.PostAsync("/upload", uploadCompleteForm);
 
            if (uploadCompleteResponse.IsSuccessStatusCode)
            {
                _logger.LogInformation("文件：{0} 上传成功，共 {1} 个片段", file.Name, chunkCount);
                return FileUploadResult.Success;
            }
            else
            {
                _logger.LogError("上传发生错误，已停止文件：{0} 的上传", file.Name);
 
                var uploadCancelContent = new Dictionary<string, string>()
                {
                    {"uploadType", "cancelUpload"},
                    {"uploadTaskCode", uploadTaskCode!},
                    {"fileName", file.Name},
                    {"hashAlgorithm", selectedHashAlgorithm},
                    {"fileChunkCount", stringChunkCount},
                    {"cancelReason", "上传过程中发生错误。"},
                };
                var uploadCancelForm = new FormUrlEncodedContent(uploadCancelContent);
                var uploadCancelResponse = await _http.PostAsync("/upload", uploadCancelForm);
 
                return FileUploadResult.Fail;
            }
        }
        else if (sliceUploadResults.Any(success => success is FileUploadResult.Fail))
        {
            return FileUploadResult.Fail;
        }
        else
        {
            return FileUploadResult.Canceled;
        }
    }
 
    /// 
    /// 异步切分要上传的文件
    /// 
如果想中途结束切分，不要在调用此方法的foreach块中使用break，请使用取消令牌，否则会出现内存泄漏
    /// 
    /// 要分片的文件
    /// 分片大小
    /// 取消令牌
    /// 已切分的文件片段数据，用完切记释放其中的内存缓冲
    private static async IAsyncEnumerable<(IMemoryOwner<byte> memory, int sliceIndex, int readBytes, long fileOffset)> SliceFileAsync(
        IBrowserFile file,
        int sliceSize,
        [EnumeratorCancellation] CancellationToken cancellation = default)
    {
        if (cancellation.IsCancellationRequested) yield break;
 
        int fileSliceIndex;
        long fileOffset;
        IMemoryOwner<byte> memory;
        await using var fileStream = file.OpenReadStream(long.MaxValue);
 
        for (fileSliceIndex = 0, fileOffset = 0, memory = MemoryPool<byte>.Shared.Rent(sliceSize);
            (await fileStream.ReadAsync(memory.Memory[..sliceSize], cancellation)) is int readBytes and > 0;
            fileSliceIndex++, fileOffset += readBytes, memory = MemoryPool<byte>.Shared.Rent(sliceSize)
        )
        {
            if(cancellation.IsCancellationRequested)
            {
                // 如果取消切分，缓冲不会返回到外部，只能在内部释放
                memory.Dispose();
                yield break;
            }
            yield return (memory, fileSliceIndex, readBytes, fileOffset);
        }
        // 切分结束后会多出一个没用的缓冲，只能在内部释放
        memory.Dispose();
    }
 
    /// 
    /// 上传结果
    /// 
    public enum FileUploadResult
    {
        /// 
        /// 失败
        /// 
        Fail = -2,
 
        /// 
        /// 取消
        /// 
        Canceled = -1,
 
        /// 
        /// 没有结果，未知结果
        /// 
        None = 0,
 
        /// 
        /// 成功
        /// 
        Success = 1
    }
}

示例使用Parallel.ForEachAsync方法并行启动多个文件和每个文件的多个片段的上传，并发量由方法的参数控制。UploadChunkedFile方法负责单个文件的上传，其中的IBrowserFile类型是.NET 6新增的文件选择框选中项的包装，可以使用其中的OpenReadStream方法流式读取文件数据，确保大文件上传不会在内存中缓冲所有数据导致内存占用问题。

UploadChunkedFile方法内部使用自适应分片大小算法，规则为片段最小1MB，最大10MB，尽可能平均分为10份。得出片段大小后向服务端请求开始上传文件，服务端成功返回后开始文件切分、校验和上传。

SliceFileAsync负责切分文件并流式返回每个片段，切分方法是惰性的，所以不用担心占用大量内存，但是这个方法只能使用取消令牌中断切分，如果在调用该方法的await foreach块中使用break中断会产生内存泄漏。切分完成后会返回包含片段数据的内存缓冲和其他附加信息。OpenReadStream需要使用参数控制允许读取的最大字节数（默认512KB），因为这里是分片上传，直接设置为long.MaxValue即可。for循环头使用逗号表达式定义多个循环操作，使循环体的代码清晰简洁。

UploadChunkedFile方法使用Parallel.ForEachAsync并行启动多个片段的校验和上传，WASM中不支持MD5和所有非对称加密算法，需要注意。完成文件的并行上传或发生错误后会检查所有片段的上传情况，如果所有片段都上传成功，就发送完成上传请求通知服务端收尾善后，否则删除临时文件。

结语

这应该是一个比较清晰易懂的分片上传示例。示例使用Blazor 和C#以非常流畅的异步代码实现了并发分片上传。但是本示例依然有许多可优化的点，例如实现断点续传，服务端如果没有收到结束请求时的兜底处理等，这些就留给朋友们思考了。

又是很久没有写文章了，一直没有找到什么好选题，难得找到一个，经过将近1周的研究开发终于搞定了。

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本文地址：https://www.cnblogs.com/coredx/p/17746162.html