是否可以覆盖 MultipartFormDataStreamProvider 以便不将上传保存到文件系统? [英] Is it possible to override MultipartFormDataStreamProvider so that is doesn't save uploads to the file system?

问题描述

我有一个 ASP.Net Web API 应用程序，它允许客户端(html 页面和 iPhone 应用程序)将图像上传到.我正在使用此 中所述的异步上传任务文章.

I have an ASP.Net Web API application that allows clients (html pages and iPhone apps) to upload images to. I am using an async upload task as described in this article.

当我想保存到文件系统时，一切都很好，因为这就是这段代码在幕后自动执行的操作.但是，我不想将上传的文件保存到文件系统.相反，我想使用适用于 .Net 的 AWS 开发工具包将上传的流传递到 Amazon S3 存储桶.

Everything works great when I want to save to the file system because that's what this code does automatically, behind the scenes it seems. But, I don't want to save the uploaded files to the file system. Instead, I want to take the uploaded stream and pass it through to an Amazon S3 bucket using the AWS SDK for .Net.

我已经设置了将流发送到 AWS 的代码.我想不通的问题是如何从 Web API 方法获取上传的内容流，而不是让它自动保存到磁盘.

I have the code set up to send the stream up to AWS. The problem I can't figure out is how to get the uploaded content stream from the Web API method instead of having it automatically save to disk.

我希望有一个我可以在 MultipartFormDataStreamProvider 中覆盖的虚拟方法，它允许我对上传的内容做一些其他的事情而不是保存到磁盘，但似乎没有.

I was hoping there would be a virtual method I could override in MultipartFormDataStreamProvider which would allow me to do something else with the uploaded content other than save to disk, but there doesn't seem to be.

有什么建议吗?

推荐答案

您可以覆盖 MultipartFormDataStreamProvider 的 GetStream 方法以返回一个流，该流不是文件流而是您的 AWS 流，但是这样做会出现一些问题(我不会此处详述).相反，您可以创建一个派生自抽象基类 MultipartStreamProvider 的提供程序.以下示例很大程度上基于 MultipartFormDataStreamProvider 和 MultipartFileStreamProvider 的实际源代码.您可以查看这里 和 此处 了解更多详情.示例如下:

You could override MultipartFormDataStreamProvider's GetStream method to return a stream which is not a file stream but your AWS stream, but there are some issues doing so(which I will not elaborate here). Instead you could create a provider deriving from the abstract base class MultipartStreamProvider. Following sample is heavily based on the actual source code of MultipartFormDataStreamProvider and MultipartFileStreamProvider. You can check here and here for more details. Sample below:

public class CustomMultipartFormDataStreamProvider : MultipartStreamProvider
{
    private NameValueCollection _formData = new NameValueCollection(StringComparer.OrdinalIgnoreCase);

    private Collection<bool> _isFormData = new Collection<bool>();

    private Collection<MyMultipartFileData> _fileData = new Collection<MyMultipartFileData>();

    public NameValueCollection FormData
    {
        get { return _formData; }
    }

    public Collection<MultipartFileData> FileData
    {
        get { return _fileData; }
    }

    public override Stream GetStream(HttpContent parent, HttpContentHeaders headers)
    {
        // For form data, Content-Disposition header is a requirement
        ContentDispositionHeaderValue contentDisposition = headers.ContentDisposition;
        if (contentDisposition != null)
        {
            // If we have a file name then write contents out to AWS stream. Otherwise just write to MemoryStream
            if (!String.IsNullOrEmpty(contentDisposition.FileName))
            {
                // We won't post process files as form data
                _isFormData.Add(false);

                 MyMultipartFileData fileData = new MyMultipartFileData(headers, your-aws-filelocation-url-maybe);
                 _fileData.Add(fileData);

                return myAWSStream;//**return you AWS stream here**
            }

            // We will post process this as form data
            _isFormData.Add(true);

            // If no filename parameter was found in the Content-Disposition header then return a memory stream.
            return new MemoryStream();
        }

        throw new InvalidOperationException("Did not find required 'Content-Disposition' header field in MIME multipart body part..");
    }

    /// <summary>
    /// Read the non-file contents as form data.
    /// </summary>
    /// <returns></returns>
    public override async Task ExecutePostProcessingAsync()
    {
        // Find instances of HttpContent for which we created a memory stream and read them asynchronously
        // to get the string content and then add that as form data
        for (int index = 0; index < Contents.Count; index++)
        {
            if (_isFormData[index])
            {
                HttpContent formContent = Contents[index];
                // Extract name from Content-Disposition header. We know from earlier that the header is present.
                ContentDispositionHeaderValue contentDisposition = formContent.Headers.ContentDisposition;
                string formFieldName = UnquoteToken(contentDisposition.Name) ?? String.Empty;

                // Read the contents as string data and add to form data
                string formFieldValue = await formContent.ReadAsStringAsync();
                FormData.Add(formFieldName, formFieldValue);
            }
        }
    }

    /// <summary>
    /// Remove bounding quotes on a token if present
    /// </summary>
    /// <param name="token">Token to unquote.</param>
    /// <returns>Unquoted token.</returns>
    private static string UnquoteToken(string token)
    {
        if (String.IsNullOrWhiteSpace(token))
        {
            return token;
        }

        if (token.StartsWith(""", StringComparison.Ordinal) && token.EndsWith(""", StringComparison.Ordinal) && token.Length > 1)
        {
            return token.Substring(1, token.Length - 2);
        }

        return token;
    }
}

public class MyMultipartFileData
{
    public MultipartFileData(HttpContentHeaders headers, string awsFileUrl)
    {
        Headers = headers;
        AwsFileUrl = awsFileUrl;
    }

    public HttpContentHeaders Headers { get; private set; }

    public string AwsFileUrl { get; private set; }
}

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