是否有可能重写MultipartFormDataStreamProvider，这样它就不会将上传保存到文件系统

您可以重写MultipartFormDataStreamProvider的GetStream方法来返回一个不是文件流而是AWS流的流，但是这样做会有一些问题(我不会在这里详细说明)。相反，您可以创建一个派生自抽象基类MultipartStreamProvider的提供程序。下面的示例主要基于MultipartFormDataStreamProvider和MultipartFileStreamProvider的实际源代码。您可以查看此处和此处了解更多详细信息。示例如下:

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; }
}

自从@KiranChalla发布了他们的答案后，在Fix 1760中引入了一个新的抽象类MultipartFormDataRemoteStreamProvider:使MultipartFormDataStreamProvider更容易处理非filestreams。为方便起见。

类的摘要很好地解释了如何使用它:

一个MultipartStreamProvider实现，适用于HTML文件上传，用于将文件内容写入远程存储Stream。流提供程序查看Content-Disposition报头字段，并根据文件名参数的存在确定一个输出远程Stream。如果在Content-Disposition报头字段中存在文件名参数，则主体部分将被写入GetRemoteStream提供的远程Stream。否则写入MemoryStream。