细谈 Web Api 图片上传，在使用 Task.ContinueWith 变量无法赋值问题的解决办法！

在使用Asp.Net Web Api 图片上传接口的时候，到网上找了一些个例子，但大多数找到都是这个版本！

[HttpPost]
public Task<Hashtable> ImgUpload()
{
// 检查是否是 multipart/form-data
if (!Request.Content.IsMimeMultipartContent("form-data"))
throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
//文件保存目录路径
string SaveTempPath = "~/SayPlaces/" + "/SayPic/SayPicTemp/";
String dirTempPath = HttpContext.Current.Server.MapPath(SaveTempPath);
// 设置上传目录
var provider = new MultipartFormDataStreamProvider(dirTempPath);
//var queryp = Request.GetQueryNameValuePairs();//获得查询字符串的键值集合
var task = Request.Content.ReadAsMultipartAsync(provider).
ContinueWith<Hashtable>(o =>
{
Hashtable hash = new Hashtable();
hash["error"] = 1;
hash["errmsg"] = "上传出错";
var file = provider.FileData[0];//provider.FormData
string orfilename = file.Headers.ContentDisposition.FileName.TrimStart('"').TrimEnd('"');
FileInfo fileinfo = new FileInfo(file.LocalFileName);
//最大文件大小
int maxSize = 10000000;
if (fileinfo.Length <= 0)
{
hash["error"] = 1;
hash["errmsg"] = "请选择上传文件。";
}
else if (fileinfo.Length > maxSize)
{
hash["error"] = 1;
hash["errmsg"] = "上传文件大小超过限制。";
}
else
{
string fileExt = orfilename.Substring(orfilename.LastIndexOf('.'));
//定义允许上传的文件扩展名
String fileTypes = "gif,jpg,jpeg,png,bmp";
if (String.IsNullOrEmpty(fileExt) || Array.IndexOf(fileTypes.Split(','), fileExt.Substring(1).ToLower()) == -1)
{
hash["error"] = 1;
hash["errmsg"] = "上传文件扩展名是不允许的扩展名。";
}
else
{
String ymd = DateTime.Now.ToString("yyyyMMdd", System.Globalization.DateTimeFormatInfo.InvariantInfo);
String newFileName = DateTime.Now.ToString("yyyyMMddHHmmss_ffff", System.Globalization.DateTimeFormatInfo.InvariantInfo);
fileinfo.CopyTo(Path.Combine(dirTempPath, newFileName + fileExt), true);
fileinfo.Delete();
hash["error"] = 0;
hash["errmsg"] = "上传成功";
}
}
return hash;
});
return task;
}

Web Api 图片上传，在使用 Task.ContinueWith 变量无法赋值问题

[HttpPost]

public Task<Hashtable> ImgUpload()

{

    // 检查是否是 multipart/form-data

    if (!Request.Content.IsMimeMultipartContent("form-data"))

        throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);

    //文件保存目录路径

    string SaveTempPath = "~/SayPlaces/" + "/SayPic/SayPicTemp/";

    String dirTempPath = HttpContext.Current.Server.MapPath(SaveTempPath);

    // 设置上传目录

    var provider = new MultipartFormDataStreamProvider(dirTempPath);

    //var queryp = Request.GetQueryNameValuePairs();//获得查询字符串的键值集合

    var task = Request.Content.ReadAsMultipartAsync(provider).

        ContinueWith<Hashtable>(o =>

        {

            Hashtable hash = new Hashtable();

            hash["error"] = 1;

            hash["errmsg"] = "上传出错";

            var file = provider.FileData[0];//provider.FormData

            string orfilename = file.Headers.ContentDisposition.FileName.TrimStart('"').TrimEnd('"');

            FileInfo fileinfo = new FileInfo(file.LocalFileName);

            //最大文件大小

            int maxSize = 10000000;

            if (fileinfo.Length <= 0)

            {

                hash["error"] = 1;

                hash["errmsg"] = "请选择上传文件。";

            }

            else if (fileinfo.Length > maxSize)

            {

                hash["error"] = 1;

                hash["errmsg"] = "上传文件大小超过限制。";

            }

            else

            {

                string fileExt = orfilename.Substring(orfilename.LastIndexOf('.'));

                //定义允许上传的文件扩展名

                String fileTypes = "gif,jpg,jpeg,png,bmp";

                if (String.IsNullOrEmpty(fileExt) || Array.IndexOf(fileTypes.Split(','), fileExt.Substring(1).ToLower()) == -1)

                {

                    hash["error"] = 1;

                    hash["errmsg"] = "上传文件扩展名是不允许的扩展名。";

                }

                else

                {

                    String ymd = DateTime.Now.ToString("yyyyMMdd", System.Globalization.DateTimeFormatInfo.InvariantInfo);

                    String newFileName = DateTime.Now.ToString("yyyyMMddHHmmss_ffff", System.Globalization.DateTimeFormatInfo.InvariantInfo);

                    fileinfo.CopyTo(Path.Combine(dirTempPath, newFileName + fileExt), true);

                    fileinfo.Delete();

                    hash["error"] = 0;

                    hash["errmsg"] = "上传成功";

                }

            }

            return hash;

        });

    return task;

}

如果只是上传，简单用是可以的，但是你可能不会发现有什么问题。但如果你在Request.Content.ReadAsMultipartAsync(provider).ContinueWith 延时Task任务里面赋值一个变量，你就会发现始终赋值不上，不信你可以试试。

例子如下：

public string UploadFile()

{

        if (Request.Content.IsMimeMultipartContent())

        {

            //Save file

            MultipartFormDataStreamProvider provider = new MultipartFormDataStreamProvider(HttpContext.Current.Server.MapPath("~/Files"));string filename = "Not set";

            Request.Content.ReadAsMultipartAsync(provider).ContinueWith(o =>

            {

                //File name
filename = "Set success";

            }, TaskScheduler.FromCurrentSynchronizationContext()); 

            return filename;

        }

        else

        {

            return "Invalid.";

        }

 }

上面的得出的结果: filename = "Not set" ;

【注意如下结论】

经测试发现如下结论，在执行 Request.Content.ReadAsMultipartAsync(provider).ContinueWith 异步延时任务的时候，先不会被立即执行。

等待 return 结束之后才会被执行。这也就是为什么返回的总是： "Not set" 。

经过几天的摸索测试，在*上找到了一个解决的办法如下：

IEnumerable<HttpContent> parts = null;

Task.Factory

    .StartNew(() => parts = Request.Content.ReadAsMultipartAsync().Result.Contents,

        CancellationToken.None,

        TaskCreationOptions.LongRunning, // guarantees separate thread

        TaskScheduler.Default)

    .Wait();

改造后就变成了这样，真的太棒了！

 public string UploadFile()

        {

            if (Request.Content.IsMimeMultipartContent())

            {

                //Save file

                MultipartFormDataStreamProvider provider = new MultipartFormDataStreamProvider(HttpContext.Current.Server.MapPath("/UploadUser/"));

                string filename = "Not set";

                IEnumerable<HttpContent> parts = null;

                Task.Factory

                    .StartNew(() =>

                    {

                        parts = Request.Content.ReadAsMultipartAsync(provider).Result.Contents;

                        filename = "Set Success";

                    },

                    CancellationToken.None,

                    TaskCreationOptions.LongRunning, // guarantees separate thread

                    TaskScheduler.Default)

                    .Wait();

                return filename;

            }

            else

            {

                return "Invalid.";

            }

        }