.Net 4，Web API和Visual Studio 2010的自定义MediaTypeFormatter上的编译错误

Question

I am implementing a custom MediaTypeFormatter and I am getting a compilation error on the overriden method ReadFromStreamAsync as shown in the code example. For work reasons I can't update to Visual Studio 2012 and use .net 4.5 unfortunately:

ImageMediaFormatter.ReadFromStreamAsync(System.Type, System.IO.Stream, System.Net.Http.HttpContent, System.Net.Http.Formatting.IFormatterLogger)': not all code paths return a value

public class ImageMediaFormatter : MediaTypeFormatter
{
    public ImageMediaFormatter()
    {
        SupportedMediaTypes.Add(new MediaTypeHeaderValue("image/jpeg"));
        SupportedMediaTypes.Add(new MediaTypeHeaderValue("image/jpg"));
        SupportedMediaTypes.Add(new MediaTypeHeaderValue("image/png"));
        SupportedMediaTypes.Add(new MediaTypeHeaderValue("multipart/form-data"));
    }

    public override bool CanReadType(Type type)
    {
        return type == typeof(ImageMedia);
    }

    public override bool CanWriteType(Type type)
    {
        return false;
    }

    public override Task<object> ReadFromStreamAsync(
        Type type, Stream stream, HttpContent request,
        IFormatterLogger formatterContext)
    {

        if (!request.IsMimeMultipartContent())
        {
            throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
        }

        var streamProvider = new MultipartMemoryStreamProvider();
        var task = request.ReadAsMultipartAsync(streamProvider)
            .ContinueWith(
               t =>
               {
                   var content = streamProvider.Contents.First(x =>
                    SupportedMediaTypes.Contains(x.Headers.ContentType));

                   string fileName = content.Headers.ContentDisposition.FileName;
                   string mediaType = content.Headers.ContentType.MediaType;

                   var imgstream = content.ReadAsStreamAsync().ContinueWith(x =>
                                                                                {
                                                                                    byte[] imagebuffer = ReadFully(x.Result);
                                                                                    return new ImageMedia(fileName, mediaType, imagebuffer);  
                                                                                });




               }
         );

        //return task.Result;

    }

    private byte[] ReadFully(Stream input)
    {
        byte[] buffer = new byte[16*1024];
        using (MemoryStream ms = new MemoryStream())
        {
            int read;
            while ((read = input.Read(buffer, 0, buffer.Length)) > 0)
            {
                ms.Write(buffer, 0, read);
            }
            return ms.ToArray();
        }
    }
}

Answer 1

好的， ReadFromStreamAsync()方法必须返回Task<object>但是您已经注释了return语句（它可能应该只是return task; ），因此显然不会编译。