前段时间我正在处理几乎相同的问题。你得到这种行为的原因是在 ASP.Net WepAPI 中没有 "out-of-the-box" formatter for multipart/form-data media-type strong>（奇怪的是，ASP.Net MVC 中有一个）。
我不记得我看到的 SO 问题、Microsoft 文档、ASP.Net 资源和文章的确切路径，但这是工作结果：
创建一个HttpPostedFileMultipart 类来处理发布的文件：
public class HttpPostedFileMultipart : HttpPostedFileBase
{
public override string FileName { get; }
public override string ContentType { get; }
public override Stream InputStream { get; }
public override int ContentLength => (int)InputStream.Length;
public HttpPostedFileMultipart(string fileName, string contentType, byte[] fileContents)
{
FileName = fileName;
ContentType = contentType;
InputStream = new MemoryStream(fileContents);
}
}
然后创建您的 MediaTypeFormatter：
public class FormMultipartEncodedMediaTypeFormatter : MediaTypeFormatter
{
private const string SupportedMediaType = "multipart/form-data";
public FormMultipartEncodedMediaTypeFormatter()
{
SupportedMediaTypes.Add(new MediaTypeHeaderValue(SupportedMediaType));
}
// can we deserialize multipart/form-data to specific type
public override bool CanReadType(Type type)
{
if (type == null) throw new ArgumentNullException(nameof(type));
return true;
}
// can we serialize specific type to multipart/form-data
public override bool CanWriteType(Type type)
{
if (type == null) throw new ArgumentNullException(nameof(type));
return false;
}
// deserialization
public override async Task<object> ReadFromStreamAsync(
Type type, Stream readStream, HttpContent content, IFormatterLogger formatterLogger)
{
if (type == null) throw new ArgumentNullException(nameof(type));
if (readStream == null) throw new ArgumentNullException(nameof(readStream));
try
{
// read content
var multipartProvider = await content.ReadAsMultipartAsync();
// fill out model dictionary
var modelDictionary = await ToModelDictionaryAsync(multipartProvider);
// apply dictionary to model instance
return BindToModel(modelDictionary, type, formatterLogger);
}
catch (Exception e)
{
if (formatterLogger == null) throw;
formatterLogger.LogError(string.Empty, e);
return GetDefaultValueForType(type);
}
}
// fill out model dictionary
private async Task<IDictionary<string, object>> ToModelDictionaryAsync(MultipartMemoryStreamProvider multipartProvider)
{
var dictionary = new Dictionary<string, object>();
foreach (var element in multipartProvider.Contents)
{
// getting element name
var name = element.Headers.ContentDisposition.Name.Trim('"');
// if we have a FileName - this is a file
// if not - pretend this is a string (later binder will transform this strings to objects)
if (!string.IsNullOrEmpty(element.Headers.ContentDisposition.FileName))
// create our HttpPostedFileMultipart instance if we have any data
if (element.Headers.ContentLength.GetValueOrDefault() > 0)
dictionary[name] = new HttpPostedFileMultipart(
element.Headers.ContentDisposition.FileName.Trim('"'),
element.Headers.ContentType.MediaType,
await element.ReadAsByteArrayAsync()
);
else
dictionary[name] = null;
else
dictionary[name] = await element.ReadAsStringAsync();
}
return dictionary;
}
// apply dictionary to model instance
private object BindToModel(IDictionary<string, object> data, Type type, IFormatterLogger formatterLogger)
{
if (data == null) throw new ArgumentNullException(nameof(data));
if (type == null) throw new ArgumentNullException(nameof(type));
using (var config = new HttpConfiguration())
{
if (RequiredMemberSelector != null && formatterLogger != null)
config.Services.Replace(
typeof(ModelValidatorProvider),
new RequiredMemberModelValidatorProvider(RequiredMemberSelector));
var actionContext = new HttpActionContext {
ControllerContext = new HttpControllerContext {
Configuration = config,
ControllerDescriptor = new HttpControllerDescriptor { Configuration = config }
}
};
// workaround possible locale mismatch
var cultureBugWorkaround = CultureInfo.CurrentCulture.Clone() as CultureInfo;
cultureBugWorkaround.NumberFormat = CultureInfo.InvariantCulture.NumberFormat;
var valueProvider = new NameValuePairsValueProvider(data, cultureBugWorkaround);
var metadataProvider = actionContext.ControllerContext.Configuration.Services.GetModelMetadataProvider();
var metadata = metadataProvider.GetMetadataForType(null, type);
var modelBindingContext = new ModelBindingContext
{
ModelName = string.Empty,
FallbackToEmptyPrefix = false,
ModelMetadata = metadata,
ModelState = actionContext.ModelState,
ValueProvider = valueProvider
};
// bind our model
var modelBinderProvider = new CompositeModelBinderProvider(config.Services.GetModelBinderProviders());
var binder = modelBinderProvider.GetBinder(config, type);
var haveResult = binder.BindModel(actionContext, modelBindingContext);
// store validation errors
if (formatterLogger != null)
foreach (var modelStatePair in actionContext.ModelState)
foreach (var modelError in modelStatePair.Value.Errors)
if (modelError.Exception != null)
formatterLogger.LogError(modelStatePair.Key, modelError.Exception);
else
formatterLogger.LogError(modelStatePair.Key, modelError.ErrorMessage);
return haveResult ? modelBindingContext.Model : GetDefaultValueForType(type);
}
}
}
最后，在您的 WebApiConfig.Register() 方法中注册此格式化程序：
public static void Register(HttpConfiguration config)
{
// ...
// add multipart/form-data formatter
config.Formatters.Add(new FormMultipartEncodedMediaTypeFormatter());
// ...
}