为什么反序列化失败
简而言之：您尝试反序列化的类型，即HttpPostedFileWrapper 及其底层HttpPostedFile，不可公开构造。
大概你的HttpPostedFileConverter实际上是一个流转换器，它的工作原理是这样的：
public class StreamConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return typeof(Stream).IsAssignableFrom(objectType);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
if (reader.TokenType == JsonToken.Null)
return null;
var bytes = serializer.Deserialize<byte[]>(reader);
return new MemoryStream(bytes);
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
var stream = (Stream)value;
var bytes = stream.ReadAllBytesAndReposition();
serializer.Serialize(writer, bytes);
}
}
public static class StreamExtensions
{
public static byte[] ReadAllBytesAndReposition(this Stream stream)
{
const int bufferSize = 4096;
using (var ms = new MemoryStream())
{
byte[] buffer = new byte[bufferSize];
int count;
var position = stream.CanSeek ? stream.Position : (long?)null;
while ((count = stream.Read(buffer, 0, buffer.Length)) != 0)
ms.Write(buffer, 0, count);
if (position != null)
{
// Restore position
stream.Position = position.Value;
}
return ms.ToArray();
}
}
}
在这种情况下，我可以为您的 MyCustomObject 类生成类似于您的 JSON，假设我还在序列化程序设置中设置了 TypeNameHandling = TypeNameHandling.Auto。但是，反序列化会失败，因为具体类型System.Web.HttpPostedFileWrapper 只有一个构造函数，它采用HttpPostedFile。来自reference source：
public class HttpPostedFileWrapper : HttpPostedFileBase {
private HttpPostedFile _file;
public HttpPostedFileWrapper(HttpPostedFile httpPostedFile) {
if (httpPostedFile == null) {
throw new ArgumentNullException("httpPostedFile");
}
_file = httpPostedFile;
}
由于构造函数是公共的，Json.NET 将调用它，但由于 JSON 中没有名为 httpPostedFile 的属性，它将为该值传递 null，从而导致抛出 ArgumentNullException。
这给我们带来了下一个问题：HttpPostedFile 没有公共构造函数。相反，要直接创建一个，您必须通过反射调用几种不同的 Microsoft 内部方法，如How to instantiate a HttpPostedFile 所示。因此，即使您为HttpPostedFileWrapper 创建了custom JsonConverter，在内部构造必要的HttpPostedFile 也是非常棘手的。 Json.NET 当然不能自动完成。
那么，你有什么选择？
选项 1：创建您自己的 HttpPostedFilesBase 子类型
您可以创建自己的HttpPostedFilesBase 子类型，它可以成功地进行序列化和反序列化，然后在序列化期间使用适当的JsonConverter 将HttpPostedFilesBase 的所有实例映射到此类型。以下类型MemoryHttpPostedFile 执行此操作：
public sealed class MemoryHttpPostedFile : HttpPostedFileBase
{
readonly string contentType;
readonly string fileName;
readonly MemoryStream inputStream;
public MemoryHttpPostedFile(string contentType, string fileName, [JsonConverter(typeof(StreamConverter))] MemoryStream inputStream)
{
if (inputStream == null)
throw new ArgumentNullException("inputStream");
this.contentType = contentType;
this.fileName = fileName;
this.inputStream = inputStream;
}
public override int ContentLength { get { return (int)inputStream.Length; } }
public override string ContentType { get { return contentType; } }
public override string FileName { get { return fileName; } }
[JsonConverter(typeof(StreamConverter))]
public override Stream InputStream { get { return inputStream; } }
//TODO: implement SaveAs()
public override void SaveAs(string filename)
{
// Implement based on HttpPostedFile.SaveAs()
// https://referencesource.microsoft.com/#System.Web/HttpPostedFile.cs,678e7f8bc95c149f
throw new NotImplementedException();
}
}
public class HttpPostedFileBaseConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return typeof(HttpPostedFileBase).IsAssignableFrom(objectType)
&& !typeof(MemoryHttpPostedFile).IsAssignableFrom(objectType);
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
var postedFile = (HttpPostedFileBase)value;
// Save position
var wrapper = new MemoryHttpPostedFile(postedFile.ContentType, postedFile.FileName, new MemoryStream(postedFile.InputStream.ReadAllBytesAndReposition()));
serializer.Serialize(writer, wrapper);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
if (reader.TokenType == JsonToken.Null)
return null;
var wrapper = serializer.Deserialize<MemoryHttpPostedFile>(reader);
return wrapper;
}
}
由于HttpPostedFilesBase 的所有子类型都映射到MemoryHttpPostedFile，因此不再需要设置TypeNameHandling.Auto。请注意，我在此答案的前面使用了 StreamConverter 和 StreamExtensions。
将转换器添加到设置中，如下所示：
var settings = new JsonSerializerSettings
{
Converters = new JsonConverter[] { new HttpPostedFileBaseConverter() },
Formatting = Formatting.Indented,
};
选项 2：实际序列化和反序列化 HttpPostedFileWrapper
为此，我们需要使用从this answer 到How to instantiate a HttpPostedFile by paracycle 的相当复杂和“hacky”的代码，如下所示：
public class HttpPostedFileBaseConverter : JsonConverter
{
class HttpPostedFileSurrogate
{
public string ContentType { get; set; }
public string FileName { get; set; }
public byte[] InputStream { get; set; }
}
public override bool CanConvert(Type objectType)
{
return typeof(HttpPostedFileBase).IsAssignableFrom(objectType);
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
var wrapper = (HttpPostedFileWrapper)value;
// Save position
var surrogate = new HttpPostedFileSurrogate
{
ContentType = wrapper.ContentType,
FileName = wrapper.FileName,
InputStream = wrapper.InputStream.ReadAllBytesAndReposition(),
};
serializer.Serialize(writer, surrogate);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
if (reader.TokenType == JsonToken.Null)
return null;
var surrogate = serializer.Deserialize<HttpPostedFileSurrogate>(reader);
var file = HttpPostedFileExtensions.ConstructHttpPostedFile(surrogate.InputStream, surrogate.FileName, surrogate.ContentType);
return new HttpPostedFileWrapper(file);
}
}
public static class HttpPostedFileExtensions
{
public static HttpPostedFile ConstructHttpPostedFile(byte[] data, string filename, string contentType)
{
// Adapted from https://stackoverflow.com/questions/5514715/how-to-instantiate-a-httppostedfile/5515134#5515134
// Get the System.Web assembly reference (they seem to be in different assemblies in different versions of .Net
var assemblies = new[] { typeof(HttpPostedFile).Assembly, typeof(HttpPostedFileBase).Assembly };
// Get the types of the two internal types we need
Type typeHttpRawUploadedContent = assemblies.Select(a => a.GetType("System.Web.HttpRawUploadedContent")).Where(t => t != null).First();
Type typeHttpInputStream = assemblies.Select(a => a.GetType("System.Web.HttpInputStream")).Where(t => t != null).First();
// Prepare the signatures of the constructors we want.
Type[] uploadedParams = { typeof(int), typeof(int) };
Type[] streamParams = { typeHttpRawUploadedContent, typeof(int), typeof(int) };
Type[] parameters = { typeof(string), typeof(string), typeHttpInputStream };
// Create an HttpRawUploadedContent instance
object uploadedContent = typeHttpRawUploadedContent
.GetConstructor(BindingFlags.NonPublic | BindingFlags.Instance, null, uploadedParams, null)
.Invoke(new object[] { data.Length, data.Length });
// Call the AddBytes method
typeHttpRawUploadedContent
.GetMethod("AddBytes", BindingFlags.NonPublic | BindingFlags.Instance)
.Invoke(uploadedContent, new object[] { data, 0, data.Length });
// This is necessary if you will be using the returned content (ie to Save)
typeHttpRawUploadedContent
.GetMethod("DoneAddingBytes", BindingFlags.NonPublic | BindingFlags.Instance)
.Invoke(uploadedContent, null);
// Create an HttpInputStream instance
object stream = (Stream)typeHttpInputStream
.GetConstructor(BindingFlags.NonPublic | BindingFlags.Instance, null, streamParams, null)
.Invoke(new object[] { uploadedContent, 0, data.Length });
// Create an HttpPostedFile instance
HttpPostedFile postedFile = (HttpPostedFile)typeof(HttpPostedFile)
.GetConstructor(BindingFlags.NonPublic | BindingFlags.Instance, null, parameters, null)
.Invoke(new object[] { filename, contentType, stream });
return postedFile;
}
}
注意我在这个答案的前面使用了StreamExtensions。这个版本的HttpPostedFileBaseConverter 也会像往常一样添加到JsonSerializerSettings.Converters。
（老实说，我不推荐这种解决方案，因为它过多地依赖于 Microsoft 实现 HttpPostedFile 的未记录内部结构，在以后的版本中很容易改变。）