c# - 将值从字符串转换为流时出错

Question

我想序列化一个自定义对象:

public class MyCustomObject
{
    public string Name { get; set; }
    public DateTime Date { get; set; }
    public List<HttpPostedFileBase> Files { get; set; }

    public MyCustomObject()
    {
        Files = new List<HttpPostedFileBase>();
    }
}

在 json 中。为此，我使用自定义转换器:

public class HttpPostedFileConverter : JsonConverter
{
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        var stream = (Stream)value;
        using (var sr = new BinaryReader(stream))
        {
            var buffer = sr.ReadBytes((int)stream.Length);
            writer.WriteValue(Convert.ToBase64String(buffer));
        }
    }

我使用 JsonSerializerSettings 序列化为 json.net 知道哪种类型的实现(对于 HttpPostedFileBase)。

        var settings = new JsonSerializerSettings();
        settings.Converters.Add(new HttpPostedFileConverter());
        settings.TypeNameHandling = TypeNameHandling.Objects;

对象已正​​确序列化，但序列化时出现此错误:

JsonSerializationException Error converting value "/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAIBAQEBAQIBAQECAgICAgQDAgICAgUEBAMEBgUGBgYFBgYGBwkIBgcJBwYGCAsICQoKCgoKBggLDAsKDA

这是我的序列化对象的值:

{
  "$type": "ConsoleApplication1.MyCustomObject, ConsoleApplication1",
  "Name": "Test2",
  "Date": "2016-11-03T12:35:14.6020154+01:00",
  "Files": [
    {
      "$type": "System.Web.HttpPostedFileWrapper, System.Web",
      "ContentLength": 1024,
      "FileName": "Pannigale.jpg",
      "ContentType": "image/jpg",
      "InputStream": "/9j/4AAQ...KKAP//Z"
    }
  ]
}

反序列化有什么问题？

编辑我已经测试了一个类来测试......现在它可以工作了:

public class TestHttpFile : HttpPostedFileBase
{
    string fullFileName = @"C:\Pictures\SBK-1299-Panigale-S_2015_Studio_R_B01_1920x1080.mediagallery_output_image_[1920x1080].jpg";
    public override int ContentLength
    {
        get
        {
            return 1024;
        }
    }

    public override string FileName
    {
        get
        {
            return "Pannigale.jpg";
        }
    }

    public override string ContentType
    {
        get
        {
            return "image/jpg";
        }
    }

    public override Stream InputStream
    {
        get
        {
            return File.OpenRead(fullFileName);
        }
    }
}

在连载中我注意到了这个区别:

 "$type": "ConsoleApplication1.TestHttpFile, ConsoleApplication1",

代替

 "$type": "System.Web.HttpPostedFileWrapper, System.Web",

但最后我不想创建包装器或其他任何东西......而且我不明白为什么它适用于这种类型而不适用于 HttpPostedFileWrapper。

Answer 1

为什么反序列化失败

简而言之:您要反序列化的类型，即 HttpPostedFileWrapper及其基础HttpPostedFile , 不可公开构建。

大概您的 HttpPostedFileConverter 实际上是一个流转换器，并且工作方式如下:

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，假设我还设置了 TypeNameHandling = TypeNameHandling.Auto在序列化器设置中。但是，反序列化将失败，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(public)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，从而导致要抛出的 ArgumentNullException。

这给我们带来了下一个问题:HttpPostedFile 没有公共(public)构造函数。 .相反，要直接创建一个，您必须通过反射调用多个不同的 Microsoft 内部方法，如 How to instantiate a HttpPostedFile 中所示。 .因此，即使您创建了一个 custom JsonConverter对于 HttpPostedFileWrapper，在内部构造必需的 HttpPostedFile 非常棘手。 Json.NET 当然不能自动完成。

那么，您有哪些选择？

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，它可以成功序列化和反序列化，然后在序列化期间使用适当的 将 HttpPostedFilesBase 的所有实例映射到该类型JsonConverter.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，我正在使用此答案前面的 StreamConverter 和 StreamExtensions。

像这样将转换器添加到设置中:

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一个 HttpPostedFileWrapper

要做到这一点，我们需要使用来自 this answer 的相当复杂和“hacky”的代码至 How to instantiate a HttpPostedFile通过 paracycle ，像这样:

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;
    }
}

请注意，我正在使用此答案前面的 StreamExtensions。此版本的 HttpPostedFileBaseConverter 也将像往常一样添加到 JsonSerializerSettings.Converters。

(老实说，我不推荐这个解决方案，因为它太依赖于 Microsoft 实现 HttpPostedFile 的未记录的内部结构，这在以后的版本中很容易改变。)