将值从字符串转换为流时出错

Question

将值从字符串转换为流时出错

Flo*_*ian 5 c# serialization json json.net deserialization

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>();
    }
}

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在json中。为此，我使用一个自定义转换器：

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));
        }
    }

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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;

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该对象已正确序列化，但我在序列化时遇到此错误：

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

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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"
    }
  ]
}

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反序列化有什么问题？

编辑我已经测试了一个类进行测试...，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);
        }
    }
}

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在序列化中，我注意到了这种差异：

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

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代替

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

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但是最后我不想创建一个包装器或其他任何东西...而且我也不明白为什么它适用于这种类型而不适用于HttpPostedFileWrapper。

Answer 1

dbc*_*dbc 5

为什么反序列化失败

简而言之：您要反序列化的类型（即HttpPostedFileWrapper其基础HttpPostedFile类型）不可公开构造。

大概您HttpPostedFileConverter实际上是一个流转换器，并且工作原理如下：

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();
        }
    }
}

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在这种情况下，MyCustomObject假设我也在TypeNameHandling = TypeNameHandling.Auto序列化程序设置中进行了设置，则可以为您的类生成类似于您的JSON的JSON 。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，该构造函数采用HttpPostedFile。从参考源：

public class HttpPostedFileWrapper : HttpPostedFileBase {

    private HttpPostedFile _file;

    public HttpPostedFileWrapper(HttpPostedFile httpPostedFile) {
        if (httpPostedFile == null) {
            throw new ArgumentNullException("httpPostedFile");
        }
        _file = httpPostedFile;
    }

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由于构造函数是公共的，因此Json.NET将对其进行调用，但是由于httpPostedFileJSON中没有命名的属性，因此它将传递null该值，从而ArgumentNullException引发。

这就引出了下一个问题：没有公共构造函数HttpPostedFile。相反，要直接创建一个，您将必须通过反射调用几种不同的Microsoft内部方法，如如何实例化HttpPostedFile所示。因此，即使您为此创建了一个自定义JsonConverter，HttpPostedFileWrapper也很难在其中构造必要的条件HttpPostedFile。Json.NET当然不能自动执行此操作。

那么，您有什么选择呢？

选项1：创建自己的 HttpPostedFilesBase

您可以创建自己的HttpPostedFilesBase可以成功进行序列化和反序列化的子类型，然后HttpPostedFilesBase在序列化过程中使用适当的将该子实例的所有实例映射到该类型JsonConverter。以下类型可以MemoryHttpPostedFile做到这一点：

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;
    }
}

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由于HttpPostedFilesBaseget的所有子类型都已映射到MemoryHttpPostedFile它，因此不再需要设置TypeNameHandling.Auto。请注意我使用StreamConverter，并StreamExtensions从早期在这个答案。

将转换器添加到如下设置中：

var settings = new JsonSerializerSettings
{
    Converters = new JsonConverter[] { new HttpPostedFileBaseConverter() },
    Formatting = Formatting.Indented,
};

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选项2：实际上是序列化和反序列化 HttpPostedFileWrapper

要做到这一点，我们将需要使用相当精巧和“哈克”从代码这个答案要如何实例化一个HttpPostedFile通过paracycle，就像这样：

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 /sf/ask/386030081/#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;
    }
}

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注意我正在StreamExtensions从此答案的前面。此版本的HttpPostedFileBaseConverter也会JsonSerializerSettings.Converters照常添加。

（老实说，我不建议使用此解决方案，因为该解决方案过于依赖Microsoft未实现的内部文档HttpPostedFile，在以后的发行版中很容易更改。）

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