Why are DriveInfo's properties missing when serializing to json string using Json.NET?

Question

Attempting to serialize DrivInfo to a Json string with this code only return the "name" property:

DriveInfo dr = new DriveInfo("C");    
string json = Newtonsoft.Json.JsonConvert.SerializeObject(dr);

The string result is only: {"_name":"C:\"}

DrivInfo is sealed so I cannot change anything. Is there a way to do it excluding wrapping?

Why would you serialize a type you don't control? What if MS decides to change the implementation of `DriveInfo`? — Kenneth K., May 11 '16 at 17:55
For internal utilisation in the company I wrote a program to dump information of the environment. So the objects are serialized then deserialized to xml files. I use serialized json string because of the limitation of XmlSerializer with default constructor. — TheBugHunter, May 11 '16 at 18:06

score 2 · Answer 1 · answered May 11 '16 at 18:41

Your difficulty is that DriveInfo implements the ISerializable interface for custom serialization, and Json.NET respects this interface by default, using it to serialize and deserialize the type. And since DriveInfo is defined entirely by the name of the drive, that's all that it's custom serialization code stores into the serialization stream.

Since you just want to dump the properties of DriveInfo and do not care about deserialization, you can disable use of ISerializable by setting DefaultContractResolver.IgnoreSerializableInterface = true. However, if you do this, you'll get an infinite recursion serializing dr.RootDirectory.Root.Root.... To work around this, you could create a JsonConverter for DirectoryInfo:

public class DirectoryInfoConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(DirectoryInfo);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        if (reader.TokenType == JsonToken.Null)
            return null;
        var token = JToken.Load(reader);
        return new DirectoryInfo((string)token);
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        writer.WriteValue(value.ToString());
    }
}

Then you could do:

DriveInfo dr = new DriveInfo("C");
var settings = new JsonSerializerSettings 
{ 
    ContractResolver = new DefaultContractResolver { IgnoreSerializableInterface = true },
    Converters = new [] { new DirectoryInfoConverter() },
};
var json = Newtonsoft.Json.JsonConvert.SerializeObject(dr, Formatting.Indented, settings);

But at this point, it's probably easier just to serialize an intermediate anonymous type:

var json = JsonConvert.SerializeObject(
    new
    {
        Name = dr.Name,
        DriveType = dr.DriveType,
        DriveFormat = dr.DriveFormat,
        IsReady = dr.IsReady,
        AvailableFreeSpace = dr.AvailableFreeSpace,
        TotalFreeSpace = dr.TotalFreeSpace,
        TotalSize = dr.TotalSize,
        RootDirectory = dr.RootDirectory.ToString(),
        VolumeLabel = dr.VolumeLabel
    },
    Formatting.Indented);

(Of course, you won't be able to deserialize it in this format.)

score 0 · Accepted Answer · answered May 11 '16 at 18:11

The class contains its own custom serialization which specifies that only the _name field should be included. The other properties aren't stored in the class. They're determined by the environment, so they can't be replaced with deserialized values.

From the source code:

private const String NameField = "_name";  // For serialization

    void ISerializable.GetObjectData(SerializationInfo info, StreamingContext context)
    {
        // No need for an additional security check - everything is public.
        info.AddValue(NameField, _name, typeof(String));
    }

Other properties are determined by actually inspecting the drive referenced by the DriveInfo. Properties like TotalFreeSpace and TotalFreeSize can change from moment to moment and may not (probably don't) apply on another computer where the class could be deserialized.

If you could serialize the whole thing and deserialize it someplace else, then it would be possible to create a deserialized instance of the class where all of the property values are wrong, because, for example, they actually describe the c: drive on another computer. But the purpose of the class is to return information about a drive on the computer where the code is executing.

If you want to pass that data around then you can always create your own class and serialize that.

Why are DriveInfo's properties missing when serializing to json string using Json.NET?

2 Answers2

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