JavaScriptSerializer.Deserialize - how to change field names

Question

Summary: How do I map a field name in JSON data to a field name of a .Net object when using JavaScriptSerializer.Deserialize ?

Longer version: I have the following JSON data coming to me from a server API (Not coded in .Net)

{"user_id":1234, "detail_level":"low"}

I have the following C# object for it:

[Serializable]
public class DataObject
{
    [XmlElement("user_id")]
    public int UserId { get; set; }

    [XmlElement("detail_level")]
    public DetailLevel DetailLevel { get; set; }
}

Where DetailLevel is an enum with "Low" as one of the values.

This test fails:

[TestMethod]
public void DataObjectSimpleParseTest()
{
    JavaScriptSerializer serializer = new JavaScriptSerializer();
    DataObject dataObject = serializer.Deserialize<DataObject>(JsonData);

    Assert.IsNotNull(dataObject);
    Assert.AreEqual(DetailLevel.Low, dataObject.DetailLevel);
    Assert.AreEqual(1234, dataObject.UserId);
}

And the last two asserts fail, since there is no data in those fields. If I change the JSON data to

 {"userid":1234, "detaillevel":"low"}

Then it passes. But I can't change the server's behaviour, and I want the client classes to have well-named properties in the C# idiom. I can't use LINQ to JSON since I want it to work outside of Silverlight. It looks like the XmlElement tags are having no effect. I don't know where I got the idea they were relevant at all, they probably aren't.

How do you do field name mapping in JavaScriptSerializer? Can it be done at all?

Solution 1

I took another try at it, using the DataContractJsonSerializer class. This solves it:

The code looks like this:

using System.Runtime.Serialization;

[DataContract]
public class DataObject
{
    [DataMember(Name = "user_id")]
    public int UserId { get; set; }

    [DataMember(Name = "detail_level")]
    public string DetailLevel { get; set; }
}

And the test is:

using System.Runtime.Serialization.Json;

[TestMethod]
public void DataObjectSimpleParseTest()
{
        DataContractJsonSerializer serializer = new DataContractJsonSerializer(typeof(DataObject));

        MemoryStream ms = new MemoryStream(Encoding.Unicode.GetBytes(JsonData));
        DataObject dataObject = serializer.ReadObject(ms) as DataObject;

        Assert.IsNotNull(dataObject);
        Assert.AreEqual("low", dataObject.DetailLevel);
        Assert.AreEqual(1234, dataObject.UserId);
}

The only drawback is that I had to change DetailLevel from an enum to a string - if you keep the enum type in place, the DataContractJsonSerializer expects to read a numeric value and fails. See DataContractJsonSerializer and Enums for further details.

In my opinion this is quite poor, especially as JavaScriptSerializer handles it correctly. This is the exception that you get trying to parse a string into an enum:

System.Runtime.Serialization.SerializationException: There was an error deserializing the object of type DataObject. The value 'low' cannot be parsed as the type 'Int64'. --->
System.Xml.XmlException: The value 'low' cannot be parsed as the type 'Int64'. --->  
System.FormatException: Input string was not in a correct format

And marking up the enum like this does not change this behaviour:

[DataContract]
public enum DetailLevel
{
    [EnumMember(Value = "low")]
    Low,
   ...
 }

This also seems to work in Silverlight.

Solution 2

By creating a custom JavaScriptConverter you can map any name to any property. But it does require hand coding the map, which is less than ideal.

public class DataObjectJavaScriptConverter : JavaScriptConverter
{
    private static readonly Type[] _supportedTypes = new[]
    {
        typeof( DataObject )
    };

    public override IEnumerable<Type> SupportedTypes 
    { 
        get { return _supportedTypes; } 
    }

    public override object Deserialize( IDictionary<string, object> dictionary, 
                                        Type type, 
                                        JavaScriptSerializer serializer )
    {
        if( type == typeof( DataObject ) )
        {
            var obj = new DataObject();
            if( dictionary.ContainsKey( "user_id" ) )
                obj.UserId = serializer.ConvertToType<int>( 
                                           dictionary["user_id"] );
            if( dictionary.ContainsKey( "detail_level" ) )
                obj.DetailLevel = serializer.ConvertToType<DetailLevel>(
                                           dictionary["detail_level"] );

            return obj;
        }

        return null;
    }

    public override IDictionary<string, object> Serialize( 
            object obj, 
            JavaScriptSerializer serializer )
    {
        var dataObj = obj as DataObject;
        if( dataObj != null )
        {
            return new Dictionary<string,object>
            {
                {"user_id", dataObj.UserId },
                {"detail_level", dataObj.DetailLevel }
            }
        }
        return new Dictionary<string, object>();
    }
}

Then you can deserialize like so:

var serializer = new JavaScriptSerializer();
serialzer.RegisterConverters( new[]{ new DataObjectJavaScriptConverter() } );
var dataObj = serializer.Deserialize<DataObject>( json );

Solution 3

Json.NET will do what you want (disclaimer: I'm the author of the package). It supports reading DataContract/DataMember attributes as well as its own to change the property names. Also there is the StringEnumConverter class for serializing enum values as the name rather than the number.

Solution 4

There is no standard support for renaming properties in JavaScriptSerializer however you can quite easily add your own:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Web.Script.Serialization;
using System.Reflection;

public class JsonConverter : JavaScriptConverter
{
    public override object Deserialize(IDictionary<string, object> dictionary, Type type, JavaScriptSerializer serializer)
    {
        List<MemberInfo> members = new List<MemberInfo>();
        members.AddRange(type.GetFields());
        members.AddRange(type.GetProperties().Where(p => p.CanRead && p.CanWrite && p.GetIndexParameters().Length == 0));

        object obj = Activator.CreateInstance(type);

        foreach (MemberInfo member in members)
        {
            JsonPropertyAttribute jsonProperty = (JsonPropertyAttribute)Attribute.GetCustomAttribute(member, typeof(JsonPropertyAttribute));

            if (jsonProperty != null && dictionary.ContainsKey(jsonProperty.Name))
            {
                SetMemberValue(serializer, member, obj, dictionary[jsonProperty.Name]);
            }
            else if (dictionary.ContainsKey(member.Name))
            {
                SetMemberValue(serializer, member, obj, dictionary[member.Name]);
            }
            else
            {
                KeyValuePair<string, object> kvp = dictionary.FirstOrDefault(x => string.Equals(x.Key, member.Name, StringComparison.InvariantCultureIgnoreCase));

                if (!kvp.Equals(default(KeyValuePair<string, object>)))
                {
                    SetMemberValue(serializer, member, obj, kvp.Value);
                }
            }
        }

        return obj;
    }


    private void SetMemberValue(JavaScriptSerializer serializer, MemberInfo member, object obj, object value)
    {
        if (member is PropertyInfo)
        {
            PropertyInfo property = (PropertyInfo)member;                
            property.SetValue(obj, serializer.ConvertToType(value, property.PropertyType), null);
        }
        else if (member is FieldInfo)
        {
            FieldInfo field = (FieldInfo)member;
            field.SetValue(obj, serializer.ConvertToType(value, field.FieldType));
        }
    }


    public override IDictionary<string, object> Serialize(object obj, JavaScriptSerializer serializer)
    {
        Type type = obj.GetType();
        List<MemberInfo> members = new List<MemberInfo>();
        members.AddRange(type.GetFields());
        members.AddRange(type.GetProperties().Where(p => p.CanRead && p.CanWrite && p.GetIndexParameters().Length == 0));

        Dictionary<string, object> values = new Dictionary<string, object>();

        foreach (MemberInfo member in members)
        {
            JsonPropertyAttribute jsonProperty = (JsonPropertyAttribute)Attribute.GetCustomAttribute(member, typeof(JsonPropertyAttribute));

            if (jsonProperty != null)
            {
                values[jsonProperty.Name] = GetMemberValue(member, obj);
            }
            else
            {
                values[member.Name] = GetMemberValue(member, obj);
            }
        }

        return values;
    }

    private object GetMemberValue(MemberInfo member, object obj)
    {
        if (member is PropertyInfo)
        {
            PropertyInfo property = (PropertyInfo)member;
            return property.GetValue(obj, null);
        }
        else if (member is FieldInfo)
        {
            FieldInfo field = (FieldInfo)member;
            return field.GetValue(obj);
        }

        return null;
    }


    public override IEnumerable<Type> SupportedTypes
    {
        get 
        {
            return new[] { typeof(DataObject) };
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

[AttributeUsage(AttributeTargets.Field | AttributeTargets.Property)]
public class JsonPropertyAttribute : Attribute
{
    public JsonPropertyAttribute(string name)
    {
        Name = name;
    }

    public string Name
    {
        get;
        set;
    }
}

The DataObject class then becomes:

public class DataObject
{
    [JsonProperty("user_id")]
    public int UserId { get; set; }

    [JsonProperty("detail_level")]
    public DetailLevel DetailLevel { get; set; }
}

I appreicate this might be a little late but thought other people wanting to use the JavaScriptSerializer rather than the DataContractJsonSerializer might appreciate it.

Solution 5

Create a class inherited from JavaScriptConverter. You must then implement three things:

Methods-

1. Serialize
2. Deserialize

Property-

1. SupportedTypes

You can use the JavaScriptConverter class when you need more control over the serialization and deserialization process.

JavaScriptSerializer serializer = new JavaScriptSerializer();
serializer.RegisterConverters(new JavaScriptConverter[] { new MyCustomConverter() });

DataObject dataObject = serializer.Deserialize<DataObject>(JsonData);

Here is a link for further information

View more solutions

Comments

[Konstantin Salavatov]

Great solution! With .Net 4.5 seems working fine for enum members with just plain [DataMember] declaration (no need for [EnumMember] etc.)

[Adrian Iftode]

I've used your code with generics like JsonConverter<T> : JavaScriptConverter so this class can be used with any type.

[Matt Smith]

What's in your JsonData? When I do this as you've written, I get a SerializationException to the effect that the Serializer is expecting a root element, as though it's expecting XML. My JSON data is {"user": "THEDOMAIN\\MDS", "password": "JJJJ"}

[Tyler StandishMan]

ASP.NET uses JavascriptSerializer when converting client json (via ajax to a WebMethod) to a .NET object, so users like me working on a website who want to handle client data have no choice but to use Microsoft's version. For serialization, we can of course use Newtonsoft, but deserialization requires this. Source: referencesource.microsoft.com/#System.Web.Extensions/Script/‌​…

[PhonicUK]

A two-line code examples showing using the attribute would be nice to see in this answer.