How to assign JsonProperty attribute to properties of classes inside DLL using Json.Net?

Question

I have a class inside a DLL which is not labelled with DataContract, JsonProperty etc. Now I want to serialize instance of the class as JSON objects, with the C# property names shortened.

For instance, the class is:

public class Foo
{
    public string SomeLengthyCSharpPropertyName { get; set; }
}

I wonder if I could create a mapping between the C# names and the json names. I cannot directly add the DataContract, JsonProperty attributes like below. Is there any workaround?

[DataContract]
public class Foo
{
    [JsonProperty("s")]
    public string SomeLengthyCSharpPropertyName { get; set; }
}

I tend not to create a another class with the same but JsonProperty-decorated properties and copy the properties to the new class and then serialize.

Answer 1

You can make your own custom ContractResolver with a dictionary of override attributes by member, then override CreateProperty() and apply the overrides to the JsonProperty returned by the base class:

public class JsonPropertyOverride
{
    public string PropertyName { get; set; }

    public bool? Ignored { get; set; }

    // Others as required from http://www.newtonsoft.com/json/help/html/T_Newtonsoft_Json_JsonPropertyAttribute.htm
    // Changing all value type properties to nullables.
}

public class OverrideContractResolver : DefaultContractResolver
{
    readonly Dictionary<MemberInfo, JsonPropertyOverride> overrides; // A private copy for thread safety.

    public OverrideContractResolver(IDictionary<MemberInfo, JsonPropertyOverride> overrides)
        : base()
    {
        if (overrides == null)
            throw new ArgumentNullException();
        this.overrides = overrides.ToDictionary(p => p.Key, p => p.Value);
    }

    protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
    {
        var property = base.CreateProperty(member, memberSerialization);
        if (property != null)
        {
            JsonPropertyOverride attr;
            if (overrides.TryGetValue(member, out attr))
            {
                if (attr.PropertyName != null)
                    property.PropertyName = ResolvePropertyName(attr.PropertyName);
                if (attr.Ignored != null)
                    property.Ignored = attr.Ignored.Value;
            }
        }
        return property;
    }
}

You could also inherit from CamelCasePropertyNamesContractResolver if you prefer.

Then use it like:

public class Foo
{
    public string SomeLengthyCSharpPropertyName { get; set; }

    public string DefaultNotIgnored { get; set; }

    [JsonIgnore]
    public string DefaultIgnored { get; set; }
}

public class TestClass
{
    public static void Test()
    {
        var foo = new Foo { SomeLengthyCSharpPropertyName = "SomeLengthyCSharpPropertyName", DefaultIgnored = "DefaultIgnored", DefaultNotIgnored = "DefaultNotIgnored" };

        var resolver = new OverrideContractResolver(new Dictionary<MemberInfo, JsonPropertyOverride> { 
            { typeof(Foo).GetProperty("SomeLengthyCSharpPropertyName"), new JsonPropertyOverride { PropertyName = "c"  } }, 
            { typeof(Foo).GetProperty("DefaultNotIgnored"), new JsonPropertyOverride { Ignored = true  } }, 
            { typeof(Foo).GetProperty("DefaultIgnored"), new JsonPropertyOverride { Ignored = false  } }, 
        });
        var settings = new JsonSerializerSettings { ContractResolver = resolver };

        var json = JsonConvert.SerializeObject(foo, settings); // Outputs {"c":"SomeLengthyCSharpPropertyName","DefaultIgnored":"DefaultIgnored"}
        Debug.WriteLine(json);

        var expectedJson = @"{ ""c"": ""SomeLengthyCSharpPropertyName"", ""DefaultIgnored"": ""DefaultIgnored"" }";
        var ok = JToken.DeepEquals(JToken.Parse(json), JToken.Parse(expectedJson));
        Debug.Assert(ok); // No assert

        var foo2 = JsonConvert.DeserializeObject<Foo>(json, settings);

        var ok2 = foo2.DefaultIgnored == foo.DefaultIgnored && foo2.SomeLengthyCSharpPropertyName == foo.SomeLengthyCSharpPropertyName;
        Debug.Assert(ok2); // No assert
    }
}