Deserialise JSON to Dictionary where the JSON string contains a null value

Question

I have a very simple class for the deserialisation of a JSON string into a dictionary

private static Dictionary<string,string> DeserializeJSON(string json)
{
    var dict = new Dictionary<string,string>();
    if (string.IsNullOrEmpty(json))
       return dict;

    var jsDict = JsonConvert.DeserializeObject<Dictionary<string,string>>(json);
    if (jsDict != null)
         dict = jsDict;
    return dict;
}

This does deserialise the string, but dies when the string contains a null value for example

["ImplementID",null,"ImplementType","Flexicoil Airseeder","SeedUnits","Kg/H","SeedApplicationRate","75","CropType","Wheat(Morombi)","Fertiliser0ID","2edd3043","FertiliserRate0","100","FertiliserRateUnit0","Kg/H"]

with the following throwback

2014-04-23 01:39:07.212 myapp[63857:70b] Ouch, something went pop : Cannot deserialize the current JSON array (e.g. [1,2,3]) into type 'System.Collections.Generic.Dictionary`2[System.String,System.String]' because the type requires a JSON object (e.g. {"name":"value"}) to deserialize correctly. To fix this error either change the JSON to a JSON object (e.g. {"name":"value"}) or change the deserialized type to an array or a type that implements a collection interface (e.g. ICollection, IList) like List that can be deserialized from a JSON array. JsonArrayAttribute can also be added to the type to force it to deserialize from a JSON array. Path '', line 1, position 1.- at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.EnsureArrayContract (Newtonsoft.Json.JsonReader reader, System.Type objectType, Newtonsoft.Json.Serialization.JsonContract contract) [0x00000] in :0 at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateList (Newtonsoft.Json.JsonReader reader, System.Type objectType, Newtonsoft.Json.Serialization.JsonContract contract, Newtonsoft.Json.Serialization.JsonProperty member, System.Object existingValue, System.String id) [0x00000] in :0 at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateValueInternal (Newtonsoft.Json.JsonReader reader, System.Type objectType, Newtonsoft.Json.Serialization.JsonContract contract, Newtonsoft.Json.Serialization.JsonProperty member, Newtonsoft.Json.Serialization.JsonContainerContract containerContract, Newtonsoft.Json.Serialization.JsonProperty containerMember, System.Object existingValue) [0x00000] in :0 at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize (Newtonsoft.Json.JsonReader reader, System.Type objectType, Boolean checkAdditionalContent) [0x00000] in :0

Is there a simple way to ignore the null value (and key) when deserializing the JSON string?

user2864740 · Accepted Answer · 2014-04-23 00:56:49Z

It has nothing to do with a null value. It is because the input represents a JSON Array (a sequence of values) and cannot be directly mapped to a Dictionary (a set of key-value pairs) as so.

Try it with a non-null value and you'll get the same result.

Cannot deserialize the current JSON array (e.g. [1,2,3]) .. because the [Dictionary] type requires a JSON object (e.g. {"name":"value"}) to deserialize correctly.

Arguably the correct solution is to get and parse JSON Object input. Compare the posted JSON with this variation, which would have deserialized fine, even with a null value.

{"ImplementID": null, "ImplementType": "Flexicoil Airseeder", "SeedUnits": "Kg/H"}

However, if you cannot get the JSON in a sensible form, simply derserialize the JSON Object as a List and then convert the List to a Dictionary as required.

var data = JsonConvert.DeserializeObject<List<string>>(json);
var dict = new Dictionary<string,string>();
for (var i = 0; i < data.length; i += 2) {
   var key = data[i];
   var value = data[i+1];
   dict[key] = value;
}

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