Serializing object with list of object arrays throws NullReferenceException using Json.NET

Question

So I have a lot of data that just needs to be stored and not configured so I've decided to store it as Json in my database.

List<fooData[]> foo = new List<fooData[]>();
List<faaData[]> faa = new List<faaData[]>();
foreach(Choice c in choices) 
{ 
    foo.Add(getFooData(c)) // Returns IEnumerable<fooData>
    faa.Add(getFaaData(c)) // Returns IEnumerable<faaData>
}
var JsonThis = MyJsonObject 
{
    fooArray = foo.ToArray(),
    faaArray = faa.ToArray()
};

string JsonString = JsonConvert.SerializeObject(JsonThis, Formatting.Indented,
                 new JsonSerializerSettings { PreserveReferencesHandling = PreserveReferencesHandling.Objects });

Fine, that worked as expected. My problem occurs when I Deserialize my Json again, no matter if I take from the database or I do it on the next line, I just get thrown a NullReferenceException on the properties of fooData & faaData.

Heres the code:

MyJsonObject obj = JsonConvert.DeserializeObject<MyJsonObject>(JsonString,
                 new JsonSerializerSettings { PreserveReferencesHandling = PreserveReferencesHandling.Objects });

JsonObject

[JsonObject(IsReference = true)]
public class MyJsonObject
{
    public fooData[][] foo { get; set; }
    public faaData[][] faa { get; set; }
}

faaData & fooData are 99% identical

[JsonObject(IsReference = true)]
public class faaData 
{
    public string faaString1 { get; set; }
    public string faaString2 { get; set; }
    public string faaString3 { get; set; }
}

serialized JsonString example:

{
  "Foo": [
    [
      {
        "FooString1": "foo string 1",
        "FooString2": "foo string 2",
        "FooString3": "foo string 3"
      },
      {
        "FooString1": "foo string 4",
        "FooString2": "foo string 5",
        "FooString3": "foo string 6"
      }
    ],
    [
      {
        "FooString1": "foo string 1",
        "FooString2": "foo string 2",
        "FooString3": "foo string 3"
      },
      {
        "FooString1": "foo string 4",
        "FooString2": "foo string 5",
        "FooString3": "foo string 6"
      }
    ]
  ],
  "Faa": [
    [
      {
        "FaaString1": "faa string 1",
        "FaaString2": "faa string 2",
        "FaaString3": "faa string 3"
      },
      {
        "FaaString1": "faa string 4",
        "FaaString2": "faa string 5",
        "FaaString3": "faa string 6"
      }
    ],
    [
      {
        "FaaString1": "faa string 1",
        "FaaString2": "faa string 2",
        "FaaString3": "faa string 3"
      },
      {
        "FaaString1": "faa string 4",
        "FaaString2": "faa string 5",
        "FaaString3": "faa string 6"
      }
    ]
  ]
}

My question: Is it not possible to Serialize MyJsonObject with arrays of arrays of objects and get it back in the same object as initially? Or Am I doing something wrong?

Can you construct a minimal reproducible example consisting of (1) a minimal definition of MyJsonObject, (2) a literal JSON string and (3) a call to DeserializeObject that fails? — Mo B.
– Mo B., Commented Dec 2, 2020 at 13:18

Saeid Amini · Accepted Answer · 2020-12-02 14:04:25Z

There is no problem with a data structure like this(Array of Array of Objects) when we are working with JSON.

Try this in your code:

List<FooData[]> foo = new List<FooData[]>()
{
    new FooData[]
    {
        new FooData() { FooString1 = "foo string 1" , FooString2 = "foo string 2", FooString3 = "foo string 3" },
        new FooData() {FooString1 = "foo string 4" , FooString2 = "foo string 5", FooString3 = "foo string 6" }
    },
    new FooData[]
    {
        new FooData() { FooString1 = "foo string 1" , FooString2 = "foo string 2", FooString3 = "foo string 3" },
        new FooData() {FooString1 = "foo string 4" , FooString2 = "foo string 5", FooString3 = "foo string 6" }
    }
};

List<FaaData[]> faa = new List<FaaData[]>()
{
    new FaaData[]
    {
        new FaaData() { FaaString1 = "faa string 1" , FaaString2 = "faa string 2", FaaString3 = "faa string 3" },
        new FaaData() {FaaString1 = "faa string 4" , FaaString2 = "faa string 5", FaaString3 = "faa string 6" }
    },
    new FaaData[]
    {
        new FaaData() { FaaString1 = "faa string 1" , FaaString2 = "faa string 2", FaaString3 = "faa string 3" },
        new FaaData() {FaaString1 = "faa string 4" , FaaString2 = "faa string 5", FaaString3 = "faa string 6" }
    }
};

var JsonThis = new 
{
    Foo = foo.ToArray(),
    Faa = faa.ToArray()
};

string JsonString = JsonConvert.SerializeObject(JsonThis, Formatting.Indented,
                    new JsonSerializerSettings { PreserveReferencesHandling = PreserveReferencesHandling.Objects });

var result = JsonConvert.DeserializeObject<MyJsonObject>(JsonString);

This is the new definition of classes. I tried to follow the C# Coding Standards and Naming Conventions

public class MyJsonObject
{
    public FooData[][] Foo { get; set; }
    public FaaData[][] Faa { get; set; }
}

public class FooData
{
    public string FooString1 { get; set; }
    public string FooString2 { get; set; }
    public string FooString3 { get; set; }
}

public class FaaData
{
    public string FaaString1 { get; set; }
    public string FaaString2 { get; set; }
    public string FaaString3 { get; set; }
}

This is the result:

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