Newtonsoft Json Error converting value {null} to type 'System.Int32'

Question

When performing an AJAX request I am getting the following error:

Error converting value {null} to type 'System.Int32'. Path '[5].tabID', line 1, position 331.

The error occurs on the second line of my processRequest (...)

public void ProcessRequest (HttpContext context) { 
    string strJson = new StreamReader(context.Request.InputStream).ReadToEnd();
    List<ElementToUpdate> elements = JsonConvert.DeserializeObject<List<ElementToUpdate>>(strJson);

    // (...)
}

The debugger says that this the content of strJson:

[{
    "bmk": "132M1",
    "state": "off",
    "type": "motor",
    "tabID": 8
}, {
    "bmk": "158M1",
    "state": "off",
    "type": "motor",
    "tabID": 8
}, {
    "bmk": "194M1",
    "state": "off",
    "type": "motor",
    "tabID": 8
}, {
    "bmk": "198M1",
    "state": "on",
    "type": "motor",
    "tabID": 8
}, {
    "bmk": "202M1",
    "state": "off",
    "type": "motor",
    "tabID": 8
}, {
    "bmk": "test-m",
    "state": "on",
    "type": "motor",
    "tabID": null
}, {
    "bmk": "158M1-2",
    "state": "off",
    "type": "motor",
    "tabID": 2
}, {
    "bmk": "100M1",
    "state": "on_right",
    "type": "screwconveyor",
    "tabID": 8
}, {
    "bmk": "152M1",
    "state": "on",
    "type": "screwconveyor",
    "tabID": 8
}, {
    "bmk": "192M1",
    "state": "on_left",
    "type": "screwconveyor",
    "tabID": 8
}, {
    "bmk": "196M1",
    "state": "off",
    "type": "screwconveyor",
    "tabID": 8
}, {
    "bmk": "2000M1",
    "state": "on_left",
    "type": "screwconveyor",
    "tabID": 8
}, {
    "bmk": "74M1",
    "state": "off",
    "type": "screwconveyor",
    "tabID": 8
}, {
    "bmk": "76M1",
    "state": "off",
    "type": "screwconveyor",
    "tabID": 8
}, {
    "bmk": "80M1",
    "state": "off",
    "type": "screwconveyor",
    "tabID": 8
}, {
    "bmk": "82M1",
    "state": "off",
    "type": "screwconveyor",
    "tabID": 8
}, {
    "bmk": "86M1",
    "state": "off",
    "type": "screwconveyor",
    "tabID": 8
}, {
    "bmk": "90M1",
    "state": "off",
    "type": "screwconveyor",
    "tabID": 8
}, {
    "bmk": "94M1",
    "state": "off",
    "type": "screwconveyor",
    "tabID": 8
}, {
    "bmk": "95M1",
    "state": "off",
    "type": "screwconveyor",
    "tabID": 8
}, {
    "bmk": "96M1",
    "state": "off",
    "type": "screwconveyor",
    "tabID": 8
}, {
    "bmk": "102Y1",
    "state": "closed",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "104Y1",
    "state": "open",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "112Y2",
    "state": "closed",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "114Y2",
    "state": "closed",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "120Y1",
    "state": "closed",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "122Y1",
    "state": "closed",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "128Y2",
    "state": "closed",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "146Y1_2",
    "state": "closed",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "148Y2",
    "state": "closed",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "156Y1",
    "state": "closed",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "180Y1",
    "state": "open",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "182Y1",
    "state": "closed",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "184Y1",
    "state": "open",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "206Y1",
    "state": "open",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "208Y1",
    "state": "closed",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "72Y2",
    "state": "closed",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "78Y2",
    "state": "open",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "84Y2",
    "state": "closed",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "88Y2",
    "state": "closed",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "92Y2",
    "state": "closed",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "95_1Y1",
    "state": "blocked",
    "type": "ventile",
    "tabID": 8
}, {
    "bmk": "17H1",
    "state": "on",
    "type": "lamp",
    "tabID": 8
}, {
    "bmk": "l1",
    "state": "on",
    "type": "lamp",
    "tabID": 8
}, {
    "bmk": "17H1-2",
    "state": "on",
    "type": "lamp",
    "tabID": 2
}, {
    "bmk": "106M1",
    "state": "on",
    "type": "elevator",
    "tabID": 8
}, {
    "bmk": "154M1",
    "state": "off",
    "type": "elevator",
    "tabID": 8
}, {
    "bmk": "164M1",
    "state": "off",
    "type": "rotaryvalve",
    "tabID": 8
}]

The class ElementToUpdate is

public class ElementToUpdate
{
public ElementType type;
public String bmk;
public string state;
public int tabID;

public ElementToUpdate()
{
}

public ElementToUpdate(ElementType type, String bmk, string state, int tabID)
{
    this.type = type;
    this.bmk = bmk;
    this.state = state;
    this.tabID = tabID;
}
}

So my question is: How to resolve this issue? If I understand the error message correctly, then it says that tabID of the 5th json object in the serialized array is null. But as you can see it isn't. Moreover ElementToUpdate.tabID isn't an Int32 but an int. Did I miss something?

The solution

In fact, my JSON string contained an element whose tabID was null. I somehow overlooked this because firstly my JSON string wasn't formatted when I checked and secondly because [5] means "6th element of the array" (which I claimed to know actually).

Solution 1

Make your tabID nullable:

public int? tabID;

Value null could not be deserialized to integer. That's your 6-th part of json (or 5-th starting with 0):

{\"bmk\":\"test-m\",\"state\":\"on\",\"type\":\"motor\",\"tabID\":null}

You could see, that tabID is null there.

Solution 2

Depending on your use case, you may also want to consider having the JSON Serializer ignore nulls

Example:

Movie movie = new Movie();
movie.Name = "Bad Boys III";
movie.Description = "It's no Bad Boys";

string included = JsonConvert.SerializeObject(movie,
    Formatting.Indented,
    new JsonSerializerSettings { });

// {
//   "Name": "Bad Boys III",
//   "Description": "It's no Bad Boys",
//   "Classification": null,
//   "Studio": null,
//   "ReleaseDate": null,
//   "ReleaseCountries": null
// }

string ignored = JsonConvert.SerializeObject(movie,
    Formatting.Indented,
    new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore });

// {
//   "Name": "Bad Boys III",
//   "Description": "It's no Bad Boys"
// }

Comments

[emerson.marini]

int = Int32. Just in case... C#, int or Int32? Should I care?

[elementzero23]

@MelanciaUK That's what I thought, too. But the error message made me unsure...

[emerson.marini]

I edited the question to expand the JSON string, so it makes it easier to see where the problem is.

[elementzero23]

Obviously in none of the objects I am passing as JSON array tabID is null.

[Maksim Simkin]

@elementzero23 One of tabID's in your JSON is null, either it was null by serialization, or your serialization was wrong.

[Carele]

Thank you for keeping the answer as is, though. I did not know these kind of things were possible.

[Victorio Berra]

Having to make all of my viewmodel's properties nullable to avoid this issue seems like the wrong way to go about this.

[Paul Schroeder]

var serializerSettings = new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore };

[LastTribunal]

This is the best answer. changing the model property to nullable is not always an option

[LastTribunal]

changing model definitions is not always an option

[PinoyDev]

I agree with @LastTribunal, tried so many times (11 tests) except for the settings from paulschoeder

[Samaritan_Learner]

This should be marked as correct answer, Because if we start changing the data type to nullable in real time, there are possible to change n number of places. Atleast in my case I was in need to change 50+places and we need to type case everywhere where already implemented using not nullable. So this is more appropriate solution to me,

[Soner from The Ottoman Empire]

Simplicity is the ultimate sophistication. Overlooking simply. +1