c# deserialize json to list results in null when json is empty?

Question

I am facing this situation and it is really strange to me.

I have class A has the following

public class A
{
   public A()
   {   
      Values = new List<B>();
   }

   public virtual IList<B> Values { get; set; }
}

I have a method that will read all A records form the db.

var all = await GetAll();

here is my method

public async Task<IList<A>> GetAll(int id1)
{
    var parameters = new[]
    {
        new SqlParameter("@ProductTypeId", SqlDbType.Int) {Value = id1},  
    };

    return (await _sqlCommandWrapper.ExecuteReaderAsync("SPName", r => new  A
        {
            values = JsonConvert.DeserializeObject<IList<B>>(r["Values"].ToString())
        }, parameters)).ToList();
}

my db column r["Values"] will return json that can be deserialized to list of B class.

Then i am using linq to query all my A:

var subitems = all .Where(a=> a.Values .Count > 0).ToList();

If my r["Values"] return data, it works fine.

however if it is not then a.Values is null ?

Any reason, however I defined in my A constructor that Values = new List<B>();

You have a public setter, so somebody must be calling it with a null value. Change the property to set an explicit field, set a break in the setter, and find out where. Probably Json.NET is doing it, in which case you can change the setter to treat an incoming null as an empty list. — dbc
– dbc, Commented Nov 17, 2014 at 5:47

Mrinal Kamboj · Accepted Answer · 2014-11-17 05:59:45Z

1

Reason for the issue is, following code which do the Json deserialization replaces the Values of type (IList<B>), you are not doing anything with the original list that is instantiated in the constructor

values = JsonConvert.DeserializeObject<IList<B>>(r["Values"].ToString())

When r["Values"].ToString() is a valid Json it gets deserialized into IList<B> which is an assignment call, now when r["Values"].ToString() generates a Json that is a mismatch or empty then the result of the following code is Null

JsonConvert.DeserializeObject<IList<B>>(r["Values"].ToString())

In case you plan to use the List instantiated in the constructor then it should be:

Values.Add(JsonConvert.DeserializeObject<B>(r["Values"].ToString()))

In this case r["Values"].ToString() generates the Json for type B not IList<B>

However this would certainly not work as is, since you are generating a new A, which is then added to IList<A> and returned as a Task result, so that part would also need modification

answered Nov 17, 2014 at 5:59

Mrinal Kamboj

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Mrinal Kamboj Over a year ago

Welcome, in case it does help in solving the exact issue at hand then please remember to mark this as an answer

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