C# linq DefaultIfEmpty() NullReferenceException Value cannot be empty. Parameter name: Source

Question

I have a problem How can I inquire in a linq correctly Get a mistake NullReferenceException every time

    public class Model
    {
        public int Nmb { get; set; }
        public string _UserId { get; set; }
    }
    public static List<Model> _models_List { get; set; }

        string user = "test name";

        int test = _models_List.Where(o => o._UserId == user).Select(o => o.Nmb).DefaultIfEmpty(0).First();

        if (test == 0)
        {         
            Model obj = new Model();
            obj._UserId = user;
            obj.Nmb = 1;
            _models_List.Add(obj);
        }

I tried to correct the code like this But I get the same mistake NullReferenceException Value cannot be empty. Parameter name: Source

    int test = _models_List.Where(o => o._UserId == user).Select(o => o.Nmb).FirstOrDefault();

Please Help

`string _UserId = _models_List.Where(o => o._UserId == user).Select(o => o._UserId).FirstOrDefault();`, can be replaced with `var userExists = _models_List.Any(o => o._UserId == user);` — Igor, Mar 27 '20 at 17:16
[edit] your question and include a [mcve]. You could provide test data for `_models_List` — Igor, Mar 27 '20 at 17:19
Thanks for your response, sir you are right But I just want to know how to inquire the correct in linq, so I attached this example — hasan81, Mar 27 '20 at 17:21

Alireza Mahmoudi · Accepted Answer · 2020-03-27T17:35:21.480

2

The list _models_List is empty, to solve the issue replace the following code:

public static List<Model> _models_List { get; set; }

with:

public static List<Model> _models_List { get; set; } = new List<Model>();

edited Mar 27 '20 at 17:35

answered Mar 27 '20 at 17:25

Alireza Mahmoudi

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C# linq DefaultIfEmpty() NullReferenceException Value cannot be empty. Parameter name: Source

1 Answers1