Why am I getting a null exception when trying to assign a value to a nested object in C# asp.net 5?

Question

I am trying to assign a value to a nested object however I get a null exception.

System.NullReferenceException: Object reference not set to an instance of an object.

code below

addingTaskScheduleDto addToReturnTask = new addingTaskScheduleDto();


userTaskDto test = new userTaskDto();
test.UserId = task.UserTasks.FirstOrDefault().UserId;
test.TaskScheduleId = task.UserTasks.FirstOrDefault().TaskScheduleId;

// null exception happens on the code below
addToReturnTask.UserTasks.Add(test);

addingTaskScheduleDto model

public class addingTaskScheduleDto
{
    public int Id { get; set; }
    public string Title { get; set; }
    public DateTime? Start { get; set; }
    public DateTime? End { get; set; }

    public List<userTaskDto> UserTasks {get; set;}
}


public class userTaskDto
{
    public int UserId { get; set; }
    public int TaskScheduleId { get; set; }
}

score 1 · Accepted Answer · answered May 31 '21 at 18:07

1

You get an exception because UserTasks is null. You need to initialize it with an empty list at some point. You could do it when you instantiate addingTaskScheduleDto like this

    public class addingTaskScheduleDto
    {
        public int Id { get; set; }
        public string Title { get; set; }
        public DateTime? Start { get; set; }
        public DateTime? End { get; set; }
    
        public List<userTaskDto> UserTasks {get; set;} = new List<userTaskDto>();
    }

answered May 31 '21 at 18:07

Hans Kilian

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21

FirstOrDefault() can also return null and you still get null exception... Best solution is to check for null before accessing the property – EGN May 31 '21 at 18:11
I copied the code above, however, I still get the null exception error. – jkeelyunreely May 31 '21 at 18:16
I just ran it and it removed the exception for me. – Hans Kilian May 31 '21 at 18:27
@EGN wouldn't the null exception then be on `test.UserId = task.UserTasks.FirstOrDefault().UserId;` rather than the line OP indicates? – Hans Kilian May 31 '21 at 18:29
1

Works for me now. It wasn't the FirstOrDefault() as I can see a value was there, it was because I didn't know about initialising a nested object. – jkeelyunreely May 31 '21 at 18:33

Why am I getting a null exception when trying to assign a value to a nested object in C# asp.net 5?

1 Answers1