Adding values to a list - NullPointerException C#

Question

I have a class which allows me to store a time as follows.

    class LockTime
{

    public int Start { get; set; }

    public int End {get; set; }

    public LockTime (int start, int end)
    {
        Start = start;
        End = end;

    }

}

In another class I declare a list of this object.

private List<LockTime> listOfTimes;

I have a method in this 2nd class to then add to this list.

LockTime theTime = new LockTime((pickerTimeStart.Value.Hour + pickerTimeStart.Value.Minute), (pickerTimeEnd.Value.Hour + pickerTimeEnd.Value.Minute));

listOfTimes.Add(theTime);

When it trys to add to the list it throws the NullPointerException. In the debugger it shows 2 values in theTime variable. I don't quite understand why it's saying NullPointer.

Also at the start where I've declared listOfTimes it is underlined blue saying that the field is never assigned to anything and will always have a null value.

This small little issue is driving me nuts, am I just completely missing something. Please help!

Have you initialized listOfItems ? listOfTimes = new List<LockTime>(); ? — Réda Mattar
– Réda Mattar, Commented Apr 26, 2014 at 19:12

Mike Zboray · Accepted Answer · 2014-04-26 19:12:08Z

6

You need to initialize the field, otherwise its default value is null because List<T> is a reference type.

Use private List<LockTime> listOfTimes = new List<LockTime>();

answered Apr 26, 2014 at 19:12

Mike Zboray

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3 Comments

JARRRRG Over a year ago

Sad thing is, I had tried = new List<LockTime> but I was missing the braces. I've wasted almost 2-3 hours pulling my hair out. Thanks for such a quick response.

Mike Zboray Over a year ago

@Jagga If you forgot the parentheses you would get an error like "A new expression requires (), [], or {} after type". What did you not understand about that? (Not a criticism of you, I'm just curious about what you thought it meant. When you know a language its hard to see how others misunderstand it.)

JARRRRG Over a year ago

I kept doing ..=new List<LockTime>(0,0); and stuff. Then I started to change the accessor from private to public and the whole lot. I basically did a load of unnessecary changes simply because I didn't know you can say or perhaps remember I need the braces. For some reason my brain kept going, constructor requires 2 ints. I dunno.

Domin8urMind · Accepted Answer · 2014-04-26 19:14:28Z

4

Seems you didn't allocate the list object .

maybe change

private List<LockTime> listOfTimes;

to:

private List<LockTime> listOfTimes = new List<LockTime>();

answered Apr 26, 2014 at 19:14

Domin8urMind

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Comments

Sam · Accepted Answer · 2014-04-26 19:12:16Z

2

You declared the list, but didn't initialize it, so it currently points to null.

private List listOfTimes = new List();

answered Apr 26, 2014 at 19:12

Sam

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Comments

shree.pat18 · Accepted Answer · 2014-04-26 19:12:19Z

2

You declare but do not initialize the list. So you are trying to call Add on what is essentially null.

answered Apr 26, 2014 at 19:12

shree.pat18

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Comments

srsyogesh · Accepted Answer · 2014-04-26 19:14:56Z

2

You have not initialized the list. The place where you are seeing the warning do something like this ,

   private List<LockTime> listOfTimes = new List <LockTime>()

answered Apr 26, 2014 at 19:14

srsyogesh

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