Exception when adding item to list<t>

Question

I always get a "An unhandled exception of type 'System.NullReferenceException' occurred in myapp.exe" when code reaches _match.Players2.Add(_hero);

I am not sure why, I initialized _match as well as _hero, when I debug I see that there are no null values. I am not sure why I am getting this exception, am I missing something?

Here is my code:

public static List<MyDotaClass.MatchHistory> GetMatchHistory(string uri)
{
    var HeroCollection = new List<MyDotaClass.DotaHero>();
    var MatchCollection = new List<MyDotaClass.MatchHistory>();

    string response = GetDotaWebResponse(uri);
    dotadata.Dota.MatchHistoryRootObject matches = JsonConvert.DeserializeObject<dotadata.Dota.MatchHistoryRootObject>(response);
    foreach (var match in matches.result.matches)
    {
        var _match = new MyDotaClass.MatchHistory();
        _match.MatchID = match.match_id.ToString();

        foreach (var player in match.players)
        {
            var _hero = new MyDotaClass.DotaHero();

            foreach(var h in heros)
            {
                if(player.hero_id.ToString().Equals(h.HeroID))
                {
                    _hero.HeroName = h.HeroName;
                    _hero.HeroNonCleanName = h.HeroNonCleanName;
                    _hero.HeroID = h.HeroID;
                    _match.Players2.Add(_hero);
                }
            }
        }
        MatchCollection.Add(_match);
    }
    return MatchCollection;
}

public class MyDotaClass
{
    public class DotaHero
    {
        public string HeroName { get; set; }
        public string HeroNonCleanName { get; set; }
        public string HeroID { get; set; }
    }

public class MatchHistory
{
    public string MatchID { get; set; }
    //public List<DotaHero> Players { get; set; }
    public List<MyDotaClass.DotaHero> Players2 { get; set; }
}

UPDATE:

For those interested in Dota, I lost all of my original code so I am rewriting and doing my best to make a tutorial out of it. http://uglyvpn.com/2014/07/21/dota-2-net-c-tool-pt1/

did you initialize Players2?

Dustin Hodges
– Dustin Hodges

2014-02-19 23:56:29 +00:00
Commented Feb 19, 2014 at 23:56 — Dustin Hodges
– Dustin Hodges, Commented Feb 19, 2014 at 23:56

p.s.w.g · Accepted Answer · 2014-02-19 23:56:51Z

Make sure the list in Players2 is initialized properly. It could be done in your foreach loop, like this:

foreach (var match in matches.result.matches)
{
    var _match = new MyDotaClass.MatchHistory();
    _match.MatchID = match.match_id.ToString();
    _match.Players2 = new List<MyDotaClass.DotaHero>();
    ...

But that's not a great design. It would be more robust to initialize it in the MatchHistory constructor, like this:

public class MatchHistory
{
    public string MatchID { get; set; }

    //public List<DotaHero> Players { get; set; }

    public List<MyDotaClass.DotaHero> Players2 { get; private set; }

    public MatchHistory() {
        this.Players2 = new List<MyDotaClass.DotaHero>();
    }
}

Note that I have also made the setter private here, as there should be no need to modify the value of this property after it's been initialized in the constructor.

Yair Nevet · Accepted Answer · 2014-02-20 00:14:27Z

1

It is look like your Players2 collection is not instantiated, so do it first:

this.Players2 = new List<MyDotaClass.DotaHero>();

and then you would be able to refer and use it:

_match.Players2.Add(_hero);

As for where to instantiate it, do it in the constructor of MatchHistory:

public MatchHistory() {
    this.Players2 = new List<MyDotaClass.DotaHero>();
}

This is a good practice, because next time, even if you will forget to instantiate it, the constructor will do it automatically for you.

Otherwise, you'd keep getting: System.NullReferenceException

UPDATE:

You can't use this in a static context, change this method in order to enjoy from the constructor-base instantiating:

public static List<MyDotaClass.MatchHistory> GetMatchHistory(string uri)

to this:

public List<MyDotaClass.MatchHistory> GetMatchHistory(string uri)

edited Feb 20, 2014 at 0:14

answered Feb 19, 2014 at 23:58

Yair Nevet

13.1k17 gold badges71 silver badges111 bronze badges

2 Comments

nGX Over a year ago

I can't use this method without changing a bunch of my code since my method is static. I get this error: Error 1 Keyword 'this' is not valid in a static property, static method, or static field initializer

Yair Nevet Over a year ago

@KPS Well, you don't have an instance(this) in a static context!

evanmcdonnal · Accepted Answer · 2014-02-20 05:05:01Z

1

You initialized the MatchHistory instance but not the Players2 property so it's still null.

You probably want to allocate it in the constructor like this although there are other options;

public MatchHistory()
{
    Players2 = new List<MyDotaClass.DotaHero>();
}

PS LoL > Dota :p

edited Feb 20, 2014 at 5:05

answered Feb 19, 2014 at 23:57

evanmcdonnal

48.5k17 gold badges111 silver badges119 bronze badges

1 Comment

nGX Over a year ago

Almost same advice as p.s.w.g gave, thanks guys! I've been wanting to play LoL but Dota gives way more eyecandy I think

Collectives™ on Stack Overflow

Exception when adding item to list<t>

3 Answers 3

Comments

2 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related