C# List + Object reference not set to an instance of an object

Question

I have this code in C#. It is returning "Object reference not set to an instance of an object" exception.

Code:

    public decimal Calculate(String id)
    {
        decimal Total=0;
        AmountDataDB getData=new AmountDataDB();
        List<AmountData> d = new List<AmountData>();
        d = getData.Amount_Details(id);
        if (d.Capacity != 0)
        {
            foreach (AmountData temp in d)//NullReference exception occurs here
            {
                Total += temp.Amount;
            }

        }
        return Total;
    }

Here, AmountDataDB and AmountData are two classes. Amount_Details returns a list of type AmountData.

Your AmountData object in your list is possibly NULL. Check it — Tony The Lion
– Tony The Lion, Commented Jun 21, 2010 at 11:10
If it was returning null, then if(d.capacity...) should throw the exception — cjk
– cjk, Commented Jun 21, 2010 at 11:15
@user372066 are you sure exception is thrown at foreach line? — apoorv020
– apoorv020, Commented Jun 21, 2010 at 11:28

leppie · Accepted Answer · 2010-06-21 11:12:22Z

4

Non-answer (advice):

Please, please x 100000, dont ever write code like:

List<AmountData> d = new List<AmountData>();
d = getData.Amount_Details(id);

It is totally pointless creating a new list only to have it overwritten in the next line.

answered Jun 21, 2010 at 11:12

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leppie

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Comments

Darin Dimitrov · Accepted Answer · 2010-06-21 11:12:32Z

4

You could use the null-coalescing operator to ensure that the d variable is never null:

List<AmountData> d = getData.Amount_Details(id) ?? new List<AmountData>();

answered Jun 21, 2010 at 11:12

Darin Dimitrov

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Comments

Femaref · Accepted Answer · 2010-06-21 11:11:41Z

1

getData.Amount_Details(id); returns null, probably because there are no records for that particular id. You probably should change the behaviour of Amount_Details() to return an empty list instead of null if there are no records.

answered Jun 21, 2010 at 11:11

Femaref

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4 Comments

Ed James Over a year ago

Surely that would crash when he called capacity on the null reference on the next line, not at the foreach? (agreed with the advice though, never return null collections!)

Femaref Over a year ago

hm, you are right. Either it has to occur in the conditional or in the foreach block (because either d or temp are null) but in the foreach-decleration? hm.

Rup Over a year ago

Is there any way that the result of Amount_Details could be a subclass of List<> that has overridden the GetEnumerator() method? I wouldn't have thought that was possible, but I can't think what else happens on the foreach line.

Femaref Over a year ago

This could be possible. But without a call stack it is impossible to say.

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