Creating object with related objects in Entity Framework 4.1

Question

I have the following database tables/EF objects

public class Transaction
{
    //some other properties
    public ICollection<TransactionItems> Items {get; set;}

}

public class TransactionItems
{
    //some properties
}

What I need to do is, create a new instance of transaction along with several instances of TransactionItems for its Items property and save all of these to my DB

I have tried the following:

Transaction trans = new Transaction();
//set its properties

Then in a foreach loop I am looping through a collection and creating a new TransactionItem for each member and attempting to add it to the trans object Item Collection

foreach(var item in myCollection)
{

     TransactionItem newItem = new TransactionItem();
     //set its properties

     //add it to the tran Item collection
    tran.TransactionItems.Add(newItem);//getting null reference here...

}

I am getting a null reference exception when I attempt to add a transactionITem to the Item collection of my Transaction object. What am I doing wrong?

SLaks · Accepted Answer · 2011-11-07 21:47:04Z

2

You need to initialize the property to hold a collection instance in the constructor:

Items = new HashSet<TransactionItems>();

answered Nov 7, 2011 at 21:47

SLaks

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2 Comments

Merlyn Morgan-Graham Over a year ago

If you define it this way be careful that your Equals and GetHashCode operators specifically take the primary keys/unique constraint properties into consideration, and not other properties.

SLaks Over a year ago

@MerlynMorgan-Graham: Or just use reference equality.

Merlyn Morgan-Graham · Accepted Answer · 2011-11-07 21:49:20Z

1

Did you ever Initialize TransactionItems in the constructor for Transaction or in your actual code?

public class Transaction
{
    public Transaction()
    {
        Items = new List<TransactionItems>();
    }

    //some other properties
    public ICollection<TransactionItems> Items {get; set;}
}

Or less preferrably (unless you also do the above):

Transaction trans = new Transaction()
{
    Items = myCollection.Select(
        item => new TransactionItem
        {
            // set its properties
        })
        .ToList();
};

answered Nov 7, 2011 at 21:49

Merlyn Morgan-Graham

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