在Entity Framework 4.1中创建具有相关对象的对象

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?

Answer 1

您需要初始化属性以在构造函数中保存集合实例：

Items = new HashSet<TransactionItems>();

Answer 2

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();
};