Loop Through a list and add to another list using linq - REPLACING FOREACH

Question

I have two Class MyObjectA and MyObjectB

public class MyObjectA    
{
    private int A_ID {get; set;}
    private int B_ID {get; set;}
    .
    .
    .
    .        
}

public class MyObjectB    
{
    private int B_ID {get; set;}
    .
    .
    .
    .

    public MyObjectB (int B_ID )
    {            
        DAL.GetMyObjectBFromDatabase(B_ID );
    }
}

Now what I am trying to do in my function is

public void DoSomething()
{

    List<MyObjectA> ListA = new List<MyObjectA>();
    List<MyObjectB> ListB = new List<MyObjectB>();

    ListA  = DAL.GetMyObjectAListFromDatabase();  

I get an error in the line below for null exception in x.B_ID. If i debug through it. ListA has count > 0 and every x.B_ID has values . but the minute the below code is executed ListA becomes null not sure why.

I REPEAT THERE IS NO NULL IN THE ListA. WHEN THE FOREACH IS EXECUTED ListA BECOMES NULL. I WASDoing THIS TO REPLACE C# FOREACH. MY LIST IS CORRECT FOR FOREACH TO WORK. sO MY QUESTION WAS WHY THIS IS HAPPENING

    ListA.ForEach(x => {
            ListB.Add(new MyObjectB(x.B_ID));
    });
}

All I am trying to do is looping through ListA to get B_ID and using B_ID to get ListB. As done in this linq. Linq style "For Each"

Please help me understand where I am going wrong

Answer 1

Your NullReferenceException is caused by x being null, that means a member of ListA is null, not that ListA itself is null.

For instance if you had called the following 3 lines instead of ListA = DAL.GetMyObjectAListFromDatabase();.

ListA.Add(new MyObjectA());
ListA.Add(null);
ListA.Add(new MyObjectA());

EDIT:

Looking at what you are doing again the correct tool for what you are doing is Select.

ListB = ListA.Select(x => new MyObject(x.B_ID)).ToList();