Unable to Add Dictionary to List>

Question

I hit a null reference exception when attempting to add a Dictionary<string,string> to a List<Dictionary<string,string>> variable. I'm unable to find the cause.

public class MyObject
{
    public List<Dictionary<string, string>> ID_Name_List { get; set; }
}

MyObject obj1 = new MyObject();

Dictionary<string, string> ID_Name_Pair = new Dictionary<string, string>();

ID_Name_Pair.Add("1", "Jane");

List<Dictionary<string, string>> ID_Name_List_2 = new List<Dictionary<string, string>>(); //For Testing

ID_Name_List_2.Add(ID_Name_Pair); //SUCCESS

obj1.ID_Name_List.Add(ID_Name_Pair); //ERROR - System.NullReferenceException: 'Object reference not set to an instance of an object.'

My obj1.ID_Name_List variable has the same List<Dictionary<string, string>> type with ID_Name_List_2

Why did my obj1.ID_Name_List hit an error while ID_Name_List_2 is successful?

Any help will be greatly appreciated

Whee do you assign a value to `ID_Name_List`? Try `public List> ID_Name_List { get; } = new List>();` or assign a value in the constructor of `MyObject`. — Klaus Gütter, Mar 07 '21 at 09:14
Nope, I used it directly from the object to add the dictionary value. Am I missing an action? — gymcode, Mar 07 '21 at 09:15
You forgot to init ID_Name_List ... obj1.ID_Name_List = new Dictionary(); — Saledan, Mar 07 '21 at 09:16
Null reference is not about type mismatch, it's just null there, you never call new on the list. — Wiktor Zychla, Mar 07 '21 at 09:16

score 1 · Accepted Answer · answered Mar 07 '21 at 09:16

1

You have to initialize ID_Name_List.

public class MyObject
{
    public List<Dictionary<string, string>> ID_Name_List { get; set; }

    public MyObject()
    {
        ID_Name_List = new List<Dictionary<string, string>>();
    }
}

answered Mar 07 '21 at 09:16

user2250152

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Geez! Thank you!! – gymcode Mar 07 '21 at 09:19

score 1 · Answer 2 · answered Mar 07 '21 at 09:24

of course you ran into a NullReferenceException. Your obj1.obj1.ID_Name_List (object) is not initialized.

try this

obj1.ID_Name_List = new List<Dictionary<string, string>();
obj1.ID_Name_List.Add(ID_Name_Pair); // will work like a charm;

alternatively,

public class MyObject
{
    public List<Dictionary<string, string>> ID_Name_List { get; set; }
    public MyObject() {
         ID_Name_List = new List<Dictionary<string, string>>();
    }
}

Unable to Add Dictionary to List>

2 Answers2