.net How to add Add to nested list object

I have the following class object
I fill a DataTable with data from my sp and the message from the DebtAllOut class
I roll through the DataTable and filling a list

public class DebtAllOut
{
	public string message { get; set; }
	public List<DebtAllDetail> debtalldetail { get; set; }
}

public class DebtAllDetail
{
	public int debtid { get; set; }
	public string name { get; set; }
}

var debtallout = new DebtAllOut();
da.SelectCommand.CommandType = CommandType.StoredProcedure;
da.SelectCommand.Parameters.AddWithValue("@ProfileID", profileid);
DataTable dt = new DataTable();
da.Fill(dt);
debtallout.message = "Providing general message";

List<DebtSingleOut> lstDebtSingleOut = new List<DebtSingleOut>();
if (dt.Rows.Count > 0)
{
  	for (int i = 0; i < dt.Rows.Count; i++)
   	{
       	DebtSingleOut debtsingleout = new DebtSingleOut();
        if (dt.Rows[i]["RowID"] != DBNull.Value) {
			debtsingleout.debtid =     Convert.ToInt32(dt.Rows[i]    ["RowID"]); 
		}
  		lstDebtSingleOut.Add(debtsingleout);
	}
}
debtallout.debtalldetail.AddRange(lstDebtSingleOut);	

The last line gives the the error. When I try to add the list to the debtalldetail nested class
debtallout.debtalldetail.AddRange(lstDebtSingleOut);

The error I receive is
Error CS1503 Argument 1: cannot convert from 'System.Collections.Generic.List<mystuff.Models.DebtSingleOut>' to 'System.Collections.Generic.IEnumerable<mystuff.Models.DebtAllDetail>'

You're trying to add a list lstDebtSingleOut of type List<DebtSingleOut> to the debtalldetail property which is of type List<DebtAllDetail>. It's not easy to understand full logic but I think you mixed up classes DebtAllDetail and DebtSingleOut.

You need to create a new DebtAllDetail object for each row and add it to the debtalldetail list or you can fix the code change the type to the necessary.

var debtallout = new DebtAllOut();
debtallout.message = "Providing general message;"

// inirialize new instance of List<DebtAllDetail> to add elements 
debtallout.debtalldetail = new List<DebtAllDetail>();

da.SelectCommand.CommandType = CommandType.StoredProcedure;
da.SelectCommand.Parameters.AddWithValue("@ProfileID", profileid);
DataTable dt = new DataTable();
da.Fill(dt);

if (dt.Rows.Count > 0)
{
    for (int i = 0; i < dt.Rows.Count; i++)
    {
        // create and initialize new DebtAllDetail object
        DebtAllDetail debtAllDetail = new DebtAllDetail();
        if (dt.Rows[i]["RowID"] != DBNull.Value)
            debtAllDetail.debtid = Convert.ToInt32(dt.Rows[i]["RowID"]);
        if (dt.Rows[i]["Name"] != DBNull.Value)
            debtAllDetail.name = dt.Rows[i]["Name"].ToString(); // or another code to retrieve the name

        // add initialized DebtAllDetail object to list
        debtallout.debtalldetail.Add(debtAllDetail);
    }
}

Given this class definition:

public class DebtAllOut
{
    public string message { get; set; }
    public List<DebtAllDetail> debtalldetail { get; set; }
}

It's important to understand the result of this line:

var debtallout = new DebtAllOut(); 

At this point, the value of debtallout.debtalldetail is still null!

This is because the class definition did not specify to actually create the list object. It only specified to create a variable that might (or might not) refer to some list variable. That is, you have a reference, but the reference doesn't refer to anything yet.

You can fix it in two ways. First, by changing the class definition:

public class DebtAllOut
{
    public string message { get; set; }
    public List<DebtAllDetail> debtalldetail { get; private set; } = new List<DebtAllDetail>();
}

(Notice the use of private. You will still be able to add or remove items from the list, because accessing members of the list, including methods like Add(), is still a get operation.)

The second fix is to manually create a new list instance:

var debtallout = new DebtAllOut(); 
debtallout.debtalldetail = new List<DebtAllDetail>();

But all that aside, we can significantly improve the code:

da.SelectCommand.CommandType = CommandType.StoredProcedure;
da.SelectCommand.Parameters.AddWithValue("@ProfileID", profileid);
DataTable dt = new DataTable();
da.Fill(dt);

var debtallout = new DebtAllOut() {
   message = "Providing general message",
   debtalldetail = new List<DebtAllDetail>()
};

for (int i = 0; i < dt.Rows.Count; i++)
{
    if (dt.Rows[i]["RowID"] != DBNull.Value) 
    {
        DebtSingleOut debtsingleout = new DebtSingleOut() {
            debtsingleout.debtid = Convert.ToInt32(dt.Rows[i]["RowID"]); 
        }
        debtallout.debtalldetail.Add(debtsingleout);
    }
}