Question

I’ve created a class witch contains a typed list and derives to another class that I created. This looks as follows:

namespace MyIntegretyCheck.Common
{
    /// <summary>
    /// Description of PolicyErrors.
    /// </summary>
    public partial class PolicyErrorEndingDates
    {
        public int ID_P {get;set;}
        public DateTime DT_S_E {get;set;}
        public DateTime DT_SC_E {get;set;}

        public List<PolicyErrorDescr> Errors {get;set;}
    }

    public partial class PolicyErrorDescr
    {
        public string Field1{get;set;}
        public string Field2 {get;set;}
        public string F1IsThanF2 {get;set;}
        public string Message {get;set;}
        public int ErrorLevel {get;set;} //0= Info | 1= Warning | 2= Error

    }
}

Now I created a typed list of PolicyErrorEndingDates, added an entry and tried then to add entries of his nested list Errors as follows:

public List<PolicyErrorEndingDates> MyPolicyEndingDates()
{

    DAL.PolicyEndingDates ped = new DAL.PolicyEndingDates();
    List<PolicyErrorEndingDates> MyErrors = new List<PolicyErrorEndingDates>();

    foreach(var m in ped.CheckTables())
    {
        bool HasError = false;
        PolicyErrorEndingDates p = new PolicyErrorEndingDates();
        p.ID_P = m.ID_P;

        if(m.DT_S_E != m.DT_SC_E)
        {
            PolicyErrorDescr e = new PolicyErrorDescr();
            HasError = true;
            e.Field1 = "DT_S_E";
            e.Field2 = "DT_SC_E";
            e.Message = "blablabla...";
            e.ErrorLevel = 3;
            p.Errors.Add(e);
        }

        if(HasError)
            MyErrors.Add(p);
    }
}

The Debugger crashed with the message Object reference not set to an instance of an object, at the line p.Errors.Add(e); inside my if. What did I do wrong? How can I create an instance of the nested list?

Answer 1

Did you assign a List<PolicyErrorDescr> instance to the Errors property?

First, I would probably make the Errors collection read-only from the outside, that is, the list can be changed, but you can’t give it a new list.

This I would do by making the setter private:

public List<PolicyErrorDescr> Errors { get; private set; }
                                            ^-----^
                                              this

Then I would create a constructor and assign a collection instance to that property from it:

public PolicyErrorEndingDates()
{
    Errors = new List<PolicyErrorDescr>();
}

This should take care of the null reference exception.