Index out of bounds when comparing two lists

Question

I searched all over the internet to find an answer for this. I am looping through two lists (list1 and list2) with nested for loops and removing duplicate records in first list based on three criteria. If all records in these two lists match each other, I get an out of bounds error. I assume it happens when I remove all the items from the first list, and when it finally reduces to 0, and does not have any records to loop through, but putting an if statement to check the count of the first list (if inbox_emails_filtered_contacts.Count > 0) does not help either. Please let me know if any of you can tell me why this errors out.

Outlook Add-in in C#.net

for (int i = 0; i < list1.Count; i++)
{
    for (int j = 0; j < list2.Count; j++)
    {
        if (list1.Count > 0)
        {
            if ((list1[i].username == registered_user)
                && (list1[i].from_email.ToLower() == list2[j].from_email.ToLower())
                && (list1[i].email_subject == list2[j].email_subject)
                && (list1[i].email_timestamp.ToLongDateString() == list2[j].email_timestamp.ToLongDateString()))
            {
                //Remove the duplicate email from inbox_emails_filtered_contacts
                list1.RemoveAt(i);
            }
        }
    }
}

Answer 1

I would suggest using while loop here. Also you need to break out of inner loop if a match is found to restart checking from the beginning.

int i = 0;
while (i < list1.Count)
{
    int found = 0;
    for (int j = 0; j < list2.Count; j++)
    {
        if ((list1[i].username == registered_user)
            && (list1[i].from_email.ToLower() == list2[j].from_email.ToLower())
            && (list1[i].email_subject == list2[j].email_subject)
            && (list1[i].email_timestamp.ToLongDateString() == list2[j].email_timestamp.ToLongDateString()))
        {
            //Remove the duplicate email from inbox_emails_filtered_contacts
            list1.RemoveAt(i);
            found = 1;
            break;
        }
   }
   if (!found)
   {
       i++;
   }
}

Answer 2

Use a reverse for when you iterate a list/array while removing item out of it (snippet "forr" in Visual) :

for (int i = list1.Count - 1; i >= 0; i--)

Answer 3

The likelyhood is that there are multiple duplicates in application_emails and so removeAt(i) is being called multiple times. Your if will never catch the case where i is the last item in a list longer than 2 and you end up removing multiple times.

Additionally you will also end up skipping emails with the remove statement. Assume i=4, if you remove at 4 index 4 will contain a new item and i will be 5 on the next iteration. You may be better off with a while loop

int i=0;
while (i < inbox_emails.Count) {
   bool foundDuplicate=false;
   for (int j=0;j<for (int j = 0; j < application_emails.Count; j++) {       
     if ((inbox_emails[i].username == registered_user)       
         && (inbox_emails[i].from_email.ToLower() == application_emails[j].from_email.ToLower())       
         && (inbox_emails[i].email_subject == application_emails[j].email_subject)       
         && (inbox_emails[i].email_timestamp.ToLongDateString() == application[j].email_timestamp.ToLongDateString()))       
        {       
           foundDuplicate=true;
           break;   // This is optional but it stops the j loop from continuing as we've found a duplicate
        }       
    }   
    if (foundDuplicate) {
       inbox_emails.RemoveAt(i);
    } else {
       i++;
    }
}

If this is a single thread you could just replace the list using Linq, make sure you have a using System.Linq

inbox_emails = inbox_emails.Where(i=> 
                 i.username != registered_user 
                 || ! application_emails.Any(j=>
                    i.from_email.ToLower() == j.from_email.ToLower()
                    && i.email_subject == j.email_subject
                    && i.email_timestamp.ToLongDateString() == j.email_timestamp.ToLongDateString()
                    )
                 ).ToList();

Answer 4

here is an implementation of an Email class

class Email:IComparable<Email>
{
    private static int _Id;

    public Email()
    {
        Id = _Id++;
    }

    public int Id { get; private set; }
    public string UserName { get; set; }
    public string Body { get; set; }
    public string Subject { get; set; }
    public DateTime TimeStamp { get; set; }

    public override int GetHashCode()
    {
        return UserName.GetHashCode() ^ Body.GetHashCode() ^ Subject.GetHashCode();
    }

    public int CompareTo(Email other)
    {
        return GetHashCode().CompareTo(other.GetHashCode());
    }

    public override string ToString()
    {
        return String.Format("ID:{0} - {1}", Id, Subject);
    }

    public override bool Equals(object obj)
    {
        if (obj is Email)
            return CompareTo(obj as Email) == 0;
        return false;
    }

}

and 2 ways of getting the diff.

        var list1 = new List<Email>();
        var ran = new Random();
        for (int i = 0; i < 50; i++)
        {
            list1.Add(new Email() { Body = "Body " + i, Subject = "Subject " + i, UserName = "username " + i, TimeStamp = DateTime.UtcNow.AddMinutes(ran.Next(-360, 60)) });
        }
        var list2 =  new List<Email>();

        for (int i = 0; i < 50; i++)
        {
            if (i % 2 == 0)
                list2.Add(new Email() { Body = "Body " + i, Subject = "Subject Modifed" + i, UserName = "username " + i, TimeStamp = DateTime.UtcNow.AddMinutes(ran.Next(-360, 60)) });
            else 
                list2.Add(new Email() { Body = "Body " + i, Subject = "Subject " + i, UserName = "username " + i, TimeStamp = DateTime.UtcNow.AddMinutes(ran.Next(-360, 60)) });

        }


        foreach (var item in list2.Intersect<Email>(list1))
        {
            Console.WriteLine(item);
        }

        foreach (var item in list1)
        {
            for (int i = 0; i < list2.Count; i++)
            {

                if (list2[i].Equals(item))
                {
                    Console.WriteLine(item);
                    list2.RemoveAt(i);
                    break;
                }
            }
        }

        Console.WriteLine(list2.Count);