Issue deleting many to many collection in Entity Framework

Question

I have two tables - QuestionOptions and Answers. There's a many to many relationship between the two. I'm trying to remove QuestionOptions pertaining to a specific Question, and need to remove the answers first. Here's my code:

using (Survey_Context _context = new Survey_Context())
{
    IQueryable<QuestionOption> questionOptions = _context.QuestionOptions.Where(qo => qo.QuestionID == questionID);
    foreach (QuestionOption questionOption in questionOptions.ToList())
    {
        foreach (Answer answer in questionOption.Answers)
            questionOption.Answers.Remove(answer);
        _context.SaveChanges();
    }
    _context.QuestionOptions.RemoveRange(questionOptions.ToList());
    _context.SaveChanges();
}

This causes the following:

Error - System.InvalidOperationException: Collection was modified; enumeration operation may not execute.

What am I missing here?

Answer 1

There are two main problems in your code.

First, the error that is causing the Invalid Operation Exception you are seeing :

You are trying to remove element of a list that you are iterating on !

You should not modify a list (I mean : add or delete elements) that you are enumerating (with a foreach, like in your code) .

That is simply not valid this way.

Second, there is an error of logic :

If you want to delete elements, then you want to delete this objects from the database context , not remove them from some list that you just created.

As suggested in comments, you can correct both of these errors in one go by replacing :

foreach (Answer answer in questionOption.Answers)
    questionOption.Answers.Remove(answer);

by

_context.Answers.RemoveRange(questionOption.Answers);