Why is this error, 'Sequence contains no elements', happening?

Question

I am getting an Invalid Operation Exception, the stack is down below. I think it is because db.Responses.Where(y => y.ResponseId.Equals(item.ResponseId)).First();

Answer 1

Check again. Use debugger if must. My guess is that for some item in userResponseDetails this query finds no elements:

.Where(y => y.ResponseId.Equals(item.ResponseId))

so you can't call

.First()

on it. Maybe try

.FirstOrDefault()

if it solves the issue.

Do NOT return NULL value! This is purely so that you can see and diagnose where problem is. Handle these cases properly.