I have the below example code, and I am interested to know how I can make this any cleaner, possibly through better use of SelectMany()
. At this point the QuestionList
property will not be null. All I want is a list of answerRows
that are not null
, but Questions
can sometimes be null
too.
IEnumerable<IQuestion> questions = survey.QuestionList
.Where(q => q.Questions != null)
.SelectMany(q => q.Questions);
if(questions == null)
return null;
IEnumerable<IAnswerRow> answerRows = questions
.Where(q => q.AnswerRows != null)
.SelectMany(q => q.AnswerRows);
if(answerRows == null)
return null;
I was interested by Jon's comment about Enumerable.SelectMany
and Null..
so I wanted to try my example with some fake data to more easily see where the error is, please see the below, specifically how I am using SelectMany()
on the result of a SelectMany()
, its clearer to me now that the problem was having to make sure you don't use SelectMany()
on a null reference, obvious when I actually read the NullReferenceException
name :( and finally put things together.
Also while doing this, I realised that the use of try { } catch() { }
in this example is useless and as usual Jon Skeet has the answer :) deferred execution..
so if you want to see the exception for row 2, comment out the relevant row 1 bits :P, sorry I couldn't figure out how to stop this error without re-writing the code example.
using System;
using System.Collections.Generic;
using System.Linq;
namespace SelectManyExample
{
class Program
{
static void Main(string[] args)
{
var questionGroupList1 = new List<QuestionGroup>() {
new QuestionGroup() {
Questions = new List<Question>() {
new Question() {
AnswerRows = new List<AnswerRow>() {
new AnswerRow(),
new AnswerRow()
}
},
// empty question, causes cascading SelectMany to throw a NullReferenceException
null,
new Question() {
AnswerRows = new List<AnswerRow>() {
new AnswerRow() {
Answers = new List<Answer>() {
new Answer(),
new Answer()
}
}
}
}
}
}
};
var questionGroupList2 = new List<QuestionGroup>() {
null,
new QuestionGroup()
};
IEnumerable<AnswerRow> answerRows1 = null;
IEnumerable<AnswerRow> answerRows2 = null;
try
{
answerRows1 = questionGroupList1
.SelectMany(q => q.Questions)
.SelectMany(q => q.AnswerRows);
}
catch(Exception e) {
Console.WriteLine("row 1 error = " + e.Message);
}
try
{
answerRows2 = questionGroupList2
.SelectMany(q => q.Questions)
.SelectMany(q => q.AnswerRows);
}
catch (Exception e)
{
Console.WriteLine("row 2 error = " + e.Message);
}
Console.WriteLine("row 1: " + answerRows1.Count());
Console.WriteLine("row 2: " + answerRows2.Count());
Console.ReadLine();
}
}
public class QuestionGroup {
public IEnumerable<Question> Questions { get; set; }
}
public class Question {
public IEnumerable<AnswerRow> AnswerRows { get; set; }
}
public class AnswerRow {
public IEnumerable<Answer> Answers { get; set; }
}
public class Answer {
public string Name { get; set; }
}
}
survey.QuestionList
.Where(l => l.Questions != null)
.SelectMany(l => l.Questions)
.Where(q => q != null && q.AnswerRows != null)
.SelectMany(q => q.AnswerRows);
I'd recommend you ensure your collections are never null
. null
can be a bit of a nuisance if you don't handle it well. You end up with if (something != null) {}
all over your code. Then use:
survey.QuestionList
.SelectMany(l => l.Questions)
.SelectMany(q => q.AnswerRows);