Null Exception when using .ToList().ForEach() in C#

Question

I am using HtmlAgilityPack to parse some data and writing statements like the below one to remove unwanted contents -

doc.DocumentNode.SelectNodes("//ul").ToList().ForEach(a => a.Remove());

This works well when there's <ul> element present in the HTML document. However, if there's no <ul> element, I get a Value cannot by null exception when converting it into .ToList().

I do not want to use the following -

doc.DocumentNode.SelectNodes("//ul")== null ? null : doc.DocumentNode.SelectNodes("//ul").ToList().ForEach(a => a.Remove());

What elegant alternatives do I have?

What do you find *inelegant* about your proposed solution? (Except, of course, that it wont compile! - `ForEach` is void and `null` on the other side won't coalesce) — Jamiec, Oct 16 '15 at 09:25
@Jamiec, I know it won't compile and just wanted to share the way I don't want to write. This approach is OK when using on a single tag, but let's say I have around 100 tags and I want to remove them. lot of code to be written and I know I can transfer this into a function but that does not server the function properly as well. — Nitesh, Oct 16 '15 at 09:33

score 6 · Accepted Answer · answered Oct 16 '15 at 09:25

6

In your example you are calling doc.DocumentNode.SelectNodes("//ul") twice. What is wrong with

var nodes = doc.DocumentNode.SelectNodes("//ul");
if(nodes != null)
   nodes.ToList().ForEach(a => a.Remove());

answered Oct 16 '15 at 09:25

Marius Bancila

15,632
8
47
86

Is remove within a foreach really recommended? – Thomas Ayoub Oct 16 '15 at 09:27
@Thomas it's what the OP is doing, and isn't what the question is about. – user1666620 Oct 16 '15 at 09:27
@Marius, I was wondering if this can be written in single statement. Please advise. – Nitesh Oct 16 '15 at 09:28
@Thomas, why is it nor recommended? Any links? – Nitesh Oct 16 '15 at 09:29
2

It can be in C# 6. But a statement should not be your goal. Your goal should be correct and simple code. – Marius Bancila Oct 16 '15 at 09:31
1

Well I have [this answer](http://stackoverflow.com/a/832183/2307070) right now, more to come after @Nitesh – Thomas Ayoub Oct 16 '15 at 09:31
@Thomas, I would wait for another link from you :) – Nitesh Oct 16 '15 at 09:35

score 4 · Answer 2 · answered Oct 16 '15 at 09:25

4

If you have C# 6 available you can use The Null Conditional Operator:

doc.DocumentNode.SelectNodes("//ul")?.ToList().ForEach(a => a.Remove());

answered Oct 16 '15 at 09:25

npinti

51,070
5
71
94

score 2 · Answer 3 · answered Oct 16 '15 at 09:33

2

I would use RemoveAll instead of a ForEach (if you can use C#6):

doc.DocumentNode.SelectNodes("//ul")?.ToList().RemoveAll();

Or

var nodes = doc.DocumentNode.SelectNodes("//ul");
if(nodes != null)
    nodes.ToList().RemoveAll();

answered Oct 16 '15 at 09:33

Thomas Ayoub

28,235
15
95
135

score 1 · Answer 4 · answered Oct 16 '15 at 09:28

1

You need to check the null condition.

var docNodes = doc.DocumentNode.SelectNodes("//ul");
if(docNodes != null)
   docNodes .ToList().ForEach(a => a.Remove());

answered Oct 16 '15 at 09:28

Galma88

2,257
6
28
47

Null Exception when using .ToList().ForEach() in C#

4 Answers4