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
– Jamiec, Commented Oct 16, 2015 at 9:25
@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
– Nitesh, Commented Oct 16, 2015 at 9:33

Marius Bancila · Accepted Answer · 2015-10-16 09:25:57Z

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, 2015 at 9:25

Marius Bancila

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7 Comments

Thomas Ayoub Over a year ago

Is remove within a foreach really recommended?

user1666620 Over a year ago

@Thomas it's what the OP is doing, and isn't what the question is about.

Nitesh Over a year ago

@Marius, I was wondering if this can be written in single statement. Please advise.

Marius Bancila Over a year ago

It can be in C# 6. But a statement should not be your goal. Your goal should be correct and simple code.

Thomas Ayoub Over a year ago

Well I have this answer right now, more to come after @Nitesh

|

npinti · Accepted Answer · 2015-10-16 09:25:42Z

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, 2015 at 9:25

npinti

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Comments

Thomas Ayoub · Accepted Answer · 2015-10-16 09:33:07Z

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, 2015 at 9:33

Thomas Ayoub

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Comments

Galma88 · Accepted Answer · 2015-10-16 09:28:49Z

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, 2015 at 9:28

Galma88

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