Assign and check for a null value at the same time?

Question

I have the following scenario:

if(xml.Descendants(ns + "Children").FirstOrDefault() != null)
{
    XElement children = xml.Descendants(ns + "Children").FirstOrDefault();
}

Is there a way I can check for null and assign the value at the same time instead of having to search for the value twice, something similar to:

//Not sure if this is correct.
if (XElement children = xml.Descendants(ns + "Children").FirstOrDefault() != null)
{

}

score 15 · Accepted Answer · edited May 23 '17 at 12:30

15

A variable assignment also returns the value. So the syntax in the form of the following will do:

SomeType someVariable;
if ((someVariable = valueToAssign) != null)
{
    // valueToAssign was not null
}

In your case:

XElement children;

if ((children = xml.Descendants(ns + "Children").FirstOrDefault()) != null)
{

}

edited May 23 '17 at 12:30

Community

1
1

answered Jun 15 '12 at 13:56

CodeCaster

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Ah i had no idea, so my syntax was correct, I just guessed lol – Xaisoft Jun 15 '12 at 13:57
3

@Xaisoft Your syntax wasn't correct, you were also declaring it in-line, which isn't valid. – Adam Houldsworth Jun 15 '12 at 13:59
1

I like this approach, I didn't notice the double brace at first. Thanks. – Xaisoft Jun 15 '12 at 14:09

score 4 · Answer 2 · answered Jun 15 '12 at 14:00

4

I would do it this way:

XElement children = xml.Descendants(ns + "Children").FirstOrDefault();
if(children != null)
{
    //use children
}

answered Jun 15 '12 at 14:00

Tim S.

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This is what I was doing, but if no Children were found, I would get an error. – Xaisoft Jun 15 '12 at 14:01
1

@Xaisoft makes no sense, because if that is the case, your code would also throw an error – Dr. Andrew Burnett-Thompson Jun 15 '12 at 14:03
No worries! You might also want to check out the null-coalescing operator: http://msdn.microsoft.com/en-us/library/ms173224.aspx – Dr. Andrew Burnett-Thompson Jun 15 '12 at 14:10

Claudio Redi · Answer 3 · 2012-06-15T14:25:42.127

2

You could just do

XElement children = xml.Descendants(ns + "Children").FirstOrDefault();

and then check for null

if (children != null) {...}

edited Jun 15 '12 at 14:25

answered Jun 15 '12 at 13:56

Claudio Redi

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If no Children elements are found, this throws a null exception. – Xaisoft Jun 15 '12 at 13:58
@Xaisoft: mhh, that should not be the case. It would be the case if you use `First` but `FirstOrDefault` should return null – Claudio Redi Jun 15 '12 at 13:59
When FirstOrDefault says it returns a default value, is that a default value you specify or is it the default value for the specified type, for example, reference variables would get null? – Xaisoft Jun 15 '12 at 14:10
@Xaisoft: default value for the specified type, null in this case. – Claudio Redi Jun 15 '12 at 14:12
@Xaisoft Likely `default(T)`, which for all reference types is `null`. – Adam Houldsworth Jun 15 '12 at 14:15

score 2 · Answer 4 · answered Nov 29 '19 at 17:48

You can use a Pattern Matching in C# 7

if(xml.Descendants(ns + "Children").FirstOrDefault() is XElement children)
{
    xml.Descendants(ns + "Children").FirstOrDefault();
}

It's a late response, but this question is showing up on top in google, and it's missing a C# 7 answer

Adam Houldsworth · Answer 5 · 2012-06-15T14:10:33.500

1

You can assign and then test the assigned value in a single statement (but not declare it):

XElement children = null;

if ((children = xml.Descendants(ns + "Children").FirstOrDefault()) != null)
{    
}

But this is functionally the same as assigning and testing afterwards:

var children = xml.Descendants(ns + "Children").FirstOrDefault();

if (children != null)
{      
}

I would favour the latter because I think it is more readable (also lets you use var).

Assignment of a null value to a variable will never itself generate an error (assuming this is just a standard local variable), subsequent use of that variable might do. So either solution above will be safe assuming xml itself isn't null.

edited Jun 15 '12 at 14:10

answered Jun 15 '12 at 13:56

Adam Houldsworth

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Yes it returns null if no children are found, but that is what I want to avoid having to check for null and then assign the value. – Xaisoft Jun 15 '12 at 13:59
@Xaisoft It is functionally the same as assigning it and then checking the variable afterwards - just it does it in one line. It isn't doing any less work. – Adam Houldsworth Jun 15 '12 at 14:00
Nice. I'm going blind, I din't notice the double brace at first, but this works great. – Xaisoft Jun 15 '12 at 14:11
@Xaisoft Yeah the braces define the precedence, the "result" of the braced code is an `XElement` that can be tested for null. – Adam Houldsworth Jun 15 '12 at 14:12
Functionally, it's mostly the same, but you can get slightly nicer behaviour if you need to pull different content in multiple if statements: `if ((children = GetChildren1()) != null) {} elseif ((children = GetChildren2()) != null) {} elseif ((children = GetChildren3()) != null){}` though depending on what you're doing there may be a better design than that. But that's just way only calling GetChildren1/2/3 as needed rather than pre-calling all 3 then doing checks. – Chris Sinclair Jun 15 '12 at 14:14
1

@ChrisSinclair I'd say functionally the same based on "assignment before condition", but your flow is a nice take on it, and perhaps the only real reason why you might do this. I also do it in while loops: `while ((line = Console.ReadLine()).Length != 0)`, because you get a "free" assignment before the check expressed in the while condition itself, as is the end result in your example. – Adam Houldsworth Jun 15 '12 at 14:17

score 0 · Answer 6 · answered Jun 15 '12 at 13:56

0

You can do this though:

null != children = xml.Descendants(ns + "Children").FirstOrDefault()

answered Jun 15 '12 at 13:56

Eren Ersönmez

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Just curious, why the null != first? – Xaisoft Jun 15 '12 at 14:02
@Xaisoft well no huge reasons -- just my taste --but if you do it the other way around you have to wrap `children = xml.Descendants(ns + "Children").FirstOrDefault()` in an extra set of parenthesis. Otherwise, the compiler will think you're trying to assign a boolean to `children` (`xml.Descendants(ns + "Children").FirstOrDefault() != null`). If you put `null != ` at the beginning you get to avoid that extra set of paranthesis -- just looks cleaner to my eyes anyway :) – Eren Ersönmez Jun 15 '12 at 15:09

Assign and check for a null value at the same time?

6 Answers6