C# Null coalesce with LINQ

Question

I have 2 classes which looks like this:

class Widget
{
    string Selected { get; set; }

    List<Option> Options { get; set; }
}

class Option
{
    string InternalCode { get; set; }

    string ExternalCode { get; set; }
}

Options gets populated dynamically with different data per client for showing ExternalCode as options

Selected gets populated with ExternalCode.

I then need to access the InternalCode which matches.

At present I am doing this:

var option = widget.Options.SingleOrDefault(o => o.ExternalCode == widget.Selected);

var internalCode = option == null ? string.Empty : option.InternalCode;

Is this possible using a single line using Null Coalesce?

Why the null check on option? it's never null or a possible NullReferenceException — Sievajet, Jan 06 '16 at 10:37
@Sievajet It can be null if no matching option is found - SingleOrDefault will return default(T) in that scenario, which will be null for this case. — RB., Jan 06 '16 at 10:38

score 18 · Accepted Answer · edited Jan 06 '16 at 18:31

18

Yes, you can use the null propagation and null coalescing operator, which suits your needs if you can use C# 6:

var option = widget.Options
             .SingleOrDefault(o => o.ExternalCode == widget.Selected)?.InternalCode
             ?? string.Empty;

The ?. will translate to your use of the option == null ? part.

edited Jan 06 '16 at 18:31

Brian

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answered Jan 06 '16 at 10:37

Patrick Hofman

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Awesome! Thank you. I couldn't figure out where to access the property. The `?.Property` is what I was missing. – Shevek Jan 06 '16 at 10:38
The code showed so far does not work in cases where there is a matching instance of `Option` with `InternalCode == null` – Michael Rätzel Jan 06 '16 at 18:13

Jon Skeet · Answer 2 · 2016-01-06T10:39:45.750

9

Sure, with a small change:

var option = widget.Options
                   .Where(o => o.ExternalCode == widget.Selected)
                   .Select(o => o.InternalCode)
                   .FirstOrDefault() ?? "";

In other words, project the sequence of matching options to a sequence of internal codes, and then take the first of those, defaulting to null... which allows you to use the null-coalescing operator on the result.

You can use the null-conditional operator as per Patrick's answer instead, but personally I'd prefer the code in this answer - I think it's simpler to understand.

edited Jan 06 '16 at 10:39

answered Jan 06 '16 at 10:38

Jon Skeet

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Is there any advantage of using `Select` instead of the null propagation operator? (besides the need of C# 6 for my answer?) – Patrick Hofman Jan 06 '16 at 10:39
@PatrickHofman: Only that I think it's simpler to understand this, IMO. Definitely a personal opinion though. – Jon Skeet Jan 06 '16 at 10:40
Yes, indeed. You can easily miss the `?`. Thanks. – Patrick Hofman Jan 06 '16 at 10:40

score 2 · Answer 3 · answered Jan 06 '16 at 10:37

2

You can do this if you wrote extension method:

public static class MonadExtensions
{
    public static TResult With<TSource, TResult>(TSource source, Func<TSource, TResult> action) where TSource : class
        {
            if (source != default(TSource))
                return action(source);
            return default(TResult);
        }
}

And use it:

var internalCode  = widget.Options.SingleOrDefault(o => o.ExternalCode == widget.Selected).With(o=>o.InternalCode)??"";

answered Jan 06 '16 at 10:37

Kirill Bestemyanov

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A good answer however it doesn't use null coalesce and it isn't 1 line! – Shevek Jan 06 '16 at 10:45
you can use monads nugget package to not write this extension method: https://www.nuget.org/packages/Monads/ and, of cause, it uses nullcoalesce operator ?? – Kirill Bestemyanov Jan 06 '16 at 10:47
I'll check out Monads – Shevek Jan 06 '16 at 10:51

Michael Rätzel · Answer 4 · 2016-01-06T13:28:55.833

0

Is this possible using a single line using Null Coalesce?

Yes, here is how:

var option = (widget.Options.SingleOrDefault(o => o.ExternalCode == widget.Selected) ?? new Option() { InternalCode = string.Empty }).InternalCode;

The other answers using a single statement I have seen so far yield wrong results in cases where there is a matching instance of Option with InternalCode == null

edited Jan 06 '16 at 13:28

answered Jan 06 '16 at 11:23

Michael Rätzel

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C# Null coalesce with LINQ

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