Setting the value of a variable based on the result of a fetch in C#

Question

var d = get(x).Title;
var e = get(y).Title;

The getting of Title gives an exception if get(y) returns null. However what I need is for e to be null if get(y) is null and there to be no exception generated. I can do this with two steps of get(y) but I want to only do the get(y) one time.

Is there some easy one line way that I can with just one get(y) and with one line of code set e to null if get(y) is null or the actual value if get(y) does not return a null?

what's more important? only calling get(y) once or not assigning an intermediate variable? — Adam, Dec 21 '11 at 16:50

score 2 · Accepted Answer · answered Dec 21 '11 at 16:45

var z = get(y);
var e = z != null ? z.Title : null;

You can also use extension methods like those:

    public static TResult IfNotNull<T, TResult>(this T obj, Func<T, TResult> selector)
        where T : class
    {
        return obj.IfNotNull(selector, default(TResult));
    }

    public static TResult IfNotNull<T, TResult>(this T obj, Func<T, TResult> selector, TResult valueIfNull)
        where T : class
    {
        if (obj == null)
            return valueIfNull;
        return selector(obj);
    }

...

    var e = get(y).IfNotNull(_ => _.Title);

Thanks Thomas. This solution looks very interesting. I tried it and it seems to work okay so far. — Samantha J T Star, Dec 21 '11 at 17:06

score 1 · Answer 2 · edited Dec 21 '11 at 16:59

1

var d = get(x) != null ? get(x).Title : null;

Since you state that only one call of get(x) get(y) is allowed you could write this as follows

  var X = get(x);
  var d = X != null ? X.Title : null;

That is the shortest way to write the code but I think a more clear way to write all of this would be to handle this with some form of an extension method of a function call since this is something that you are having to do to multiple variables

Example Extension Methods

public static TypeOfTitle GetTitle(this TypeOfget x)
{
   return x != null ? x.Title : null;
}

Then call this as follows

var d = get(x).GetTitle();

edited Dec 21 '11 at 16:59

Anthony Pegram

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answered Dec 21 '11 at 16:44

msarchet

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This won't compile... both operands of the ?? operator must have the same type – Thomas Levesque Dec 21 '11 at 16:45
WOW I have never seen this operator before !! – Samantha J T Star Dec 21 '11 at 16:45
Samantha needs to return Title if get(y) is null, null otherwise. Your code will return get(y) and if that's null, return get(y).Title. – Bryan Boettcher Dec 21 '11 at 16:45
-1 - Even if this compiled, if `get(x)` returns a `null`, then `get(x).Title` will try to execute, resulting in a `NullReferenceException`. – Oded Dec 21 '11 at 16:46
I noticed and fixed it...completely wrong. woops – msarchet Dec 21 '11 at 16:46
the OP stated only one call of get(y) was allowed. You're calling get(y) twice if it isn't null. – Adam Dec 21 '11 at 16:47
But is this not back to the needing to do get(x) twice ? – Samantha J T Star Dec 21 '11 at 16:48
@codesparkle changed after I wrote my answer correcting – msarchet Dec 21 '11 at 16:48
@SamanthaJ I've corrected my code for those issues. – msarchet Dec 21 '11 at 16:50
Your edit does not include a valid extension method, nor a valid invocation of one. – Anthony Pegram Dec 21 '11 at 16:55
@AnthonyPegram thanks, it just needed the this? and yes it does – msarchet Dec 21 '11 at 16:57
No, the invocation was still wrong, but I have corrected that for you rather than be pedantic. Missing parentheses. – Anthony Pegram Dec 21 '11 at 17:00
Thank you. I think both this answer and the one by Thomas will do what I needed. I'm not actually sure which to mark correct as both look good. – Samantha J T Star Dec 21 '11 at 17:04

score 1 · Answer 3 · edited May 23 '17 at 11:48

You can use an inline conditional statement (or, ternary operator), like so:

var e = get(y) != null ? get(y).Title : null;

However, you might want to take the 'primitive' route (I know, I know, one-liners are all the rage), if for nothing else but to minimise the calls to get (which you do state as a concern), which would simply be:

string e = null;
var somethingOrOther = get(y);
if (somethingOrOther != null) {
    e = somethingOrOther;
}

To my knowledge, I'm afraid it's one or the other - someone else might know something I don't, or Jon Skeet might concoct some evil for you, but this is where I stand (anything else would seem only to be variations of the same approaches).

Furthermore, since now being aware of a possible duplicate, but for completeness, I'll quote Eric Lippert's on "null if object is null, or object.member if object is not null":

There's no short form for that; implementing one is a fairly frequently requested feature.

But this needs get(y) to be performed twice :-( – Samantha J T Star Dec 21 '11 at 16:48 — Samantha J T Star, Dec 21 '11 at 16:48

score 0 · Answer 4 · answered Dec 21 '11 at 16:44

0

You'll have to use a terinary operator:

var e = (get(y) == null ? null : get(y).Title).

If get(y) is an expensive operation, you'll want to cache it in another variable.

answered Dec 21 '11 at 16:44

Bryan Boettcher

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Still hoping for a solution with no other variable. The ?? below looked good but another poster said it would not work :-( – Samantha J T Star Dec 21 '11 at 16:47
1

@SamanthaJ: You can't do it without another variable unless you call get(y) twice. Even the null coalescing operator (written improperly below) requires two calls to get(y). – Bryan Boettcher Dec 21 '11 at 16:48
@SamanthaJ, there have been calls for a `?.` operator, which would theoretically allow you to say `get(x)?.Title` without a null check, but there have been no announcements about whether to include that in a future version of C#, and it is obviously not there now. – Anthony Pegram Dec 21 '11 at 16:48
1

@SamanthaJ: unless this is code golf, use another variable. – Bryan Boettcher Dec 21 '11 at 16:49

Setting the value of a variable based on the result of a fetch in C#

4 Answers4