Return value or null, without null reference exception linq

Question

I want to return the category Name or just null within the projection below, but I get a null reference exception, which makes sense if result.CategoryId is null.

return results.Select(i =>
new Model
{
  Score = i.Score
 ,Date = i.InstanceResult.TestDate
 ,Category = categories.Where(c=>c.Id.Equals(i.result.CategoryId)).SingleOrDefault().Name
});

categoryId is int on in the categories list, but CategoryId is a nullable int in the result object. — Rayshawn
– Rayshawn, Commented Apr 7, 2014 at 12:51

Ash Burlaczenko · Accepted Answer · 2014-04-07 16:02:47Z

1

You need to check if there's a category with that Id then decide what value to assign.

return results.Select(i =>
new Model
{
     Score = i.Score
     ,Date = i.InstanceResult.TestDate
     ,Category = categories.Any(c => c.Id.Equals(i.result.CategoryId)) ?
                 categories.First(c => c.Id.Equals(i.result.CategoryId)).Name : 
                 null
});

You could tidy this up with an extension method like so

...
Category = categories.GetNameFromId(i.result.CategoryId)
...

public static string GetNameFromId(this IEnumerable<Category> categories, string id)
{
    return categories.Any(c => c.Id.Equals(id)) ?
                 categories.First(c => c.Id.Equals(id)).Name : 
                 null
}

edited Apr 7, 2014 at 16:02

answered Apr 7, 2014 at 12:46

Ash Burlaczenko

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Comments

Selman Genç · Accepted Answer · 2014-04-07 12:47:15Z

1

You can use FirstOrDefault like this but you need to execute same query twice, if you want to do that in one statement:

Category = categories
           .FirstOrDefault(c=> c.Id.Equals(i.result.CategoryId)) != null ? 
           categories
           .FirstOrDefault(c=> c.Id.Equals(i.result.CategoryId)).Name : null;

answered Apr 7, 2014 at 12:47

Selman Genç

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1 Comment

aevitas Over a year ago

FirstOrDefault returns default(T) if it can't find a matching element, which means you can do Categories != null ? Category.Name : null right away if it's a reference type.

Venom · Accepted Answer · 2014-04-07 13:27:33Z

0

return results.Select(i =>
                {
                    var singleOrDefault = categories.SingleOrDefault(c => c.Id.Equals(i.result.CategoryId));
                    return new Model
                        {
                            Score = i.Score,
                            Date = i.InstanceResult.TestDate,
                            Category = singleOrDefault != null ? singleOrDefault.Name : null
                        };
                });

answered Apr 7, 2014 at 13:27

Venom

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Comments

Relax · Accepted Answer · 2014-04-07 12:44:59Z

-1

categories.Where(c=>c.Id.Equals(i.result.CategoryId)).SingleOrDefault()

returns null if nothing has been found. If you then try to access ".Name" it will throw an exception like you have got.

Try to make it that way:

Category singleCategory = categories.Where(c=>c.Id == i.result.CategoryId).SingleOrDefault();
if (singleCategory != null)
{
    // Do something with the Properties
}

answered Apr 7, 2014 at 12:44

Relax

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