selecting a property from FirstOrDefault in case FirstOrDefault returns null

Question

I want to make following statement:

var block = blocksById.FirstOrDefault(X => X.Value == tracResult.ID).Key

My question is how to deal

Answer 1

Since the introduction of the null conditional operators (?), it's as simple as:

var block = blocksById.FirstOrDefault(X => X.Value == tracResult.ID)?.Key;

Keep in mind that in this scenario, even if Value is an int, block will be of type Nullable<int>, and hence, can be null.

However if what you want to assign a default value if null is returned you can make use of null coalescing operator (??) is this way:

var block = blocksById.FirstOrDefault(X => X.Value == tracResult.ID)?.Key ?? 6;

Nevertheless, for more complex Select statements, the following is still valid...

---

Some time ago...

Split it in Where and Select:

var block = blocksById.Where(x => x.Value == tracResult.ID)
                      .Select(x => x.Key)
                      .FirstOrDefault();

That way you won't get a NullRefferenceException if the FirstOrDefault returns null.

Alternative you can give it a default value like this:

var block = blocksById.Where(x => x.Value == tracResult.ID)
                      .Select(x => x.Key)
                      .FirstOrDefault() ?? somedefaultvalue;

Or as @Silvermind stated for non nullable to nullable types (int's):

var block = blocksById.Where(x => x.Value == tracResult.ID)
                      .Select(x => (int?) x.Key)
                      .FirstOrDefault() ?? somedefaultvalue;

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Update: some people seems to have doubts about this being a valid use case and argue that the check for null has to be done later on in the program.

While in a lot of circumstances this is the case, is doesn't always have to be true, or if it is, it might be more convenient this way.

Some examples:

//one liner, null check delegated (not my favorite): 
return SomeCalculatedValue(collection.Where(condition).Select(selection).FirstOrDefault());

//or a mapping (shown with default value):
var result = collection.Where(condition)
                       .Select(c => 
                           { 
                                Foo = c.Foo,
                                Bar = c.Bar
                           }).FirstOrDefault()) ?? new { Foo = "New", Bar = "Empty"};