在LINQ中返回null而不是默认值

Question

I have LINQ query which has to retreive some DateTime value. Somethimes I don't have match for and I have to return NULL for that DateTime value instead default value for DateTime.

How can I avoid that and return NULL instead defaul value?

My LINQ:

CreatedDate = ctaMatch.Select(d => d.CreatedDate).DefaultIfEmpty().FirstOrDefault()

In DefaultIfEmpty I can put only DateTime.

Answer 1

Cast it to DateTime? that will cause DefaultIfEmpty creates a default collection that contains null value if the collection is empty.

CreatedDate = ctaMatch
    .Select(d => (DateTime?)d.CreatedDate)
    .DefaultIfEmpty()
    .FirstOrDefault()

PS: The DefaultIfEmpty can be omitted, because it's followed by FirstOrDefault .

Answer 2

You could use the null-conditional operator , ?. on the object hosting your date property:

DateTime? date = ctaMatch.FirstOrDefault()?.CreatedDate;

If FirstOrDefault() returns null for your collection, the null-conditional operator will return null for the CreatedDate property.

Alternatively, you could select your dates and cast them explicitly to Nullable<DateTime> .

DateTime? date = ctaMatch.Select(d => (DateTime?)d.CreatedDate).FirstOrDefault();

... thereby giving it a default value of null .

Answer 3

Alternative syntax... get the First element if Any exist; otherwise use null :

DateTime? CreatedDate = ctaMatch.Any() ? ctaMatch.First().CreatedDate : (DateTime?)null;