“序列不包含匹配元素”，而不是null

Question

The myCollection contains no element with Id == 10 :

var myVar1 = myCollection.Where(q => q.Id == 10);

In the above case the myVar1 represents just the empty collection.

But why in the following example I get a Sequence contains no matching element exception instead of just null in the myVar2 ?

var myVar2 = myCollection.First(q => q.Id == 10);

How to explain it correctly?

Answer 1

如果要第一个匹配项，请使用FirstOrDefault如果没有，请使用null。

var myVar2 = myCollection.FirstOrDefault(q => q.Id == 10);

Answer 2

Because First() expects one and only one result to be returned. It isn't meant to handle a one or no results.

You need FirstOrDefault() for that.