使用LINQ和Entity框架检查c＃var中的空值

Question

I'm quite new to LINQ & Entity framework as well as the var keyword in c# so please pardon me if this sounds like a 'newbie' question.

I have problems checking for null values after doing something like this:

var entry = myDB.Entries.Where(e => e.Email == entry.Email);

Even when the email does not exist in the database, entry does not equate to null.

So instead of if (entry == null) i had to do if (entry.Count() < 1) to check for existing Entry before i execute my next batch of statements. Is there any reason why the variable wouldn't be considered null?

Answer 1

In your example, entry will never be null . What you think of as null is in fact an IEnumerable<Entry> with no items.

If you want to check if there is at least one entry with your criteria, you normally do something like:

var entries = myDB.Entries.Where(e => e.Email == entry.Email);
if (entries.Any()) {
    // ...
}

If you know that there will be at most one entry, then you can also do:

var entry = myDB.Entries.Where(e => e.Email == entry.Email).SingleOrDefault();
if (entry != null) {
    // ...
}

This is closer to what you imagined, but will throw an exception if there is more than one matching entry.

Answer 2

“var”关键字使得可以在运行时基于赋值实现任何类型，因此当您使用“Where”查询时，var条目变为“IEnumerable”，由Where返回，这就是您必须检查计数的原因。

Answer 3

In VB

Dim entry = myDB.Entries.Where(Function(e) e.Email = entry.Email).SingleOrDefault()

If entry IsNot Nothing Then

' we have a value

else

' we dont have a value

End If