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?
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.
“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。
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
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