如果我想在我的数据库中找到确切的搜索结果，我该用什么代替“FirstOrDefault（）”？

Question

I am currently using the following code within my controller:

Instructor instructor = db.Instructors.FirstOrDefault(
    o => o.UserName == User.Identity.Name);

to select someone by username. My understanding is that I will have trouble using "FirstOrDefault()" if I have user's with similar user names (ie searching for "MrUser" when I have users named "MrUserOne" "MrUserTwo" and "MrUser" may yield "MrUserOne" because it was the 'first' search result to show up), If I am correct in my understanding of future difficulties with "FirstOrDefault," what should I use in it's place?

Or am I wrong in my understanding of how FirstOrDefault will work?

Answer 1

It appears you don't quite understand how the == operator works in this scenario. Assuming the UserName and Name values are both string then == will do an exact ordinal match. The name "MrUser" won't match "MrUserOne" at all. It will only match "MrUser" .

That being said this code will return the first user whose Name value exactly matches o.UserName or null if none do.

Answer 2

JaredPar is quite correct in what he's saying. But you can also use SingleOrDefault if what you're expecting is to be exactly one result. You can read more about that in this Stackoverflow question: LINQ: When to use SingleOrDefault vs. FirstOrDefault() with filtering criteria

Answer 3

When you use ==, you're already doing an exact search result. The expression

o => o.UserName == User.Identity.Name

is translated to

WHERE UserName = [Username]

on the database side. This is not to be confused with a statement like

o => o.UserName.Contains(User.Identity.Name)

which would translate to

WHERE UserName like '%[Username]%'

See http://msdn.microsoft.com/en-us/library/bb738681.aspx for how string functions are translated for Linq to Entities.

That being said, your FirstOrDefault will give you the first name that exactly matches the username you provided. So your example would match the first user with the name "MrUser".

Answer 4

Your query will just return the first user whose UserName exactly matches User.Identity.Name (it will not match users that start with User.Identity.Name , as you seem to be saying).

So if there are 2 users with identical names, the return value will just be the first one. And the "first" one might be different, depending on how the database feels on returning the results to you (since you are not specifying an order).

As a side note, you should get familiar with these 4 similar methods, they are worth knowing:

                   >1 result     0 results
First()            return 1st    throw
FirstOrDefault()   return 1st    return null
Single()           throw         throw
SingleOrDefault()  throw         return null