.ToList（）。First（）和.Single之间的优点对于c＃中的IQueryable

Question

So I am running a linq query on a NOSQL database that should return only a single object if one exists with that Id or nothing if nothing exists, but it is possible there are two objects with the same id in the database (if someone else messed up.) Currently I have it implemented as follows:

(from c in [IQueryableThing] where c.Id.Equals(id)).ToList().First()

I have also considered the alternative

(from c in [IQueryableThing] where c.Id.Equals(id)).Single()

I assume .Single() is faster but I am concerned that neither of these handles the case that there is no object with the correct id in the database. I don't care if there is more than one returned. I only want one of them. Eventually I will implement something to return the most recently modified.

Basically my question is what is the best way to solve this problem of converting a queryable to a single instance in a way that handles the cases where there is no object with the correct id and there is more than one object with the correct id.

Answer 1

It sounds like you just want to call FirstOrDefault() , which does exactly what the name implies.
Default means null .

You should not call ToList() ; that will needlessly download all of the results from the database.

Answer 2

Using IQueryable.Single is more efficient:

• Calling ToList enumerates the entire result set and stores each item in an single, contiguous array in memory. The First then just takes the first item from the array.
• Calling IQueryable.Single evaluates only the first item in the list, and if no items are returned, throws an exception. It then checks to see if there are any more items in the result set, and throws an exception if there are any.

If you'd like to handle the case where no items exist in the set which match your criteria, but don't care if there are more than one, I'd strongly recommend using FirstOrDefault instead:

var result = (from c in [IQueryableThing] where c.Id.Equals(id)).FirstOrDefault();
if (result == null)
{
    // no items found
}

This will translate to a TOP 1 in SQL, and simply return the first item in the result set, or if the result set is empty, it will simply return null (or the default value if you're dealing with structs).

Answer 3

The .ToList() shouldn't make a difference, it will just enumerate the queryable into an in-memory list. For your situation, you have a number of options. The two you've shown behave very differently:

• .First() - This will return the first element in the enumeration, or throw an exception if there are none.
• .Single() - This will return the only element in the enumeration, or throw an exception if there are none or if there are more than one.
• .FirstOrDefault() - This will return the first element in the enumeration, or a default value (which is null for reference types) if there are none.
• .SingleOrDefault() - This will return the only element in the enumeration, or a default value (which is null for reference types) if there are none, or throw an exception if there are more than one.

It sounds like you want to use .FirstOrDefault() . Check if the result is null . If it is, there were no matching elements. If it isn't, you have the first matching element.

Answer 4

I believe you are looking for Enumerable.FirstOrDefault

Returns the first element of a sequence, or a default value if the sequence contains no elements.

Answer 5

You should use .FirstOrDefault(c => c.ID.Equals(id));

It will return default if nothing is found (default is null for classes) or the first one it finds.

Answer 6

In case you are sure that ONE and only ONE item exists and you will want to use Single()

If case you are not sure how many items will be returned used First() Or FirstOrDefault().

The difference is that Single() will throw an exception if more than one item exists. Very usefull if you want to trap insertion errors etc on a database without unique constraints.