Which method performs better: .Any() vs .Count() > 0?

问题

in the System.Linq namespace, we can now extend our IEnumerable\'s to have theAny() and Count() extension methods.

I was told recently that if i want to check that a collection contains 1 or more items inside it, I should use the .Any() extension method instead of the .Count() > 0 extension method because the .Count() extension method has to iterate through all the items.

Secondly, some collections have a property (not an extension method) that is Count or Length. Would it be better to use those, instead of .Any() or .Count() ?

yea / nae ?

回答1:

If you are starting with something that has a .Length or .Count (such as ICollection<T>, IList<T>, List<T>, etc) - then this will be the fastest option, since it doesn't need to go through the GetEnumerator()/MoveNext()/Dispose() sequence required by Any() to check for a non-empty IEnumerable<T> sequence.

For just IEnumerable<T>, then Any() will generally be quicker, as it only has to look at one iteration. However, note that the LINQ-to-Objects implementation of Count() does check for ICollection<T> (using .Count as an optimisation) - so if your underlying data-source is directly a list/collection, there won't be a huge difference. Don't ask me why it doesn't use the non-generic ICollection...

Of course, if you have used LINQ to filter it etc (Where etc), you will have an iterator-block based sequence, and so this ICollection<T> optimisation is useless.

In general with IEnumerable<T> : stick with Any() ;-p

回答2:

Note: I wrote this answer when Entity Framework 4 was actual. The point of this answer was not to get into trivial .Any() vs .Count() performance testing. The point was to signal that EF is far from perfect. Newer versions are better... but if you have part of code that's slow and it uses EF, test with direct TSQL and compare performance rather than relying on assumptions (that .Any() is ALWAYS faster than .Count() > 0).

---

While I agree with most up-voted answer and comments - especially on the point Any signals developer intent better than Count() > 0 - I've had situation in which Count is faster by order of magnitude on SQL Server (EntityFramework 4).

Here is query with Any that thew timeout exception (on ~200.000 records):

con = db.Contacts.
    Where(a => a.CompanyId == companyId && a.ContactStatusId <= (int) Const.ContactStatusEnum.Reactivated
        && !a.NewsletterLogs.Any(b => b.NewsletterLogTypeId == (int) Const.NewsletterLogTypeEnum.Unsubscr)
    ).OrderBy(a => a.ContactId).
    Skip(position - 1).
    Take(1).FirstOrDefault();

Count version executed in matter of milliseconds:

con = db.Contacts.
    Where(a => a.CompanyId == companyId && a.ContactStatusId <= (int) Const.ContactStatusEnum.Reactivated
        && a.NewsletterLogs.Count(b => b.NewsletterLogTypeId == (int) Const.NewsletterLogTypeEnum.Unsubscr) == 0
    ).OrderBy(a => a.ContactId).
    Skip(position - 1).
    Take(1).FirstOrDefault();

I need to find a way to see what exact SQL both LINQs produce - but it's obvious there is a huge performance difference between Count and Any in some cases, and unfortunately it seems you can't just stick with Any in all cases.

EDIT: Here are generated SQLs. Beauties as you can see ;)

ANY:

exec sp_executesql N'SELECT TOP (1) 
[Project2].[ContactId] AS [ContactId], 
[Project2].[CompanyId] AS [CompanyId], 
[Project2].[ContactName] AS [ContactName], 
[Project2].[FullName] AS [FullName], 
[Project2].[ContactStatusId] AS [ContactStatusId], 
[Project2].[Created] AS [Created]
FROM ( SELECT [Project2].[ContactId] AS [ContactId], [Project2].[CompanyId] AS [CompanyId], [Project2].[ContactName] AS [ContactName], [Project2].[FullName] AS [FullName], [Project2].[ContactStatusId] AS [ContactStatusId], [Project2].[Created] AS [Created], row_number() OVER (ORDER BY [Project2].[ContactId] ASC) AS [row_number]
    FROM ( SELECT 
        [Extent1].[ContactId] AS [ContactId], 
        [Extent1].[CompanyId] AS [CompanyId], 
        [Extent1].[ContactName] AS [ContactName], 
        [Extent1].[FullName] AS [FullName], 
        [Extent1].[ContactStatusId] AS [ContactStatusId], 
        [Extent1].[Created] AS [Created]
        FROM [dbo].[Contact] AS [Extent1]
        WHERE ([Extent1].[CompanyId] = @p__linq__0) AND ([Extent1].[ContactStatusId] <= 3) AND ( NOT EXISTS (SELECT 
            1 AS [C1]
            FROM [dbo].[NewsletterLog] AS [Extent2]
            WHERE ([Extent1].[ContactId] = [Extent2].[ContactId]) AND (6 = [Extent2].[NewsletterLogTypeId])
        ))
    )  AS [Project2]
)  AS [Project2]
WHERE [Project2].[row_number] > 99
ORDER BY [Project2].[ContactId] ASC',N'@p__linq__0 int',@p__linq__0=4

COUNT:

exec sp_executesql N'SELECT TOP (1) 
[Project2].[ContactId] AS [ContactId], 
[Project2].[CompanyId] AS [CompanyId], 
[Project2].[ContactName] AS [ContactName], 
[Project2].[FullName] AS [FullName], 
[Project2].[ContactStatusId] AS [ContactStatusId], 
[Project2].[Created] AS [Created]
FROM ( SELECT [Project2].[ContactId] AS [ContactId], [Project2].[CompanyId] AS [CompanyId], [Project2].[ContactName] AS [ContactName], [Project2].[FullName] AS [FullName], [Project2].[ContactStatusId] AS [ContactStatusId], [Project2].[Created] AS [Created], row_number() OVER (ORDER BY [Project2].[ContactId] ASC) AS [row_number]
    FROM ( SELECT 
        [Project1].[ContactId] AS [ContactId], 
        [Project1].[CompanyId] AS [CompanyId], 
        [Project1].[ContactName] AS [ContactName], 
        [Project1].[FullName] AS [FullName], 
        [Project1].[ContactStatusId] AS [ContactStatusId], 
        [Project1].[Created] AS [Created]
        FROM ( SELECT 
            [Extent1].[ContactId] AS [ContactId], 
            [Extent1].[CompanyId] AS [CompanyId], 
            [Extent1].[ContactName] AS [ContactName], 
            [Extent1].[FullName] AS [FullName], 
            [Extent1].[ContactStatusId] AS [ContactStatusId], 
            [Extent1].[Created] AS [Created], 
            (SELECT 
                COUNT(1) AS [A1]
                FROM [dbo].[NewsletterLog] AS [Extent2]
                WHERE ([Extent1].[ContactId] = [Extent2].[ContactId]) AND (6 = [Extent2].[NewsletterLogTypeId])) AS [C1]
            FROM [dbo].[Contact] AS [Extent1]
        )  AS [Project1]
        WHERE ([Project1].[CompanyId] = @p__linq__0) AND ([Project1].[ContactStatusId] <= 3) AND (0 = [Project1].[C1])
    )  AS [Project2]
)  AS [Project2]
WHERE [Project2].[row_number] > 99
ORDER BY [Project2].[ContactId] ASC',N'@p__linq__0 int',@p__linq__0=4

Seems that pure Where with EXISTS works much worse than calculating Count and then doing Where with Count == 0.

Let me know if you guys see some error in my findings. What can be taken out of all this regardless of Any vs Count discussion is that any more complex LINQ is way better off when rewritten as Stored Procedure ;).

回答3:

Since this is a rather popular topic and answers differ, I had to take a fresh look on the problem.

Testing env: EF 6.1.3, SQL Server, 300k records

Table model:

class TestTable
{
    [Key]
    public int Id { get; set; }

    public string Name { get; set; }

    public string Surname { get; set; }
}

Test code:

class Program
{
    static void Main()
    {
        using (var context = new TestContext())
        {
            context.Database.Log = Console.WriteLine;

            context.TestTables.Where(x => x.Surname.Contains("Surname")).Any(x => x.Id > 1000);
            context.TestTables.Where(x => x.Surname.Contains("Surname") && x.Name.Contains("Name")).Any(x => x.Id > 1000);
            context.TestTables.Where(x => x.Surname.Contains("Surname")).Count(x => x.Id > 1000);
            context.TestTables.Where(x => x.Surname.Contains("Surname") && x.Name.Contains("Name")).Count(x => x.Id > 1000);

            Console.ReadLine();
        }
    }
}

Results:

Any() ~ 3ms

Count() ~ 230ms for first query, ~ 400ms for second

Remarks:

For my case, EF didn't generate SQL like @Ben mentioned in his post.

回答4:

EDIT: it was fixed in EF version 6.1.1. and this answer is no more actual

For SQL Server and EF4-6, Count() performs about two times faster than Any().

When you run Table.Any(), it will generate something like(alert: don't hurt the brain trying to understand it)

SELECT 
CASE WHEN ( EXISTS (SELECT 
    1 AS [C1]
    FROM [Table] AS [Extent1]
)) THEN cast(1 as bit) WHEN ( NOT EXISTS (SELECT 
    1 AS [C1]
    FROM [Table] AS [Extent2]
)) THEN cast(0 as bit) END AS [C1]
FROM  ( SELECT 1 AS X ) AS [SingleRowTable1]

that requires 2 scans of rows with your condition.

I don't like to write Count() > 0 because it hides my intention. I prefer to use custom predicate for this:

public static class QueryExtensions
{
    public static bool Exists<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate)
    {
        return source.Count(predicate) > 0;
    }
}

回答5:

It depends, how big is the data set and what are your performance requirements?

If it's nothing gigantic use the most readable form, which for myself is any, because it's shorter and readable rather than an equation.

回答6:

About the Count() method, if the IEnumarable is an ICollection, then we can't iterate across all items because we can retrieve the Count field of ICollection, if the IEnumerable is not an ICollection we must iterate across all items using a while with a MoveNext, take a look the .NET Framework Code:

public static int Count<TSource>(this IEnumerable<TSource> source)
{
    if (source == null) 
        throw Error.ArgumentNull("source");

    ICollection<TSource> collectionoft = source as ICollection<TSource>;
    if (collectionoft != null) 
        return collectionoft.Count;

    ICollection collection = source as ICollection;
    if (collection != null) 
        return collection.Count;

    int count = 0;
    using (IEnumerator<TSource> e = source.GetEnumerator())
    {
        checked
        {
            while (e.MoveNext()) count++;
        }
    }
    return count;
}

Reference: Reference Source Enumerable

回答7:

You can make a simple test to figure this out:

var query = //make any query here
var timeCount = new Stopwatch();
timeCount.Start();
if (query.Count > 0)
{
}
timeCount.Stop();
var testCount = timeCount.Elapsed;

var timeAny = new Stopwatch();
timeAny.Start();
if (query.Any())
{
}
timeAny.Stop();
var testAny = timeAny.Elapsed;

Check the values of testCount and testAny.

回答8:

If you are using the Entity Framework and have a huge table with many records Any() will be much faster. I remember one time I wanted to check to see if a table was empty and it had millions of rows. It took 20-30 seconds for Count() > 0 to complete. It was instant with Any().

Any() can be a performance enhancement because it may not have to iterate the collection to get the number of things. It just has to hit one of them. Or, for, say, LINQ-to-Entities, the generated SQL will be IF EXISTS(...) rather than SELECT COUNT ... or even SELECT * ....

来源：https://stackoverflow.com/questions/305092/which-method-performs-better-any-vs-count-0