Paging query results is timeless topic and some days ago I got question about how to do it with Entity Framework Core. Using my previous work on paging I wrote some simple extension methods that work with IQueryable<T> interface. There’s also method that maps entities to models.
Paging classes
Let’s start with paging classes. I typically use two classes:
- PagedResultBase – abstract base class for paged results, used in views and view components,
- PagedResult<T> – strongly typed class for results and result set properties.
These two classes are here:
public abstract class PagedResultBase
{
public int CurrentPage { get; set; }
public int PageCount { get; set; }
public int PageSize { get; set; }
public int RowCount { get; set; }
public int FirstRowOnPage
{
get { return (CurrentPage - 1) * PageSize + 1; }
}
public int LastRowOnPage
{
get { return Math.Min(CurrentPage * PageSize, RowCount); }
}
}
public class PagedResult<T> : PagedResultBase where T : class
{
public IList<T> Results { get; set; }
public PagedResult()
{
Results = new List<T>();
}
}
These classes can be extended with other useful methods and properties if needed. It’s just a basic and minimal set needed to get paging done in code and in views.
Paging query results
This is the universal extension method for IQueryable<T> that returns one page of results and some numbers that describe the result set.
public static PagedResult<T> GetPaged<T>(this IQueryable<T> query,
int page, int pageSize) where T : class {
var result = new PagedResult<T>();
result.CurrentPage = page;
result.PageSize = pageSize;
result.RowCount = query.Count();
var pageCount = (double)result.RowCount / pageSize;
result.PageCount = (int)Math.Ceiling(pageCount);
var skip = (page - 1) * pageSize;
result.Results = query.Skip(skip).Take(pageSize).ToList();
return result; }
NB! Don’t change IQueryable<T> to IEnumerable<T> because otherwise regular Count() method of LINQ is called instead of Entity Framework one.
This method can be called without specifying types. Example is here.
var paged = dataContext.Measurements.GetPaged(1, 5);
// Do something with results
This extension method works with evert IEnumerable that supports Count(), Skip(), Take() and ToList() methods. I just mention these methods to point out that there can be LINQ providers that doesn’t support some of these methods but this is rare case.
Paging and mapping query results
Web applications often use model classes for data presentation. These classes have no relations with databases and they carry mostly information needed to render a view. AutoMapper provides some extension methods for IQueryable that can be also used for paging. Here is the paging extension that creates paged result with model type.
public static PagedResult<U> GetPaged<T, U>(this IQueryable<T> query,
int page, int pageSize) where U: class
{
var result = new PagedResult<U>();
result.CurrentPage = page;
result.PageSize = pageSize;
result.RowCount = query.Count();
var pageCount = (double)result.RowCount / pageSize;
result.PageCount = (int)Math.Ceiling(pageCount);
var skip = (page - 1) * pageSize;
result.Results = query.Skip(skip)
.Take(pageSize)
.ProjectTo<U>()
.ToList();
return result;
}
As this method needs more than one generic parameter these parameters must be specified. This results in longer lines of code that are still well readable like shown below.
var paged = dataContext.GetPaged<Measurement,TempMeasurementModel>(1, 5);
// Do something with results
NB! This method expects that AutoMapper type mappings are already defined. Don’t forget to define mappings before calling this method!
References
Some older writings covering the same PagedResult class:
View Comments (28)
The code can be improved to parameterise the skip and take values.
See here https://www.simple-talk.com/dotnet/net-tools/entity-framework-performance-and-what-you-can-do-about-it/#crayon-5874c4815dda9144613486
Great article
Please note that if you use this method with Entity Framework you have to make sure to sort the query to avoid this error: The method 'Skip' is only supported for sorted input in LINQ to Entities. The method 'OrderBy' must be called before the method 'Skip'.
My mistake!
My previous comment was with about Entity Framework 6.
I do not know if Entity Framework Core has the same behavior.
pacoweb, it's different with EF Core. EF Core doesn't need ordered queryable for paging.
Wil, there's no versions of Skip() and Take() with lambda in Entity Framework Core.
Thanks
Doesn't this code actually make 2 trips to the database? One for the .Count() call, and one for the .Skip(...).Take(...).ToList() call? If so, anyway to avoid that?
Yes, it takes two queries to database. Count() gives back the number of rows that match query and second query takes just requested page of results.
If you don't need Count() you can remove it from method and from PagedResult class ending up with paging where user can go to next page until empty page is returned.
I fail to see how the base class is useful for anything.
Stilgar, example of base class here: http://gunnarpeipman.com/2016/07/building-pager-view-component/ In pager view component, by example, I don't need to mess with generics when using this base class that carries minimal needed information to get pager rendered.
thank you
how to pass EF Query using a Function inside the form :
public object Get_result("which best type?" query)
{
// Result
int currentPage = Public_CurrentPage;
int CountPerPage = Convert.ToInt32(PAG_CountPerPage.Text);
var result = mainPAG.GetPaged(query, currentPage, CountPerPage);
return result;
}
This paging solution expects IQeuryable that is translated to SELECT query. As long as it is possible to create SELECT with function everything should work. If you need paging inside function then this solution doesn't work for you. Based on information you delivered it is not possible to know what function and how you are using.
so, can you please show another solution that we don't need to use extension method so, i can paging inside function
If you want to do paging inside database then here is one good source: https://www.sqlshack.com/introduction-pagination-sql-server/
Hi Gunnar, thank you for the great article.
I'm using .NET Core 2.2 and Automapper 8.
I had to explicitly pass the Automapper map logic into the paging extension method
public static PagedResult GetPaged(this IQueryable query, int page, int pageSize, IMapper _mapper) where U : class
to use with
.ProjectTo(_mapper.ConfigurationProvider)
Then I call the method like so:
// var viewModel = dataContext..GetPaged(1, 5, _mapper);
I could of course be some sort of problem with my own project setup, but I thought I would bring it to your attention in any case.
Keep up the good work.
I just updated my paging samples to latest .NET/.NET Core: https://github.com/gpeipman/DotNetPaging
To use ProjectTo() you don't need instance of mapper necessarily. Here's how you should initialize AutoMapper 8 in Startup class of web application:
services.AddAutoMapper(GetType().Assembly);
Mapper.Initialize(cfg =>
{
cfg.AddProfile();
});
This is what worked for me with ASP.NET Core 2.2
Thanks for the information on the initialisation of Automapper in the Startup class. Thanks :)
Hi Gunnar,
i'm sorry but i'm new
if i have 25,000 record in the database and i defined to show 20 record per page , this code will load only the 20 record from the database or it will load 25,000 record then show 20 of them.
Hi,
Code here is written specially to avoid situations where million rows are loaded to memory and 10 of these are displayed. With this solution paging is done in database. If your page size is 20 then maximum 20 rows is read from database.
Loved your code.
Very very clean. :)
Hi Gunnar, thank you for the nice article and code. Exactly what I needed for a .NET Core 2.2, project with Automapper 8.1.1 and database with 28 million records. Works like a charm!
with this way, do you meet problem about memory allocation for STATIC ...
Please be more specific. I don't understand your question.
public static PagedResult....
This is method static, it has safe when using multi-thread
Although static it is thread-safe. It is extension method meaning that it gets instance of query.
Here is my writing that illustrates that extension method is actually clever illusion: https://gunnarpeipman.com/csharp/extension-methods/
Hi,
Is there an example passing dynamic filters?
Where (x => x ....)
GetPaged() works with IQueryable and it's therefore up to you how you build up query. Important things is to build the whole query in code before calling GetPaged().
Example:
var query = _dbContext.Invoices.Include(i => i.Customer) if(query.DueDate.HasValue) { query = query.Where(i => i.DueDate <= query.DueDate.Value); } if(query.CustomerId.HasValue) { query = query.Where(i => i.Customer.Id == query.CustomerId.Value); } var invoices = query.OrderBy(i => i.InvoiceNo).GetPaged(page, pageSize);In case of EF Core check always out the resulting SQL query as this can be ugly and not very performant.
Dear Gunnar,
When using paging with AutoMapper ProjectTo, assuming an example of Parents and Childs, wish to acquire parents list with childCount (automapper projects automatically to childCount property).
The issue happens that when paginating, several round trips to count child happens along with ProjectTo, if I remove paging, I have a single (wished) round trip to get parents and counting child.
Can you advise what to do in order to avoid those roundtrips (that is happening one count(*) for each parent record).
Thanks