Entity Framework Core – Custom Functions (Using HasDbFunction)

In the previous post (Custom Functions - using IMethodCallTranslator) we looked at the more flexible approach that requires some boilerplate code. For the usage of a custom function in a project, especially if there are no plans to make the function public then the sacrifice of some (not required) flexibility to reduce the amount of code is fully acceptable.

In diesem Artikel:

Pawel Gerr ist Architekt und Consultant bei Thinktecture. Er hat sich auf .NET Core Backends spezialisiert und kennt Entity Framework von vorne bis hinten.
  1. Entity Framework Core: ROW_NUMBER Support

  2. Entity Framework Core: Custom Functions (using IMethodCallTranslator)
  3. Entity Framework Core: Custom Functions (using HasDbFunction)

As in the previos post we will use the extension method RowVersion with the ORDER BY part only as an example. The actual code can be found on Azure DevOps: Thinktecture.EntityFrameworkCore

Create a static method "RowNumber"

For this approach the method RowNumber must a static method containing the ORDER BY parameters only, i.e. the method cannot be an extension method for DbFunctions like before.

					public static class DbFunctionsExtensions
   // will throw at runtime because EF tries to translate DbFunctions as well
   public static long RowNumber(this DbFunctions _, object orderBy)
      throw new InvalidOperationException("...");

   // works as expected
   public static long RowNumber(object orderBy)
      throw new InvalidOperationException("This method is for use with Entity Framework Core only and has no in-memory implementation.");

Due to new method signature of RowNumber the usage is slightly different as well.

  .Select(i => new
                RowNumber = DbFunctionsExtensions.RowNumber(i.ProductId)

Introduction of custom function to EF

In the previous approach we had to implement IMethodCallTranslatorIMethodCallTranslatorPlugin and IDbContextOptionsExtension to introduce a new function to EF during the configuration of the (SqlServer)DbContextOptionsBuilder. This time we will skip the IMethodCallTranslator and use the ModelBuilder during OnModelCreating.

The configuration of a new function is encapsulated in an extension method AddRowNumberSupport.

					public class DemoDbContext : DbContext

   protected override void OnModelCreating(ModelBuilder modelBuilder)


I was not totally honest with you at the beginning. It is true that we don’t need custom implementation of IMethodCallTranslator (SqlServerRowNumberTranslator) but we need some code from it. I will pretend that the SqlServerRowNumberTranslator does not exist and copy the required code to the extension method AddRowNumberSupport

Like before we fetch a MethodInfo of the method RowNumber first.

					public static class ModelBuilderExtensions
   private static readonly MethodInfo _rowNumberMethod 
            = typeof(DbFunctionsExtensions)
                         new[] { typeof(object) });

Then we use HasDbFunction and HasTranslation to introduce the function to EF. Inside of HasTranslation we (re)use the RowNumberExpression from the previous post.

					public static ModelBuilder AddRowNumberSupport(this ModelBuilder modelBuilder)
                  .HasTranslation(expressions =>
                                  var orderByParams = ExtractParams(expressions.First());

                                  return new RowNumberExpression(orderByParams);

      return modelBuilder;

The method ExtractParams is the same as in the SqlServerRowNumberTranslator.

					private static ReadOnlyCollection<Expression> ExtractParams(Expression parameter)
      if (parameter is ConstantExpression constant
          && constant.Value is IEnumerable<Expression> enumerable)
         return enumerable.ToList().AsReadOnly();

      return new List<Expression> { parameter }.AsReadOnly();


This approach is easier to grasp and the difference in the amout of code is not that big to base a decision on that alone. Use the approach you like the best because the change from one approach to another is easy and requires very litte time.


Aktuelle Artikel, Screencasts, Webinare und Interviews unserer Experten für Sie

Verpassen Sie keine Inhalte zu Angular, .NET Core, Blazor, Azure und Kubernetes und melden Sie sich zu unserem kostenlosen monatlichen Dev-Newsletter an.

Newsletter Anmeldung
Diese Artikel könnten Sie interessieren
Database Access with Sessions

Data Access in .NET Native AOT with Sessions

.NET 8 brings Native AOT to ASP.NET Core, but many frameworks and libraries rely on unbound reflection internally and thus cannot support this scenario yet. This is true for ORMs, too: EF Core and Dapper will only bring full support for Native AOT in later releases. In this post, we will implement a database access layer with Sessions using the Humble Object pattern to get a similar developer experience. We will use Npgsql as a plain ADO.NET provider targeting PostgreSQL.
Old computer with native code

Native AOT with ASP.NET Core – Overview

Originally introduced in .NET 7, Native AOT can be used with ASP.NET Core in the upcoming .NET 8 release. In this post, we look at the benefits and drawbacks from a general perspective and perform measurements to quantify the improvements on different platforms.

Optimize ASP.NET Core memory with DATAS

.NET 8 introduces a new Garbage Collector feature called DATAS for Server GC mode - let's make some benchmarks and check how it fits into the big picture.

Incremental Roslyn Source Generators: High-Level API – ForAttributeWithMetadataName – Part 8

With the version 4.3.1 of Microsoft.CodeAnalysis.* Roslyn provides a new high-level API - the method "ForAttributeWithMetadataName". Although it is just 1 method, still, it addresses one of the biggest performance issue with Source Generators.

Integrating AI Power into Your .NET Applications with the Semantic Kernel Toolkit – an Early View

With the rise of powerful AI models and services, questions come up on how to integrate those into our applications and make reasonable use of them. While other languages like Python already have popular and feature-rich libraries like LangChain, we are missing these in .NET and C#. But there is a new kid on the block that might change this situation. Welcome Semantic Kernel by Microsoft!

.NET 7 Performance: Regular Expressions – Part 2

There is this popular quote by Jamie Zawinski: Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems."

In this second article of our short performance series, we want to look at the latter one of those problems.