How do I write EF.Functions extension method?

Question

I see that EF Core 2 has EF.Functions property EF Core 2.0 Announcement which can be used by EF Core or providers to define methods that map to database functions or operators so that those can be invoked in LINQ queries. It included LIKE method that gets sent to the database.

But I need a different method, SOUNDEX() that is not included. How do I write such a method that passes the function to the database the way DbFunction attribute did in EF6? Or I need to wait for MS to implement it? Essentially, I need to generate something like

SELECT * FROM Customer WHERE SOUNDEX(lastname) = SOUNDEX(@param)

Answer 1

Adding new scalar method to EF.Functions is easy - you simply define extension method on DbFunctions class. However providing SQL translation is hard and requires digging into EFC internals.

However EFC 2.0 also introduces a much simpler approach, explained in Database scalar function mapping section of the New features in EF Core 2.0 documentation topic.

According to that, the easiest would be to add a static method to your DbContext derived class and mark it with DbFunction attribute. E.g.

public class MyDbContext : DbContext
{
    // ...

    [DbFunction("SOUNDEX")]
    public static string Soundex(string s) => throw new Exception();
}

and use something like this:

string param = ...;
MyDbContext db = ...;
var query = db.Customers
    .Where(e => MyDbContext.Soundex(e.LastName) == MyDbContext.Soundex(param));

You can declare such static methods in a different class, but then you need to manually register them using HasDbFunction fluent API.