How to use Entity Framework where clause with a dynamic string

Question

I am trying to build an Entity Framework linq query to return the parent object from a child object.

The models look like this:

public class Parent
{
   Guid Id {get; set;}
   List<Child> Children {get; set;}
}

public class Child
{
    Guid Id {get; set;}
}

And my query looks like this:

string _foreignKeyName = "Children"
Guid existingChildId = "{some existing guid}"

var parent = _context.Set<Parent>()
                     .Include(_foreignKeyName)
                     .Where(x => x.Children // <--- I would like to make "Children" dynamic
                     .Where(y => y.Id == existingChildId).Any()) 
                     .FirstAsync();

Is there anyway to make the reference to "Children" dynamic and use {_foreignKeyName} instead?

I've looked at Expression Trees and Dynamic Linq, but I'd much rather keep it in standard linq if possible.

Thanks!

Standard LINQ won't allow this. If you really want this (and sacrifice a lot of clarity in your code) you have to build expressions or use dynamic LINQ. — Gert Arnold
– Gert Arnold, Commented Apr 16, 2020 at 20:54
I was worried this might be the case. I tried a few Expression examples and just couldn't get this translate into one. — Eric Longstreet
– Eric Longstreet, Commented Apr 16, 2020 at 20:58

Jonathan Magnan · Accepted Answer · 2020-04-17 11:49:57Z

1

Disclaimer: I'm the owner of the project C# Eval Expression

If you want to keep the syntax similar to LINQ, I would recommend our library. The LINQ Dynamic part is free.

All you have to do is calling "WhereDynamic" instead of "Where" and keep using the same exact syntax.

string _foreignKeyName = "Children"
Guid existingChildId = "{some existing guid}"

var parent = _context.Set<Parent>()
                     .Include(_foreignKeyName)
                     .WhereDynamic(x => "x.Children")
                     .Where(y => y.Id == existingChildId).Any()) 
                     .FirstAsync();

LINQ Dynamic: https://eval-expression.net/linq-dynamic

answered Apr 17, 2020 at 11:49

Jonathan Magnan

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1 Comment

Eric Longstreet Over a year ago

That's pretty cool. I ended up refactoring my db to avoid the awkward query which allowed me to keep it generic. I'll keep this library in mind for the future thought, pretty clean approach. Thanks for the solution anyway.

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