Select only specific fields with Linq (EF core)

Question

Select only specific fields with Linq (EF core)

.net-core c# entity-framework-core

Question

I have a DbContext where I would like to run a query to return only specific columns, to avoid fetching all the data.
The problem is that I would like to specify the column names with a set of strings, and I would like to obtain an IQueryable of the original type, i.e. without constructing an anonymous type.

Here is an example:

// Install-Package Microsoft.AspNetCore.All
// Install-Package Microsoft.EntityFrameworkCore

using Microsoft.EntityFrameworkCore;
using System;
using System.Linq;

public class Person {
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class TestContext : DbContext {
    public virtual DbSet<Person> Persons { get; set; }
    public TestContext(DbContextOptions<TestContext> options) : base(options) {
    }
}

class Program {
    static void Main(string[] args) {

        var builder = new DbContextOptionsBuilder<TestContext>();
        builder.UseInMemoryDatabase(Guid.NewGuid().ToString());
        var context = new TestContext(builder.Options);

        context.Persons.Add(new Person { FirstName = "John", LastName = "Doe" });
        context.SaveChanges();

        // How can I express this selecting columns with a set of strings? 
        IQueryable<Person> query = from p in context.Persons select new Person { FirstName = p.FirstName };
    }
}

I would like to have something like this method:

static IQueryable<Person> GetPersons(TestContext context, params string[] fieldsToSelect) {
    // ...
}

Is there a way I can do this?

Paolo Tedesco

Popular Answer

I was able to do this with the package https://github.com/StefH/System.Linq.Dynamic.Core so easily.

Here is an example code.

use namespacing, using System.Linq.Dynamic.Core;

//var selectQuery = "new(Name, Id, PresentDetails.RollNo)";

var selectQuery = "new(Name, Id, PresentDetails.GuardianDetails.Name as GuardianName)";

var students = dbContext.Students
    .Include(s => s.PresentDetails)
    .Include(s => s.PresentDetails.GuardianDetails)
    .Where(s => s.StudentStatus == "Admitted")
    .Select(selectQuery);

Ajeesh Joshy

View more on Stack Overflow

Ivan Stoev · Accepted Answer · 2/6/2019 9:15:32 AM

Since you are projecting (selecting) the members of the type T to the same type T, the required Expression<Func<T, T>> can relatively easy be created with Expression class methods like this:

public static partial class QueryableExtensions
{
    public static IQueryable<T> SelectMembers<T>(this IQueryable<T> source, params string[] memberNames)
    {
        var parameter = Expression.Parameter(typeof(T), "e");
        var bindings = memberNames
            .Select(name => Expression.PropertyOrField(parameter, name))
            .Select(member => Expression.Bind(member.Member, member));
        var body = Expression.MemberInit(Expression.New(typeof(T)), bindings);
        var selector = Expression.Lambda<Func<T, T>>(body, parameter);
        return source.Select(selector);
    }
}

Expression.MemberInit is the expression equivalent of the new T { Member1 = x.Member1, Member2 = x.Member2, ... } C# construct.

The sample usage would be:

return context.Set<Person>().SelectMembers(fieldsToSelect);

Select only specific fields with Linq (EF core)

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