仅使用 Linq(EF 核心)选择特定字段 [英] Select only specific fields with Linq (EF core)

问题描述

我有一个 DbContext，我想在其中运行查询以仅返回特定列，以避免获取所有数据.
问题是我想用一组字符串指定列名，并且我想获得原始类型的 IQueryable，即不构造匿名类型.

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.

这是一个例子:

// 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 };
    }
}

我想要这样的方法:

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

有没有办法做到这一点?

Is there a way I can do this?

推荐答案

由于您将 T 类型的成员投影(选择)到同一类型 T，使用 Expression 类方法可以相对容易地创建所需的 Expression，如下所示:

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 是 new T { Member1 = x.Member1, Member2 = x.Member2, ... } C# 构造的表达式.

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

示例用法为:

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

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