Wrong property assignment with Unions in InMemoryProvider

[wischi-chr]

Description

There seems to be a bug in the way unions are handled by the in-memory provider. The expression used inside the first select seems to determine how the other source (provided as parameter to Union()) is interpreted/assigned which causes wrong assignments. Data from a specific "column" is assigned to an unrelated (wrong) property. This only seems to happen with the InMemoryProvider (tested it with SQL Server Provider as well and it worked as intended).

Provider and version information

EF Core version: 3.1.3 / 3.1.10 / 5.0.0 (these are the versions I tested and that showed the same behaviour)
Database provider: Microsoft.EntityFrameworkCore.InMemory
Target framework: .NET 3.1
Operating system: Win10
IDE: Visual Studio Enterprise 2019 16.7.7

Code (All-in-one file) that reproduces the issue

// nuget: Microsoft.EntityFrameworkCore.InMemory

using Microsoft.EntityFrameworkCore;
using System;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Threading.Tasks;

namespace EFCoreUnionBug
{
    class Program
    {
        static async Task Main()
        {
            await AddTestData();
            await TriggerBug();
        }

        static async Task TriggerBug()
        {
            using var context = new SampleContext();

            // Because the assigned properties are swapped the union that is executed (in-memory)
            // assigns the wrong value (different column) to the property.
            //
            // There are similar issues if a different subset of properties are assigned
            // inside the select statement. It looks like the first select determines
            // the columns that are queried/assigned for the second query.

            var userQuery = context.User
                .Select(u => new CommonSelectType()
                {
                    // 1. FirstName, 2. LastName
                    FirstName = u.Forename,
                    LastName = u.Surname,
                });

            var customerQuery = context.Customer
                .Select(c => new CommonSelectType()
                {
                    // 1. LastName, 2. FirstName
                    LastName = c.FamilyName,
                    FirstName = c.GivenName,
                });

            var result = await userQuery.Union(customerQuery).ToListAsync();

            Console.WriteLine($"result[0].FirstName = {result[0].FirstName}");
            Console.WriteLine($"result[0].LastName  = {result[0].LastName}");
            Console.WriteLine($"result[1].FirstName = {result[1].FirstName}");
            Console.WriteLine($"result[1].LastName  = {result[1].LastName}");

            // Output:
            // =======
            // result[0].FirstName = Peter
            // result[0].LastName  = Smith
            // result[1].FirstName = Doe     <--- wrong
            // result[1].LastName  = John    <--- 
        }

        static async Task AddTestData()
        {
            using var context = new SampleContext();

            context.User.Add(new User()
            {
                Forename = "Peter",
                Surname = "Smith",
            });

            context.Customer.Add(new Customer()
            {
                GivenName = "John",
                FamilyName = "Doe",
            });

            await context.SaveChangesAsync();
        }
    }

    public class CommonSelectType
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
    }

    public class SampleContext : DbContext
    {
        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseInMemoryDatabase(nameof(SampleContext));
        }

        public virtual DbSet<User> User { get; set; }
        public virtual DbSet<Customer> Customer { get; set; }
    }

    public class User
    {
        [Key]
        public int Key { get; set; }

        public string Forename { get; set; }
        public string Surname { get; set; }
    }

    public class Customer
    {
        [Key]
        public int Key { get; set; }

        public string GivenName { get; set; }
        public string FamilyName { get; set; }
    }
}