Setting the IsModified flag to false on a property leaves original value untouched so the change is detected again

[AndersKung]

using Microsoft.EntityFrameworkCore;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

//Microsoft.EntityFrameworkCore.1.1.1

namespace ConsoleApplication
{
    class Program
    {
        static void Main(string[] args)
        {
            var context = new Context();
            var user = new Users();
            user.Id = 1;

            context.Update(user);

            user.UserName = "name";
            user.City = "city";

            OnUpdating(context, user);

            // ↓↓↓Exception↓↓↓
            // The property 'UserName' on entity type 'Users' is marked as null, but this cannot be saved because the property is marked as required.
            context.SaveChanges(); 
            
        } 

        static void OnUpdating(Context context, Users user)
        {
            context.Entry(user).Property(x => x.UserName).IsModified = false;
            var isModified = context.Entry(user).Property(x => x.UserName).IsModified; // still true
        }
    }

    public class Users
    {
        public int Id { get; set; }
        public string UserName { get; set; }
        public string City { get; set; }
    }
    public class Context : DbContext
    {
        public virtual DbSet<Users> Users { get; set; }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Users>(b =>
            {
                b.HasKey(r => r.Id);

                b.Property(x => x.UserName).IsRequired();
            });
        }
        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer("Server=.\\SQLEXPRESS;Initial Catalog=Test;Trusted_Connection=True;MultipleActiveResultSets=true");
        }
    } 
}

http://stackoverflow.com/questions/12661881/exclude-property-on-update-in-entity-framework

[ajcvickers]

Notes for triage: Confirmed this is a bug, but not a regression. The original value is being left as-is when setting the IsModified flag. This causes DetectChanges to detect that the current and original values are different, and so the property is marked as modified again.

Workaround: When setting the original value to be the same as the current value before marking the property as not modified. For example:

var propertyEntry = context.Entry(user).Property(x => x.UserName);
propertyEntry.OriginalValue = user.UserName;
propertyEntry.IsModified = false;