System.AggregateException发生 - 集合被修改

Question

我在实体框架的核心1.1以下实体：

public class Question { 
    public Int32 Id { get; set; } 
    public Int32 AuthorId { get; set; } 
    public String Content { get; set; } 
    public virtual User Author { get; set; } 
    public virtual ICollection<Answer> Answers { get; set; } 
} 

public class Answer { 
    public Int32 Id { get; set; } 
    public Int32 QuestionId { get; set; } 
    public Int32 AuthorId { get; set; } 
    public String Content { get; set; } 
    public virtual Question Question { get; set; } 
    public virtual User Author { get; set; } 
} 

public class User : IdentityUser<Int32> { 
    public virtual ICollection<Answer> Answers { get; set; } = new List<Answer>(); 
    public virtual ICollection<Question> Questions { get; set; } = new List<Question>(); 
} 

public class Context : IdentityDbContext<User, Role, Int32> { 
    public DbSet<Answer> Answers { get; set; } 
    public DbSet<Question> Questions { get; set; } 

    public Context(DbContextOptions<Context> options) : base(options) { } 

    protected override void OnModelCreating(ModelBuilder builder) { 
    base.OnModelCreating(builder); 

    builder.Entity<Question>(b => 
     b.ToTable("Questions"); 
     b.HasKey(x => x.Id); 
     b.Property(x => x.Content).HasMaxLength(400).IsRequired(true); 
     b.HasOne(x => x.Author).WithMany(x => x.Questions).HasForeignKey(x => x.AuthorId).IsRequired(true).OnDelete(DeleteBehavior.Restrict); 
    }); 

    builder.Entity<Answer>(b => 
     b.ToTable("Answers"); 
     b.HasKey(x => x.Id); 
     b.Property(x => x.Content).HasMaxLength(4000).IsRequired(true); 
     b.HasOne(x => x.Author).WithMany(x => x.Answers).HasForeignKey(x => x.AuthorId).IsRequired(true).OnDelete(DeleteBehavior.Restrict); 
     b.HasOne(x => x.Question).WithMany(x => x.Answers).HasForeignKey(x => x.AuthorId).IsRequired(true).OnDelete(DeleteBehavior.Cascade);   
    }); 
    } 
} 

我试图创建一个问题列表（在下面的代码我只加一）：

List<Question> questions = new List<Question>(); 

    questions.Add(new Question { 
     Approved = DateTime.UtcNow, 
     Author = context.Users.First(), 
     Content = "question", 
     Created = DateTime.UtcNow, 
     IsApproved = true, 
     Level = context.Levels.First(), 
     Updated = DateTime.UtcNow, 
     Answers = new List<Answer> { 
     new Answer { 
      Approved = DateTime.UtcNow, 
      Author = context.Users.Last(), 
      Content = "answer", 
      Created = DateTime.UtcNow, 
      IsApproved = true, 
      Updated = DateTime.UtcNow 
     } 
     } 
    }); 

    context.AddRange(questions); 
    await context.SaveChangesAsync(); 

当添加我得到以下错误的问题：

System.AggregateException occurred 
InvalidOperationException: Collection was modified; enumeration operation may not execute. 

这是工作EntityFramework 1.0.1。迁移到1.1时出现错误。

如果我在Answer中的代码行Author = context.Users.Last(),注释，那么它工作正常。

我错过了什么吗？

Answer 1

通常在保存时将整个对象附加为导航属性并不是一个好主意，因为它可能会插入新记录。我的猜测是，它试图在创建可能导致用户列表处于集合修改状态的问题时添加用户。

最好的办法是在创建对象之前先获取第一个和最后一个用户，然后再创建对象。

var firstUser = context.Users.First(); 
var lastUser = context.Users.Last(); 

List<Question> questions = new List<Question>(); 

questions.Add(new Question { 
    Approved = DateTime.UtcNow, 
    Author = firstUser, 
    Content = "question", 
    Created = DateTime.UtcNow, 
    IsApproved = true, 
    Level = context.Levels.First(), 
    Updated = DateTime.UtcNow, 
    Answers = new List<Answer> { 
    new Answer { 
     Approved = DateTime.UtcNow, 
     Author = lastUser, 
     Content = "answer", 
     Created = DateTime.UtcNow, 
     IsApproved = true, 
     Updated = DateTime.UtcNow 
    } 
    } 
}); 

context.AddRange(questions); 
await context.SaveChangesAsync();