Question

我在Entity Framework Core 1.1中有以下实体：

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);        
    });
  }
}

我尝试创建一个问题列表（在下面的代码中我只添加一个）：

    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();

添加问题时，我收到以下错误：

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

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

如果我在Answer中注释代码行Author = context.Users.Last(),，那么它可以正常工作。

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不是一个好主意，因为它可能会插入新记录。我的猜测是它在创建问题时尝试添加用户，这可能导致用户列表处于集合修改状态。

最好的方法是在创建对象之前先获得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();

发生System.AggregateException - 修改了集合

1 个答案: