System.AggregateException發生 - 集合被修改

Question

我在實體框架的核心1.1以下實體：

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);   
    }); 
    } 
} 

我試圖創建一個問題列表（在下面的代碼我只加一）：

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(); 

當添加我得到以下錯誤的問題：

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

這是工作EntityFramework 1.0.1。遷移到1.1時出現錯誤。

如果我在Answer中的代碼行Author = context.Users.Last(),註釋，那麼它工作正常。

我錯過了什麼嗎？

Answer 1

通常在保存時將整個對象附加爲導航屬性並不是一個好主意，因爲它可能會插入新記錄。我的猜測是，它試圖在創建可能導致用戶列表處於集合修改狀態的問題時添加用戶。

最好的辦法是在創建對象之前先獲取第一個和最後一個用戶，然後再創建對象。

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();