将实体附加到数据库上下文时实例化枚举器后修改了集合

Question

To attach an entity to database context in entity framework, I use the following:

var student = GetStudent();
if (!_dbContext.Students.Local.Any(e => e.ID == student.ID))
    _dbContext.Attach(student);

This code sometimes (eg, 1 out of 3) fails with the following error raised when Attach is called:

System.InvalidOperationException: Collection was modified after the enumerator was instantiated.

Simply re-running this piece of code when exception is thrown, may resolve the issue. However, I interested in knowing why I see this error, what causes it, and how I can resolve it?

(This code belongs to a big project, and I am not sure which parts can help identify the cause of this issue; if you suspect anything, let me know and will share related code.)

Answer 1

If you are re-running the project and it is working as expected i assume this might be related to caching. Could you please add AsNoTracking() method like this.

var student = GetStudent();
if (!_dbContext.Students.AsNoTracking().Local.Any(e => e.ID == student.ID))
    _dbContext.Attach(student);

Answer 2

I can't give you exact answer for this, but exception explains whats happening. In simple terms your collection is getting changed by other thread when you are trying to attach.

This usually happens when you are using parallel programming. By Debugging you have to identify whats happening underneath of your code and try to resolve it.