无法跟踪实体类型“xTestType”的实例，因为已跟踪具有相同键的此类型的另一个实例？

Question

I'm trying to delete multiple rows from a table. But it gives the following error after the first iteration. I can see primary key Id as 0 on all the xTestType object. that might be the issue. Why is it always giving Id 0 .

foreach (var temp in oldxDetails.TestTypes)
{
  if (deleteTestTypes.Contains(input.Id))
  {
    var xTestType = new xTestType
    {
      xId = xId,
      TestTypeMasterId = temp.Id
    };

    await _xTestRepository.DeleteAsync(xTestType);
  }
}

Exception:

The instance of entity type 'xTestType' cannot be tracked because another instance of this type with the same key is already being tracked. When adding new entities, for most key types a unique temporary key value will be created if no key is set (ie if the key property is assigned the default value for its type). If you are explicitly setting key values for new entities, ensure they do not collide with existing entities or temporary values generated for other new entities. When attaching existing entities, ensure that only one entity instance with a given key value is attached to the context.

Answer 1

When you fetch data from database, and iterate over it, like:

var dataObject = await dbContext.Table.where(x=>x.UserId == model.UserId).TolistAsync();
foreach(var item in dataObject)
{ 

}

do not create another object, pass the fetched object directly to Delete or use it to update, because DbContext is tracking the objects it has fetched, not the ones you create. for example:

//Wrong code
var dataObject = await dbContext.Table.where(x=>x.UserId == model.UserId).TolistAsync();
foreach(var item in dataObject)
{ 
   var x=new DataObject()
   {
      x=item.Id
   };
   dbContext.Table.Remove(x);
}

you must pass the originally fetched instance to Remove() method, see:

var dataObject = await dbContext.Table.where(x=>x.UserId == model.UserId).TolistAsync();
foreach(var item in dataObject)
{ 
    dbContext.Table.Remove(item);
}

Answer 2

The issue exists because the entity framework is tracking xTestType when you fetched all of them. There are two approaches to handle the situation.

Approach 1:

DbContext.Entry(xTestTypeOld).State = EntityState.Deleted;  // where xTestTypeOldis record from which you are taking xId

Approach 2 :

DbContext.Entry(xTestTypeOld).State = EntityState.Detached;
DbContext.Entry(xTestType).State = EntityState.Deleted;

I would say the first approach is the best one.