csharp EF Core 同一个实体被多次 tracked

具体体现为：

he instance of entity type ‘XXXX’ cannot be tracked because another instance with the same key value for {‘Key’} is already being tracked. When attaching existing entities, ensure that only one entity instance with a given key value is attached.

原因：在使用 ef 查询后，再使用 ef 更新参与了查询的实体，就会报此错误。

重现

// _selectedCategory 是 db.Category 里的一个实例
    
var relatedDevices = db.Devices
    .Include(d => d.Category)
    .Where(d => d.Category == _selectedCategory)
    .ToList();


db.Categories.Remove(_selectedCategory);

解决：AsNoTracking

// 下面位置添加 .AsNoTracking()
    
var relatedDevices = db.Devices.AsNoTracking()
    .Include(d => d.Category)
    .Where(d => d.Category == _selectedCategory)
    .ToList();

db.Categories.Remove(_selectedCategory);