Entity Framework - ASP.NET Dynamic Data - page 61

Product product = context.Products.First(p => p.ProductName==”Mukuzani”);
product.Category = context.Categories.First(c => c.CategoryName==
”Beverages”);
context.SaveChanges();
In the next scenario, an existing Product entity is deleted, and its state changes from
Unchanged to Deleted as soon as the DeleteObject method of the Products entity set is
called. At this point, the entity has been marked for deletion but not deleted yet in the
database. When the SaveChanges method of the context is called, the row storing the
entity is “physically” deleted from the Products table. The deleted Product entity is also
removed from the ObjectContext , and trying to get its ObjectStateEntry will result in an
InvalidOperationException :
Product product = context.Products.First(p => p.ProductName == “Mukuzani”);
context.Products.DeleteObject(product);
context.SaveChanges();
Optimistic Concurrency Control
Most enterprise database applications require access by multiple simultaneous users. When
many people have access to the same data records, there is always a chance of two people
trying to modify the same record.
The following code illustrates this scenario by creating two different instances of the
NorthwindEntities context, retrieving two different copies of the same Product entity,
making different changes to it and trying to save the changes. Obviously, you wouldn't
write code like this in a real-world application— context1 and context2 simply illustrate
the actions performed by two different users running your application on two different
computers:
using (NorthwindEntities context1 = new NorthwindEntities())
using (NorthwindEntities context2 = new NorthwindEntities())
{
Product product1 = context1.Products.First(p => p.ProductName == “Chai”);
Product product2 = context2.Products.First(p => p.ProductName == “Chai”);
product1.UnitsInStock = 50;
product2.UnitsInStock = 20;
context1.SaveChanges();
context2.SaveChanges();
}