Over the last couple of weeks, I have covered concurrency checks in SQLite and Postgres. This week I’m expanding my Entity Framework sample repo to include a SQL Server based example project in order to demo concurrency checks. for SQL Server. The repo with the SQL Server project added, but before the concurrency checks can be found here.

Note that all the changes in this post will be made under the SqlServer folder.

Context Changes and Data Migration

SQL Server has a built-in column type of rowversion which is automatically generated with a unique binary number when a row is inserted or updated. In order to utilize the rowversion column type via Entity Framework, we need to add a new column to our model with the Timestamp attribute like the following from the sample code in the Contact model.

[Timestamp] 
public byte[] Timestamp { get; set; }

Next, from a command prompt in the same directory as your project file using the following .NET CLI command to generate a migration for the above change.

dotnet ef migrations add ContactTimestamp --context ContactsDbContext

Then, use the following command to apply the migration to your database.

dotnet ef database update --context ContactsDbContext

Testing it out

For a quick test add a ConcurrencyTest razor page under the Contacts directory. This function is going to ensure a specific contact exists, then pull the contact from two different DBContexts, make a mutation on the resulting contact objects, then attempt to save. The first save will work and the second should fail. Please note that this function isn’t an example of how things should be done just a quick and dirty way to prove that the concurrency check is happening.

public void OnGet()
{
    var context1 = new ContactsDbContext(new DbContextOptionsBuilder<ContactsDbContext>()
                                         .UseSqlServer("yourConnectionString")
                                         .Options);
    var contactFromContext1 = context1.Contacts
                              .FirstOrDefault(c => c.Name == "Test");

    if (contactFromContext1 == null)
    {
        contactFromContext1 = new Contact
                              {
                                  Name = "Test"
                              };

        context1.Add(contactFromContext1);
        context1.SaveChanges();
    }

    var context2 = new ContactsDbContext(new DbContextOptionsBuilder<ContactsDbContext>()
                                         .UseSqlServer("yourConnectionString")
                                         .Options);
    var contactFromContext2 = context2.Contacts
                              .FirstOrDefault(c => c.Name == "Test");

    contactFromContext1.Address = DateTime.Now.ToString();
    contactFromContext2.Address = DateTime.UtcNow.ToString();

    context1.SaveChanges();
    context2.SaveChanges();
}

Run the application and hit the ConcurrenctTest route which is https://localhost:44324/Contacts/ConcurrencyTest for my test. The following is the resulting exception.

An unhandled exception occurred while processing the request.

DbUpdateConcurrencyException: Database operation expected to affect 1 row(s) but actually affected 0 row(s). Data may have been modified or deleted since entities were loaded. See http://go.microsoft.com/fwlink/?LinkId=527962 for information on understanding and handling optimistic concurrency exceptions.

Also published on Medium.