Microsoft's release notes highlights for Preview 3 include:

• Visual Studio now offers .NET Framework 4.7.2 development tools to supported platforms with 4.7.2 runtime included.
• We improved performance during project unload/reload and branch switching.
• With added support for Azure Functions, you now have a new target host in the Configure Continuous Delivery to Azure dialog.
• Git and TFS status now updates properly for external file changes in .NET Core projects.
• We added new productivity features, such as code cleanup, invert-if refactoring, Go to Enclosing Block, Multi-Caret support, and new keyboard profiles.
• C++ enhancements include Template IntelliSense, convert macro to constexpr lightbulbs, and experimental in-editor code analysis squiggles.
• You can now use cross-language debugging with Python 3.7.0rc1.
• Performance Profiling now offers the ability to pause/resume data collection and adds a new .NET Object Allocation Tracking tool.
• We included improvements for Android incremental builds in the Xamarinsupport for Xcode 9.4.
•  

In summary, the most important issues here are:

• The Aggregate’s main task is to protect invariants (business rules, the boundary of immediate consistency)
• In a multi-threaded environment, when multiple threads are running simultaneously on the same Aggregate, a business rule may be broken
• A way to solve concurrency conflicts is to use Pessimistic or Optimistic concurrency techniques
• Pessimistic Concurrency involves the use of a database transaction and a locking mechanism. In this way, requests are processed one after the other, so basically concurrency is lost and it can lead to deadlocks.
• Optimistic Concurrency technique is based on versioning database records and checking whether the previously loaded version has not been changed by another thread.
• Entity Framework Core supports Optimistic Concurrency. Pessimistic Concurrency is not supported
• The Aggregate must always be treated and versioned as a single unit
• Domain events are an indicator, that state was changed so Aggregate version should be changed as well 

public class AggregateRootBase : Entity, IAggregateRoot
{
    private int _versionId;

    public void IncreaseVersion()
    {
        _versionId++;
    }
}

internal sealed class OrderEntityTypeConfiguration : IEntityTypeConfiguration<Order>
{
    public void Configure(EntityTypeBuilder<Order> builder)
    {
        builder.Property("_versionId").HasColumnName("VersionId").IsConcurrencyToken();
 
        //...
    }
}

var order = await _ordersContext.Orders.FindAsync(orderId);
order.AddOrderLine(request.ProductCode); 
var domainEvents = DomainEventsHelper.GetAllDomainEvents(order);
if (domainEvents.Any())
{
    order.IncreaseVersion();
}
await _ordersContext.SaveChangesAsync();