Every software engineer working with SQL databases needs to know about transactions. And since most of the time the SQL database will be abstracted by an ORM like EF Core, it's important to understand how you can work with transactions using the available abstractions.

So today, I'll show you how to work with transactions in EF Core.

Here's what we will cover:

• Default transaction behavior
• Creating transactions
• Using existing transactions

Default Transaction Behavior

What is the default EF Core transaction behavior?

By default, all changes made in a single call to SaveChanges are applied in a transaction. If any of the changes fail, the entire transaction is rolled back and no changes are applied to the database. Only if all changes are successfully persisted to the database, the call to SaveChanges can complete.

This is a wonderful feature of SQL databases and it saves us many headaches. We don't have to think about the databases remaining in an inconsistent state, because database transactions can do the work for us.

Let's take a look at an example.

using var context = new ShoppingContext();

context.LineItems.Add(new LineItem
{
    ProductId = productId,
    Quantity = quantity
});

var stock = context.Stock.FirstOrDefault(s => s.ProductId == productId);

stock.Quantity -= quantity;

context.SaveChanges();

Because we are adding a LineItem, and in the same scope reducing the Stock quantity, the call to SaveChanges will apply both changes inside of a transaction. We can guarantee that the database will remain in a consistent state.

Creating Transactions With EF Core

What if you want to have more control over transactions when working with EF Core?

You can manually create a transaction by accessing the Database facade available on a DbContext instance and calling BeginTransaction.

Here's an example where we have multiple calls to SaveChanges. In the default scenario, both calls would run in their own transaction. This leaves the possibility of the second call to SaveChanges failing, and leaving the database in an inconsistent state.

using var context = new ShoppingContext();
using var transaction = context.Database.BeginTransaction();

try
{
    context.LineItems.Add(new LineItem
    {
        ProductId = productId,
        Quantity = quantity
    });

    context.SaveChanges();

    var stock = context.Stock.FirstOrDefault(s => s.ProductId == productId);

    stock.Quantity -= quantity;

    context.SaveChanges();

    // When we commit the changes, they will be applied to the databases.
    // The transaction will auto-rollback when it is disposed,
    // if any command fails.
    transaction.Commit();
}
catch (Exception)
{
    transaction.Rollback();
}

We call BeginTransaction to manually start a new database transaction. This will create a new transaction and return it, so that we can Commit the transaction when we want to complete the operation. You also want to add a try-catch block around your code, so that you can Rollback the transaction if there are any exceptions.

Using Existing Transactions With EF Core

Creating a transaction using the EF Core DbContext isn't the only option. You can create a SqlTransaction instance and pass it to EF Core, so that the changes applied with EF Core can be committed inside the same transaction.

Here's what I mean:

using var sqlConnection = new SqlTransaction(connectionString);
sqlConnection.Open();

using var transaction = sqlConnection.BeginTransaction();

try
{
    using var context = new ShoppingContext();

    // Tell EF Core to use an existing transaction.
    context.UseTransaction(transaction);

    context.LineItems.Add(new LineItem
    {
        ProductId = productId,
        Quantity = quantity
    });

    context.SaveChanges();

    var stock = context.Stock.FirstOrDefault(s => s.ProductId == productId);

    stock.Quantity -= quantity;

    context.SaveChanges();

    transaction.Commit();
}
catch (Exception)
{
    transaction.Rollback();
}

In Summary

EF Core has excellent support for transactions and it's very easy to work with.

You have three options available:

• Rely on the default transaction behavior
• Create a new transaction
• Use an existing transaction

Most of the time, you want to rely on the default behavior and not have to think about it.

As soon as you need to perform multiple SaveChanges calls, you should manually create a transaction, and manage the transaction yourself.

See you next week, and have an excellent Saturday.

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