What is the default transaction isolation level for SQL Server with ADO.NET?

Question

What is the default transaction isolation level for SQL Server with ADO.NET? I am using a default installation of SQL Server and just the normal System.Data.SqlClient classes.

Answer 1

READ COMMITTED is the default isolation level for the Microsoft SQL Server Database Engine.

Source:

• Customizing Transaction Isolation Level @ MSDN.

Here is how it compares to other isolation levels:

• Transaction Isolation Levels @ MSDN.

The MSDN documentation for SqlConnection.BeginTransaction() also states Read committed

... To reset the isolation level to the default (READ COMMITTED) ...

• MSDN article

Answer 2

The accepted answer by hkf gives the correct answer for transactions started manually with SqlConnection.BeginTransaction(). Here, the default level is ReadCommitted.

However, this is not the only way to start a new transaction in ADO.NET: Transactions can also be created automatically by using the classes from the System.Transactions namespace, in particular, by creating a TransactionScope.

Contrary to transactions started manually, transactions created by the System.Transactions infrastructure (and, thus, by a TransactionScope) are Serializable.

See below link for more information:

• IsolationLevel Enumeration @ MSDN.