You are writing a middle-tier component for an enterprise application named XYZApplication. The component opens a SqlConnection and uses a SqlTransaction object to control multiple SQL Server stored procedure calls that update the same data. To prevent concurrency anomalies such as phantom reads when two users update the same data concurrently, you must choose the highest level of isolation that still uses standard SQL Server isolation levels. Which isolation level should you specify when beginning the transaction?

Introduction / Context:
Transaction isolation levels control how visible intermediate changes made by concurrent transactions are to each other. In SQL Server and ADO.NET, choosing the correct isolation level can prevent issues such as dirty reads, non-repeatable reads and phantom reads. This question focuses on choosing the proper IsolationLevel value in a SqlTransaction to provide the highest standard level of protection against concurrency anomalies in a middle-tier component.

Given Data / Assumptions:

• The code uses SqlConnection and SqlTransaction in a .NET middle-tier component.
• Several stored procedures are executed under a single transaction.
• Two users might attempt to update the same data at the same time.
• You want the highest standard isolation level to prevent phantom reads and other anomalies.
• You must specify an IsolationLevel value in cn.BeginTransaction.

Concept / Approach:
SQL Server supports several isolation levels: ReadUncommitted, ReadCommitted, RepeatableRead and Serializable, among others such as Snapshot. ReadCommitted prevents dirty reads but does not prevent all non-repeatable reads or phantom rows. Serializable is the strictest of the standard isolation levels; it ensures that transactions behave as if they were executed one after another, preventing phantom reads by effectively locking the range of rows being accessed. In ADO.NET, the correct way to select this level is to pass IsolationLevel.Serializable to BeginTransaction.

Step-by-Step Solution:
1. Open a SqlConnection named cn to the SQL Server database. 2. Begin a transaction by calling cn.BeginTransaction and pass the desired IsolationLevel enumeration. 3. To prevent phantom reads, choose IsolationLevel.Serializable, which locks the relevant key ranges during the transaction. 4. Execute the necessary stored procedures using the SqlTransaction object so they participate in the same transaction. 5. Commit or roll back the transaction after all procedures finish, ensuring consistent and isolated updates.

Verification / Alternative check:
You can verify the effect by running concurrent tests. Under IsolationLevel.Serializable, attempts by another transaction to insert or modify rows that would affect the same logical set of data will be blocked until the first transaction finishes. In contrast, under IsolationLevel.ReadCommitted, phantom rows can still appear, because new rows that satisfy the query conditions can be inserted by another transaction between repeated reads.

Why Other Options Are Wrong:
Option A is incorrect because passing a string such as "ReadCommitted" is not the correct overload for BeginTransaction; it expects an IsolationLevel enumeration, not a string. Option B uses IsolationLevel.ReadCommitted, which allows phantom reads because it only guarantees that committed data is read, not that the set of rows remains stable throughout the transaction. Option D is similar to Option A and incorrectly passes a string "Serializable" instead of the proper enumeration type, so it does not represent valid C# code for this context.

Common Pitfalls:
A common mistake is defaulting to ReadCommitted without considering higher isolation when strong consistency is needed, leading to subtle data anomalies. Another pitfall is using overly strict isolation levels everywhere, which can reduce concurrency and performance. Developers may also misunderstand phantom reads, thinking they are prevented by ReadCommitted, when only Serializable or specific row versioning strategies fully prevent them. Understanding both the business requirements and the behavior of each isolation level is critical when designing robust data access code.

Final Answer:
You should call BeginTransaction with IsolationLevel.Serializable, as in tr = cn.BeginTransaction(IsolationLevel.Serializable); to obtain the highest standard isolation level and prevent phantom reads.