Can you explain what the transaction isolation level means in a database management system?

The transaction isolation level in a DBMS determines how isolated a transaction is from changes made by other transactions. It affects data visibility and prevents issues such as:

• Dirty Read — reading data that has been modified but not yet committed by another transaction.
• Non-repeatable Read — when a repeated read of the same data set within a transaction yields different results due to changes made by another transaction.
• Phantom Read — when a re-execution of a query results in new rows being added by another transaction.

Main isolation levels:

1. Read Uncommitted — the lowest level, allows dirty reads.
2. Read Committed — prevents dirty reads but allows non-repeatable reads.
3. Repeatable Read — prevents dirty and non-repeatable reads but allows phantom reads.
4. Serializable — the strictest level, fully isolates transactions, preventing all the issues listed.

In C#, when working with databases via ADO.NET or Entity Framework, you can set the transaction isolation level, for example:

using (var transaction = connection.BeginTransaction(IsolationLevel.Serializable))
{
    // database operations
    transaction.Commit();
}