6. Transactions

await using var conn = new SqlConnection(cs);
await conn.OpenAsync();
await using var tx = (SqlTransaction)await conn.BeginTransactionAsync(IsolationLevel.ReadCommitted);
try
{
    var debit = new SqlCommand("UPDATE Accounts SET Balance = Balance - @amt WHERE Id = @from AND Balance >= @amt", conn, tx);
    debit.Parameters.Add("@amt", SqlDbType.Decimal).Value = amount;
    debit.Parameters.Add("@from", SqlDbType.Int).Value = fromId;
    if (await debit.ExecuteNonQueryAsync() != 1) throw new InvalidOperationException("Insufficient funds");

    var credit = new SqlCommand("UPDATE Accounts SET Balance = Balance + @amt WHERE Id = @to", conn, tx);
    credit.Parameters.Add("@amt", SqlDbType.Decimal).Value = amount;
    credit.Parameters.Add("@to", SqlDbType.Int).Value = toId;
    await credit.ExecuteNonQueryAsync();

    await tx.CommitAsync();
}
catch
{
    await tx.RollbackAsync();
    throw;
}