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I referred to this question here but it does not address the question that I have with regards to my question. I have a C# implementation of an in-memory database that I came up with during a recent interview. I was told that my code was okay in terms of getting the job done but could be more efficient dealing with transactions. I am not sure how I could improve this. I would greatly appreciate any suggestions.

The database should support GET, SET, DELETE, COUNT as basic operations. END signifies that the user is not going to issue any more commands. Input is read from stdin.

eg.

SET a 10
GET a //returns 10
COUNT a //returns 1
DELETE a
GET a //returns 0.

A transaction is the equivalent of the DB transactions support the above operations in an atomic manner. It is initiated by a BEGIN command, rollback means rolling back the active transaction, since rolling transactions are supported, commit writes all open transactions to memory. A rollback issued with no transactions open or a commit issued with no transactions open will result in a "NO TRANSACTIONS" being printed.

BEGIN
SET a 10
GET a
BEGIN
SET a 20
GET a
ROLLBACK
GET a
ROLLBACK
GET a
END

Here are my classes:

public class Operation

 private readonly Dictionary<string, int> valueStore;
 private readonly Dictionary<int, int> valueCount;

 public Operation()
 
 valueStore = new Dictionary<string, int>();
 valueCount = new Dictionary<int, int>();
 
 //Used for copying over old data for supporting transactions
 public Operation(Operation operation)
 
 valueStore = new Dictionary<string, int>(operation.valueStore);
 valueCount = new Dictionary<int, int>(operation.valueCount);
 
 //set a variable to a value in the datastore and update counts
 internal void Set(string variable, int value)
 
 if (!valueStore.ContainsKey(variable))
 
 valueStore.Add(variable, value);
 
 else
 
 valueCount[valueStore[variable]] -= 1;
 valueStore[variable] = value;
 
 if (!valueCount.ContainsKey(value))
 
 valueCount.Add(value, 1);
 
 else
 
 valueCount[value] += 1;
 
 
 //Get value from datastore, return null if not present
 internal void Get(string variable)
 
 if (valueStore.ContainsKey(variable))
 Console.WriteLine(valueStore[variable]);
 else
 Console.WriteLine("NULL");
 
 //Get count from datastore, return 0 if not present
 internal void Count(int value)
 
 if (valueCount.ContainsKey(value))
 Console.WriteLine(valueCount[value]);
 else
 Console.WriteLine("0");
 
 //Delete value from data store and update count.
 internal void Delete(string variable)
 
 if (valueStore.ContainsKey(variable))
 
 int value = valueStore[variable];
 valueCount[value] -= 1;
 valueStore.Remove(variable);
 
 else
 Console.WriteLine("Variable does not exist");
 

 /*
 * We need to override equals to compare two operations 
 * because when we have two begins, 1 commit followed by
 * a rollback we should technically have no transactions
 * to rollback, because the last committed and current 
 * transactions are both same
 */ 

 public bool Equals(Operation other)
 
 if (valueStore.Keys.Count == other.valueStore.Keys.Count)
 
 foreach (string variable in valueStore.Keys)
 
 if (other.valueStore.ContainsKey(variable)
 && other.valueStore[variable] == valueStore[variable])
 continue;
 else
 return false;
 
 
 else
 return false;
 return true;
 

 public override int GetHashCode()
 
 unchecked
 
 int hash = 17;
 hash = hash * 31 + valueStore.GetHashCode();
 hash = hash * 31 + valueCount.GetHashCode();
 return hash;
 
 


public class Transaction

 private readonly Stack<Operation> transactions;

 public Transaction()
 
 transactions = new Stack<Operation>();
 

 internal Operation Begin(Operation operation)
 
 transactions.Push(operation);
 return new Operation(operation);
 

 internal Operation Commit(Operation operation)
 
 transactions.Clear();
 transactions.Push(operation);
 return new Operation(operation);
 

 internal Operation Rollback(Operation operation)
 
 if (transactions.Count != 0)
 
 Operation lastCommitted = transactions.Pop();
 if (lastCommitted.Equals(operation))
 
 Console.WriteLine("NO TRANSACTION");
 transactions.Push(lastCommitted);
 
 return lastCommitted;
 
 else
 
 Console.WriteLine("NO TRANSACTION");
 return operation;
 
 


public class Database

 private Operation commands;
 private Transaction transaction;

 public Database()
 
 commands = new Operation();
 transaction = new Transaction();
 

 public void HandleInput()
 
 Console.WriteLine("Hello user, enter some commands");
 string line = "";
 while ((line = Console.ReadLine()) != null)
 
 if (line == "END")
 break;
 else
 
 HandleUserInput(line);
 
 
 

 private void HandleUserInput(string inputLine)
 
 /*
 * Need to use exceptions for parsing and use try catch block for FormatException
 * Assuming that input commands will be valid(eg. Get will have 2 params, 
 * Count will have 2 params etc.)
 */ 
 string[] parameters = inputLine.Split(' ');
 string op = parameters[0];
 string variable;
 int value;
 switch (op.ToUpper())
 
 case "GET":
 variable = parameters[1];
 commands.Get(variable);
 break;

 case "SET":
 variable = parameters[1];
 value = int.Parse(parameters[2]);
 commands.Set(variable, value);
 break;

 case "DELETE":
 variable = parameters[1];
 commands.Delete(variable);
 break;

 case "COUNT":
 value = int.Parse(parameters[1]);
 commands.Count(value);

 break;

 case "BEGIN":
 commands = transaction.Begin(commands);
 break;

 case "ROLLBACK":
 commands = transaction.Rollback(commands);
 break;

 case "COMMIT":
 commands = transaction.Commit(commands);
 break;

 default:
 Console.WriteLine("Invalid operation: " + op + "nTry again.");
 break;