Question

I have the code below which is intended to build a dictionary object with the following construction Dictionary<string<Dictionary<string, string>. For some reason each time I add an item to it the key text is correct but the value (dictionary) overwrites the previous one. This is probably better explained like this:

Iteration 1

• key1, dictionary1

Iteration 2

• key1, dictionary2
• key2, dictionary2

Iteration 3

• key1, dictionary3

• key2, dictionary3

• key3, dictionary3

What is causing this and how can I fix this code to stop it overwriting the dictionary in each entry?

QueryNameUserQueryString = new Dictionary<string, string>();
DialectDictionary = new Dictionary<string, Dictionary<string, string>>();

while (dataBaseConnection.NextRecord())
                {
                if (QueryNameUserQueryString != null)
                    {
                    QueryNameUserQueryString.Clear();
                    }

                string dialect = dataBaseConnection.GetFieldById (0);

                //If no dialect then carry out next iteration
                if (String.IsNullOrEmpty (dialect))
                    {
                    continue;
                    }

                try
                    {
                    dataBaseConnection2.ExecutePureSqlQuery ("SELECT * FROM " + sqlFactoryTable + " WHERE SQL_FACTORY_DIALECT = '" + dialect + "'");
                    }
                catch
                    {
                    dataBaseConnection.Close();
                    dataBaseConnection2.Close();

                    throw;
                    }

                //Do we have query strings for this dialect?
                if (!dataBaseConnection2.HasRows())
                    {
                    continue;
                    }

                //loop through query strings
                while (dataBaseConnection2.NextRecord())
                    {
                    string queryName = dataBaseConnection2.GetFieldById (2);
                    string queryString = dataBaseConnection2.GetFieldById (3);
                    string user = dataBaseConnection2.GetFieldById (4);

                    //create composite key for dictionary
                    string compositeKey = dialect + "," + queryName + "," + user;

                    if (QueryNameUserQueryString != null)
                        {
                        //Construct our query string dictionary
                        QueryNameUserQueryString.Add (compositeKey, queryString);
                        }
                    }

                //If we have a query string dictionary
                if (QueryNameUserQueryString != null)
                    {
                    //Ensure m_dialect dictionary is not null
                    if (DialectDictionary != null)
                        {
                        //Construct our dictionary entry for this dialect
                        DialectDictionary.Add (dialect, QueryNameUserQueryString);
                        }
                    }
                }
            }

Answer 1

You seem to be using the same instance of QueryNameUserQueryString on every iteration. When it’s added to the DialectDictionary, it’s added as a reference – not a copy of the original.

To “properly” solve the issue, I would move the declaration of your QueryNameUserQueryString variable inside the while-scope. That way you would make sure that it can only exist in the scope of a single iteration, not across several. When it’s added to the DialectDictionary, the reference lives on in that dictionary and you’re safe to leave the scope.