My article Optimize your application spoke about web-based application and this is the

time to speak about windows-based application. Couple of questions had been

arisen regarding first article and I hope that all your questions have been

answered now.

Obviously we cannot apply caching to windows-based application. But we can use a collection object to store all data what we need. The ideal one is HashTable that is a dictionary type collection (implemented with IDictionary interface) that associates a key to a value and it is optimized dictionary for fast retrieval (Read more about HashTable) .The below code show you the way of doing it.

public class Common
{
public Common()
{
}
//Hash table to keep combo items
static Hashtable dropdownItems = Hashtable.Synchronized(new Hashtable());
//Add dropdownItems
public static void AddDropDownItems(string name, object item)
{
dropdownItems.Add(name, items);
}
//Get dropdownItems
public static object GetDropDownItems(string name)
{
if (dropdownItems.Contains(name))
return dropdownItems[name];
else
return null;
}
}
 You can see that this is a simple class named Common that holds data for drop-down lists in HashTable named dropdownItems. There are two static methods that are used to add drop-down list data to HashTable and get data for drop-down list. Now let's see the way of using it.

Let's say that we have a windows form that has drop-down list for employee's names. The natural way of loading employee data into drop-down list is, make a connection to the SQL Server and fill data to the data set, and bind the data table to the drop-down list. Your code might looks like this.

SqlConnection sqlConnection = new SqlConnection("Server=(local);Database=Northwind;Uid=sa;Pwd=");
sqlConnection.Open();
SqlCommand sqlCommand = new SqlCommand("SELECT EmployeeID, LastName FROM Employees", sqlConnection);
DataSet dataSet = new DataSet();
SqlDataAdapter sqlDataAdapter = new SqlDataAdapter(sqlCommand);
sqlDataAdapter.Fill(dataSet, "Employees");
DropDownListEmployees.DataSource = dataSet.Tables["Employees"];
In order to stop the re-querying for same data, let's store data in HashTable by using Common class we created. Here is the code for it.

Common.AddDropDownItems("Employees", dataSet.Tables["Employees"]);
The method AddDropDownItems accepts two parameters: name and item. The parameter "name" is used as key for HashTable and "item" is the value for the key. You can see that we send employee's data to the HashTable with the key named "Employees". That's all. If same data is needed again (for same win-form or different win-form), you can simply get data from HashTable without re-querying the database. Here is the complete code.
//Get data from hashtable
object dropdownItems = Common.GetDropDownItems("Employees");
//If data is not available in hashtable
if (dropdownItems==null)
{
SqlConnection sqlConnection = new SqlConnection("Server=(local);Database=Northwind;Uid=sa;Pwd=");
sqlConnection.Open();

SqlCommand sqlCommand = new SqlCommand("SELECT EmployeeID, LastName FROM Employees", sqlConnection);
DataSet dataSet = new DataSet();
SqlDataAdapter sqlDataAdapter = new SqlDataAdapter(sqlCommand);

sqlDataAdapter.Fill(dataSet, "Employees");
DropDownListEmployees.DataSource = dataSet.Tables["Employees"];

//Save data in hashtable for future reference
Common.AddDropDownItems("Employees", dataSet.Tables["Employees"]);
}
else
DropDownListEmployees.DataSource = dropdownItems;

DropDownListEmployees.DisplayMember= "LastName";
DropDownListEmployees.ValueMember = "EmployeeID";
How simple is this? Again, I must say that do not apply this scenario for data that are frequently modified because then you need to remove old data from HashTable and add new data to the HashTable. I think that this is ideal for data that loads into to drop-down. Ok, it is up to you to decide to what type of data should be applied this scenario. Do it and see. If you have questions with this, please let me know. Enjoy.