ASP.NET Dropdown List Control: Eight Ways to Bind Data

The sample downloadable solution (zip) is entirely developed using Visual Studio.NET 2003 Enterprise Architect on Windows Server 2003 Standard Edition.  But, I am confident that it would work with other versions of Windows (which support .NET 1.1) as well.

What are the ways to bind data to a drop down list?

I don’t think there exists any meaningful application which does not have at least one “dropdownlist” control.  Every user definitely knows what a “dropdownlist” is.  The most important issue is that a “dropdownlist” can maintain two sets of values inside. 

The first set of values display generally the “textual” information (which is visible to the user).  The second set of values is “hidden” from the user.  The user would never know about the “hidden” information.  This flexibility is particularly necessary when you want to display only “student names” to the user and work with “regdno” behind the scenes.  Of course, the user may not identify any student just with the “regdno”!

Not every scenario needs the two sets of values.  Some may need only one set of values (both “text” and “value” would be the same).  The “dropdownlist” control in ASP.NET supports both of these scenarios. 

The process of attaching the “dropdownlist” control to a source of data is nothing but “data binding.”  This process of “binding” can occur in several ways including databases, arrays, and so on. In this article, we will mainly look at the following ways to bind (or place) information into a “dropdownlist” control.

• Binding to a Simple Array
  
• Binding to an ArrayList
  
• Binding to a hashtable
  
• Binding to a data table
  
• Binding to a data set
  
• Binding to a data view
  
• Binding to a set of objects
  
• Binding manually

The above are the most frequently used “data binding” mechanisms when we work with a “dropdownlist” in ASP.NET.  Now, let us go through each of those methodologies.

Binding to a Simple Array

This is the simplest of all.  It is generally used with only one set of values (rather than with two sets of values).  Let us go through the following code first:

Private Sub btnSimpleArray_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSimpleArray.Click

The first statement clears all items in the “dropdownlist.”  The second statement creates a new array “months()” which can hold any number of strings (assigned during the declaration).  It is simultaneously filled with a set of month names.  “datasource” is a simple property of “dropdownlist” which can work with several types of “collections”. 

At this moment, we assigned a string array “month()” as the data source.  The last statement would make the “dropdownlist” fetch all the information from data source “month()” and fill itself.  That’s it. We are finished.

I also added two more routines, to check the “text” and “value” of a selected item in a particular “dropdownlist.”  Let us go through the two routines available.

The above routine displays only the “text” (visible to the user) of the selected item within the “dropdownlist.”  Let us see the other:

The above routine displays only the “value” (which is hidden from the user) of the selected item within the “dropdownlist.”  In the case of “Binding a Simple Array,” there does not exist two sets of data.  It has only one set of data.  And thus both the “text” and “value” of the selected item of the “dropdownlist” would be same (which is “text” itself).

Binding to an ArrayList

Working with an “ArrayList” is also quite simple and straightforward.  Not only that, it has lot of flexibilities that a simple array does not have.  For example, we can remove, clear or do any manipulations within the “ArrayList” (which is a bit tedious to have in a simple array). 

Another beauty of an ArrayList is that it can contain any type of “object” (not just strings or integers).  We shall examine this feature later. 

Now, let us go through the following code:

I hope it is quite straightforward.  You declared a new “ArrayList” and started adding elements to it using the predefined method “add.”   But, make sure that even the “ArrayList” can contain only one set of items (and not two sets of items at the moment).

So, even now, the “text” and “value” of any selected item within the “dropdownlist” would still be the same.  

Binding to a Hashtable

Coming to the concept of “Hashtable”, it is very similar to an “ArrayList.”  But, it stores a “pair” of information (something like a key and a value).  This gives us the opportunity to have two sets of data!

Let us go through the following code.

Private Sub btnHastable_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnHastable.Click

Let me explain part by part.  Let us first consider the following statements:

I am creating a new “hashtable” and adding “items” to it.  Each “item” needs to have a “key” and a “value.”  From the above statement, I am adding a single student (item) with his regdno (key) and name (value).  So, you now understand the concept of two sets of data. 

Further proceeding, we have:

Binding to data table, data set and data view

At the moment, for this article, I am trying to fetch the information from the database and then fill either the data table or data set.  Actually, you can even create your own “offline” table structure and add rows to a “data table” (or even a “data set”).  I leave that concept to the programmers to investigate those techniques, because it is beyond the scope of this article.

Now, let us connect to the database, fetch the information into the data table (or data set) and then bind it to the “dropdownlist.”

I think the above two methods should be quite familiar to every programmer.  Working with “data view” would also be very similar to that of “data table” or “data set,” except that we can “sort” or “filter” the data.  Let us look into that.

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

Binding to a set of objects

If you have your own structure (defined in the form of “class” or “structure”), you can still use them to create several objects and together bind to the “dropdownlist.”

At this moment, I just created a simple class (CStudent) which can hold a “regdno” and “sname”.  I also defined two read/write properties to “set-get” the property values.  Let us go through the class first.

Once you create the above class, we need to declare objects (along with values) to work with those classes.  Make sure that a “class” is simply a specification (or user-defined data type).  It does not allocate any memory to hold the values (unless you create the objects).  The following code creates a few objects based on the class “CStudent” and binds it to the “dropdownlist.”

As I mentioned earlier, the “ArrayList” can hold any type of object.  And now, you can see how helpful it is!

Binding Manually

Most programmers should already know about this concept.  We can manually add different types of items to the “dropdownlist.”

Let us go through the following:

Even within the above program, I tried to give you several ways of adding “one set” or “two sets” of information manually to a “dropdownlist.”

Any comments, suggestions, bugs, errors, feedback etc. are highly appreciated at jag_chat@yahoo.com