Developer .NET UPDATE--Data Binding--February 20, 2004

Dealing with Data Binding

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Developer .NET Perspectives

Dealing with Data Binding

In "Handling Events that Fire Unexpectedly" (http://www.winnetmag.com/windows/article/articleid/41717/41717.html), I discussed how events fire somewhat unexpectedly for a ComboBox when data binding is involved. At the core of this problem are differences in how the ComboBox behaves, depending on whether data is bound or loaded into the control.

I also showed you how to add items to the ComboBox's Items collection both ways: through the DataSource property (data binding) and through the Items.Add method (data loading). As you witnessed, the DataSource method causes two unexpected events to fire. So, what's causing the unexpected events to fire and how can you limit their impact?

The first unexpected event fires because when Visual Studio .NET creates the comboBox1_SelectedIndexChanged method for you, it places the event-handler code inside the InitializeComponent method, which Visual Studio .NET also generates for you. You can find the following line within the generated code, which is, by default, collapsed:this.comboBox1.SelectedIndexChanged += new System.EventHandler(this.comboBox1_SelectedIndexChanged);

This code ensures the handler is set up before the FormLoad method gets called. The result is that the FormLoad method fires the event when you set the ComboBox's DataSource property. When Visual Studio .NET adds the data to the ComboBox behind the scenes, an event initially fires because you've changed the array of elements.

The second unexpected event fires for a different reason. Instead of the initial selection being -1 (which represents no selection), the first item is automatically selected, which results in the event being fired. You can see the difference in these two firings by watching the ComboBox's Text property value change in the debugger.

So what are the options if you want data binding to behave more like the behavior that occurs when you use the Items.Add method? You have two options, and neither is very clean. The first option is to move the generated line of code out of the form's InitializeComponent method. If you don't set the event until after the ComboBox loads, the event won't fire. However, you now have a new problem: You've selected the first item in your ComboBox but you aren't executing the code associated with that selection. In addition, this solution presumes you're loading your ComboBox only in conjunction with the form loading; if you need to reload your ComboBox, this solution doesn't work.

A better solution is to create a form-level Boolean variable that's named something like "m_bLoading." You set this variable to true before the call to bind a new array of data, then reset it to false after the ComboBox loads. In the SelectedIndexChanged method, you must check to see whether the variable is true before you proceed to your custom code. To implement this solution, change the SelectedIndexChanged method so that it looks similar to

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