Coding a Data-Bound Custom Control in .NET framework

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Once the BarChartControl control has been coded and debugged (by making it public and allowing a containing form to populate the data directly), you can create the data-bound containing control that creates and populates an instance of the BarChartControl for each row of analysis data In this example I created this as a user control to be able to automatically provide a scroll bar to use when more rows of data (BarChartControl instances) are added that can be presented within the control's size Because the control is supposed to support data binding in a fashion similar to other data-bound controls, it will need DataSource and DataMember properties The control presentation's coupling to part of the individual data items in the bound collection (the need to use one of the fields in each data item as a label for the row) requires that the control also needs a property that lets users of the control specify what the field or property name is that contains the row identifier This is similar in concept to the function of the DisplayMember and ValueMember properties on a ComboBox or ListBox control, and I will call the propertyRowIdentifierMember
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The basic skeleton of the class definition is shown in Listing 88
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Listing 88 DataBoundBarChartControl Class
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[ToolboxBitmap(typeof(DataBoundBarChartControl), "IDesignIconbmp")] public partial class DataBoundBarChartControl : UserControl, ISupportInitialize { private object m_DataSource; private string m_DataMember; private string m_RowIdMember; private bool m_Initializing = false; public DataBoundBarChartControl() { InitializeComponent(); } [Editor("SystemWindowsFormsDesignDataMemberListEditor, SystemDesign, Version=2000, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(UITypeEditor))] public string DataMember { get { return m_DataMember; } set { m_DataMember = value; UpdateDataBinding();
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} } [AttributeProvider(typeof(IListSource))] public object DataSource { get { return m_DataSource; } set { m_DataSource = value; UpdateDataBinding(); } } public string RowIdentifierMember { get { return m_RowIdMember; } set { m_RowIdMember = value; UpdateDataBinding(); } } private void UpdateDataBinding() { } public void BeginInit() { m_Initializing = true; } public void EndInit() { m_Initializing = false; UpdateDataBinding(); } }
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The DataBoundBarChartControl class is derived from UserControl and implements the ISupportInitialize interface, which lets it control its data binding when in an initialization phase The ToolboxBitmap attribute is applied to the control class and sets what the icon is that will be used for the control when it is added to the designer's Toolbox The file name specified is the name of a 16 x 16 bitmap that is added to the project and marked as an embedded resource The DataSource and DataMember properties are declared in a fashion similar to that described for the FilteredGrid example, except that in this case you aren't containing a binding source that you can delegate to Instead, this control saves the DataSource and DataMember properties in member variables on the control class that it can use to do the data binding whenever it is appropriate The set portions of these properties also call the UpdateDataBinding method, which is where the actual data-binding code lives (We will get to the implementation of that method shortly) A RowIdentifierMember property is also exposed to identify which property on the data items should be used for rendering the labels next to each bar chart instance
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Initializing and Updating the Data Bindings
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The implementation of the ISupportInitialize, BeginInit, and EndInit methods prevents data binding from occurring during initialization by setting a flag that is checked by the UpdateDataBinding method (m_Initializing), and data binding is deferred until initialization is complete by calling that method explicitly from the EndInit method The implementation of the UpdateDataBinding method requires you to reach into the data source, locate the data collection specified by the data member (if there is one), and iterate through the data items to render the bar charts Although that doesn't sound overly complicated, it can get very involved if you consider that the data source could be a data set, a data table, an array, a custom collection, a list of lists of lists, and so on Covering all those different scenarios can get very complex It basically involves taking the data source and trying to cast it to one of the various interfaces described in 7 Once you discover what kind of a collection it is, based on the interfaces it implements, you can use the interface implementations and the DataMember to determine the actual list of data that you want to bind to Luckily, most of the grunt work of doing this has been exposed through a static method on the ListBindingHelper class called GetList Listing 89 shows the implementation of the UpdateDataBinding method
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