The ISupportInitialize Interface: Supporting Designer Initialization in Visual Studio .NET

Generate Code 128 in Visual Studio .NET The ISupportInitialize Interface: Supporting Designer Initialization
The ISupportInitialize Interface: Supporting Designer Initialization
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The ISupportInitialize interface lets controls defer acting on values set on interdependent properties until the container tells the control that all values have been set This avoids having a control try to take actions based on a property's setting if those actions might fail if another property needs to be set first, and those properties can be set in any order The Windows Forms designer uses this interface, so the code it generates to set properties doesn't need any insight into the correct order to set interdependent properties Sometimes components have interdependent properties that all need to be logically set at the same time for things to work correctly But because only one line of code can execute at a time, supporting this notion of having multiple properties set simultaneously becomes a problem For example, if you specify a DataMember property for aBindingSource component or aDataGridView control, that property provides information about what part of the object that you set as the DataSource property should be used for data binding Any change to the DataMember property necessitates refreshing the data bindings However, the DataMember property doesn't understand this unless the DataSource has been set first You can't be sure that they will be set in the right order, with DataSource first and then DataMember second Also, what if you want to take a component or control that was already bound to some other data source and change it to a new data source You might change the DataSource property first, or you might change theDataMember property first When the designer writes code for you based on interactions in the designer such as selections in Smart Tags or setting properties in the Properties window, there is no way to be sure what order the code will be written to initialize those properties So there needs to be a way to signal a control or component to tell it that you will be entering a period of initialization, and then notify it again when you are done with that period of initialization If you can do that, then the component can defer enforcing any interdependencies or using the values of any of the properties until you signal it that initialization is complete This is precisely what the ISupportInitialize interface is designed for ISupportInitialize defines two methods: BeginInit and EndInit Neither takes any parameters or returns anything; they are just signal methods to the implementing class of when initialization is starting and when it is complete from some consuming code's perspective The Visual Studio designer is aware of this interface and looks for it on any component or control that you drag and drop onto a designer surface If it sees that something you added through the designer implements this interface, the designer adds calls to BeginInit and EndInit that bracket the setting of any properties for that component in the designer-generated code Doing so ensures that the order that properties are set by the designer is not important, just that it properly signals when it is starting to set properties, and when it is done setting them The following code shows a trimmed down version of the InitializeComponent method from the designer code file for a form in a Windows Forms application private void InitializeComponent() { // Code to create component instances omitted // BeginInit calls ((SystemComponentModelISupportInitialize) (thisbindingSource1))BeginInit(); ((SystemComponentModelISupportInitialize) (thisnorthwindDataSet1))BeginInit(); // Property initializationorder not important thisbindingSource1DataMember = "Customers"; thisbindingSource1DataSource = thisnorthwindDataSet1; thisnorthwindDataSet1DataSetName = "NorthwindDataSet"; // EndInit calls ((SystemComponentModelISupportInitialize) (thisbindingSource1))EndInit(); ((SystemComponentModelISupportInitialize) (thisnorthwindDataSet1))EndInit(); }
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Notice that it calls BeginInit on each component at the beginning of the initialization phase (after casting the component variable to the ISupportInitialize reference type), then sets properties, and then callsEndInit Doing this allows the control or component to internally defer acting on the properties that are being set until EndInit is called, which avoids the challenges of setting interdependent properties in the correct order Listing 72 shows a simple implementation of ISupportInitialize on a class that contains a string collection For demonstration purposes, the class is designed to support initialization by caching any values that are set on the StringCollection property until initialization is complete To support this, the class does a number of things
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It implements the ISupportInitialize interface and its methods BeginInit and EndInit Member variables are defined to hold the primary string collection that the class encapsulates, as well as a flag to indicate when the class is being initialized, and another string collection to hold onto a temporary copy of a value that is being set for the string collection during initializing The StringCollection property sets block checks to see if the class instance is being initialized through the flag, and if so, places any values set for that property into a temporary copy If the class isn't initializing, then it just writes the value into the primary string collection member variable The implementation of BeginInit sets the flag to indicate to the rest of the class that it is in initialization mode The EndInit method copies the reference to the last value set forStringCollection from the temporary variable into the primary string collection variable and resets the flag
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You will see a real-world implementation of ISupportInitialize in 8 for a custom data-bound control
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