C# 4 and .NET 4

764 ❘ ChaPTer 28 mAnAGed extensibility frAmewOrk
}
}
}
ImportsSatisfied(this, new ImportEventArgs {
StatusMessage = "ICalculator import successful" });
code snippet WPFCalculator/CalculatorImport.cs
The event of the CalculatorImport is connected to an event handler on creation of the CalculatorImport
to write a message to a TextBlock for displaying status information:
private void InitializeContainer()
{
var catalog = new DirectoryCatalog(
Properties.Settings.Default.AddInDirectory);
container = new CompositionContainer(catalog);
calcImport = new CalculatorImport();
calcImport.ImportsSatisfied += (sender, e) =>
{
textStatus.Text += String.Format("{0}\n", e.StatusMessage);
};
container.ComposeParts(calcImport);
}
InitializeOperations();
code snippet WPFCalculator/MainWindow.xaml.cs
lazy loading of Parts
By default, parts are loaded from the container, for example, by calling the extension method
ComposeParts() on the CompositionContainer. With the help of the Lazy class, the parts can
be loaded on first access. The type Lazy allows the late instantiation of any type T and defines the
properties IsValueCreated and Value. IsValueCreated is a Boolean that returns the information if the
contained type T is already instantiated. Value initializes the contained type T on first access and returns
the instance.
The import of an add-in can be declared to be of type Lazy, as shown in the Lazy
example.
[Import(typeof(ICalculator))]
public Lazy Calculator { get; set; }
code snippet WPFCalculator/CalculatorImport.cs
Calling the imported property also requires some changes to access the Value property of the Lazy
type. calcImport is a variable of type CalculatorImport. The Calculator property returns
Lazy. The Value property instantiates the imported type lazily and returns the
ICalculator interface, where now the GetOperations() method can be invoked to get all supported
operations from the calculator add-in.
private void InitializeOperations()
{
Contract.Requires(calcImport != null);
Contract.Requires(calcImport.Calculator != null);
var operators = calcImport.Calculator.Value.GetOperations();
foreach (var op in operators)
{
var b = new Button();
b.Width = 40;
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