C# 4 and .NET 4

C a t a l o g s ❘ 767
To remove exports dynamically, the Unchecked event is set in the CheckBox control that ’ s within
the MenuItem header. The ICalculatorExtension can be accessed from the Tag property of the
MenuItem control because it was set in the previous code snippet. To remove a part from the container,
a CompositeBatch object must be created that contains parts for removal. The RemovePart() method of
the CompositeBatch class requires a ComposablePart object; there ’ s no overload for attributed objects.
With the help of the class AttributedModelServices from an attributed object, a ComposablePart can be
created with the CreatePart() method. The CompositonBatch is then passed to the Compose() method of
the CompositionContainer to remove the part from it:
menuCheck.Unchecked += (sender1, e1) = >
{
MenuItem mi = (sender1 as CheckBox).Tag as MenuItem;
ICalculatorExtension ext = mi.Tag as ICalculatorExtension;
ComposablePart part =
AttributedModelServices.CreatePart(ext);
var batch = new CompositionBatch();
batch.RemovePart(part);
container.Compose(batch);
MenuItem parentMenu = mi.Parent as MenuItem;
parentMenu.Items.Remove(mi);
};
Removing parts is only possible if the AllowRecomposition property of the Import
attribute is set to true.
With an export provider, you can get information on exports added and removed by implementing a handler
to the ExportsChanged event. The parameter e of type ExportsChangedEventArgs contains a list of added
exports and removed exports that are written to a TextBlock control:
var container = new CompositionContainer(catalog);
container.ExportsChanged += (sender, e) = >
{
var sb = new StringBuilder();
};
foreach (var item in e.AddedExports)
{
sb.AppendFormat("added export {0}\n", item.ContractName);
}
foreach (var item in e.RemovedExports)
{
sb.AppendFormat("removed export {0}\n", item.ContractName);
}
this.textStatus.Text += sb.ToString();
CaTalogs
A catalog defi nes where MEF searches for requested parts. The sample application uses a DirectoryCatalog
to load the assemblies with parts from a specifi ed directory. With the DirectoryCatalog , you can get
change information with the Changed event and iterate through all added and removed defi nitions. The
DirectoryCatalog does not itself register to fi le system changes. Instead, you need to invoke the Refresh()
method of the DirectoryCatalog , and if changes were made since the last read, the Changing and Changed
events are fi red.
private void InitializeContainer()
{
catalog = new DirectoryCatalog(Properties.Settings.Default.AddInDirectory);
container = new CompositionContainer(catalog);
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