C# 4 and .NET 4

exports ❘ 761
}
}
Thread.Sleep(3000);
return x + y;
code snippet Operations/Operation2.cs
Because there is more than one exported Add method available, the import definition must be changed. The
attribute ImportMany is used if more than one export of the same name and type is available. This attribute
is applied to an array or IEnumeration interface. ImportMany is explained with more detail in the
next section. For accessing metadata, an array of Lazy can be used. The class Lazy
is new in .NET 4 and is used to support lazy initialization of types on first use. Lazy
derives from Lazy and supports, in addition to the base class, access to metadata information with
the Metadata property. In the sample, the method is referenced by the delegate Func, which is the first generic parameter of Lazy. The second generic parameter is
IDictionary for the metadata collection. The ExportMetadata attribute can be used
multiple times to add more than one capability, and always consists of a key of type string and a value
of type object.
[ImportMany("Add", typeof(Func))]
public Lazy[]
AddMethods { get; set; }
//[Import("Add", typeof(Func))]
//public Func Add { get; set; }
code snippet SimpleCalculator/Calculator.cs
The call to the Add method is now changed to iterate through the collection of Lazy elements. With the Metadata property, the key for
the capability is checked; if the speed capability has the value fast, the operation is invoked by using the
Value property of Lazy to get to the delegate:
case "+":
// result = Add(operands[0], operands[1]);
foreach (var addMethod in AddMethods)
{
if (addMethod.Metadata.ContainsKey("speed") &&
(string)addMethod.Metadata["speed"] == "fast")
result = addMethod.Value(operands[0], operands[1]);
}
// result = operands[0] + operands[1];
break;
Instead of using the attribute ExportMetadata, you can create a custom export attribute class that derives
from ExportAttribute. The class SpeedExportAttribute defines an additional Speed property that is of
type Speed:
using System;
using System.ComponentModel.Composition;
namespace Wrox.ProCSharp.MEF
{
public enum Speed
{
Fast,
Slow
}
[MetadataAttribute]
[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class)]
public class SpeedExportAttribute : ExportAttribute
www.it-ebooks.info