C# 4 and .NET 4

114 ❘ ChaPTer 5 Generics
Let ’ s start this example with a generic document manager. The document manager is used to read and write
documents from a queue. Start by creating a new Console project named DocumentManager and add the
class DocumentManager < T > . The method AddDocument() adds a document to the queue. The read - only
property IsDocumentAvailable returns true if the queue is not empty:
using System;
using System.Collections.Generic;
namespace Wrox.ProCSharp.Generics
{
public class DocumentManager < T >
{
private readonly Queue < T > documentQueue = new Queue < T > ();
public void AddDocument(T doc)
{
lock (this)
{
documentQueue.Enqueue(doc);
}
}
}
}
public bool IsDocumentAvailable
{
get { return documentQueue.Count > 0; }
}
code snippet DocumentManager/DocumentManager.cs
default Values
Now you add a GetDocument() method to the DocumentManager < T > class. Inside this method the type T
should be assigned to null . However, it is not possible to assign null to generic types. The reason is that
a generic type can also be instantiated as a value type, and null is allowed only with reference types. To
circumvent this problem, you can use the default keyword. With the default keyword, null is assigned
to reference types and 0 is assigned to value types:
public T GetDocument()
{
T doc = default(T);
lock (this)
{
doc = documentQueue.Dequeue();
}
return doc;
}
T he default keyword has multiple meanings depending on the context where it is
used. The switch statement uses a default for defi ning the default case, and with
generics default is used to initialize generic types either to null or 0 depending on if
it is a reference or value type.
C ons t raint s
If the generic class needs to invoke some methods from the generic type, you have to add constraints.
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