C# 4 and .NET 4

Generics features ❘ 115
With DocumentManager, all the titles of the documents should be displayed in the
DisplayAllDocuments() method. The Document class implements the interface IDocument with the
properties Title and Content:
public interface IDocument
{
string Title { get; set; }
string Content { get; set; }
}
public class Document: IDocument
{
public Document()
{
}
public Document(string title, string content)
{
this.Title = title;
this.Content = content;
}
}
public string Title { get; set; }
public string Content { get; set; }
code snippet DocumentManager/Document.cs
For displaying the documents with the DocumentManager class, you can cast the type T to the interface
IDocument to display the title:
public void DisplayAllDocuments()
{
foreach (T doc in documentQueue)
{
Console.WriteLine(((IDocument)doc).Title);
}
}
code snippet DocumentManager/DocumentManager.cs
The problem is that doing a cast results in a runtime exception if type T does not implement the interface
IDocument. Instead, it would be better to define a constraint with the DocumentManager class
that the type TDocument must implement the interface IDocument. To clarify the requirement in the name
of the generic type, T is changed to TDocument. The where clause defines the requirement to implement the
interface IDocument:
public class DocumentManager
where TDocument: IDocument
{
This way you can write the foreach statement in such a way that the type TDocument contains the property
Title. You get support from Visual Studio IntelliSense and from the compiler:
public void DisplayAllDocuments()
{
foreach (TDocument doc in documentQueue)
{
Console.WriteLine(doc.Title);
}
}
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