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The sealed Keyword

C# supplies another keyword, sealed, that prevents inheritance from occurring. When you mark a

class as sealed, the compiler will not allow you to derive from this type. For example, assume you

have decided that it makes no sense to further extend the MiniVan class:

// This class cannot be extended!

sealed class MiniVan : Car

{

}

If you (or a teammate) were to attempt to derive from this class, you would receive a compile-

time error:

// Error! Cannot extend

// a class marked with the sealed keyword!

class DeluxeMiniVan

: MiniVan

{}

Most often, sealing a class makes the best sense when you are designing a utility class. For

example, the System namespace defines numerous sealed classes. You can verify this for yourself

by opening up the Visual Studio 2008 Object Browser (via the View menu) and selecting the

System.String type defined within the mscorlib.dll assembly. Notice in Figure 6-1 the use of the

sealed keyword highlighted in the Summary window.

Figure 6-1.

The base class libraries define numerous sealed types.

Thus, just like the MiniVan, if you attempted to build a new class that extends System.String,

you will receive a compile-time error: