The System.Object Class

One of the most important reference types in the .NET Framework is the Object class in the System namespace. To fully appreciate the significance of the System.Object class requires that you understand inheritance, which is described in Chapter 12, “Working with Inheritance.” For the time being, simply accept that all classes are specialized types of System.Object, and that you can use System.Object to create a variable that can refer to any reference type. System.Object is such an important class that C# provides the object keyword as an alias for System.Object. In your code, you can use object or you can write System.Object; they mean exactly the same thing.

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Use the object keyword in preference to System.Object. It's more direct and it's also consistent with using other keywords that have longer synonyms, as you'll discover in Chapter 9, “Creating Value Types with Enumerations and Structs.” If your program contains a using System; directive, you could also write Object with a capital O (without the System. prefix), but it's best to consistently use the simpler keyword object.

In the following example, the variables c and o both refer to the same Circle object. The fact that the type of c is Circle and the type of o is object (the alias for System.Object) in effect provides two different views of the same object:

Circle c; 
c = new Circle(42); 
 
object o; o = c;