Types and Base Types

Non-interface types that do not specify a base type get System.Object as their base type.  Declare a type as sealed prohibits the use of the type as a base type. Declareing a type as abstract, on the other hand, disallows direct instantiation of the type and makes it useful only as a base type.

The policy for dealing with collisions between reused method names and reused field names might be different. The CLR supports two basic policies to use when the base and derived types have a method of the same name: hide-by-name and hide-by-signature. When one declares a method using hide-by-signature, only the base method with the same name and the same signature will be hidden. In contrast, when one declares a method using hide-by-name, the derived method hides all methods in the base type that have the same name, no matter what their signature may be. For method name, in C#, always use hide-by-signature. Field name always use hide-by-name.

When the CLR allocates a new object, it calls the constructor method from the most-derived type. If a base type constructor causes a virtual method call to be invoked, the most-derived type’s method will be dispatched even though the derived type’s constructor has probably not completed execution. To avoid this problem, you are strongly encouraged to avoid virtual method calls in a constructor of a non-sealed type.

In the face of instance field declarations with initialize expressions, the compiler-generated .ctor will first call all field initializers in order of declaration. Once the derived type’s field initializers have been called, the derived constructor calls the base type constructor, using the programmer-provided parameters if the base constructor was used. Once the base type’s constructor has completed execution, the derived constructor resumes execution at the body of the constructor. This means that when the base type’s constructor executes, the derived type’s constructor body has not even begun to execute.