Only methods declared virtual, override or abstract can be overridden in derived classes. A method declared sealed in a base class cannot be overridden in a derived class. Methods that are declared private are implicitly sealed, because it’s impossible to override them in a derived class (though the derived class can declare a new method with the same signature as the private method in the base class). Methods that are declared static also are implicitly sealed, because static methods cannot be overridden either. A derived-class method declared both override and sealed can override a base-class method, but cannot be overridden in derived classes further down the inheritance hierarchy.

A sealed method’s declaration can never change, so all derived classes use the same method implementation, and calls to sealed methods (and non-virtual methods) are resolved at compile time—this is known as static binding. Since the compiler knows that sealed methods cannot be overridden, it can often optimize code by removing calls to sealed methods and replacing them with the expanded code of their declarations at each method-call location—a technique known as inlining the code.

A class that’s declared sealed cannot be a base class (i.e., a class cannot extend a sealed class). All methods in a sealed class are implicitly sealed. Class string is a sealed class. This class cannot be extended, so apps that use strings can rely on the functionality of string objects as specified in the Framework Class Library.