Common Type System—Runtime Type Checking

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Common Type System Type Systems Intro § Types and Objects § Type Unification § Reference, Value Types § Accessibility, Visibility § Type Members § Fields § Methods § Locals Overloading Argument Passing Style Pass-by-Value and Pass-by-Reference Output Parameters Managed Pointers Variable Argument Methods Methods, Subclassing Virtuals, Overriding New Slots Exception Handlers Constructors, Instantiation § Default Constructors Value Construction Constructor Chaining Field Initialization Type Initializers Unhandled Exceptions Properties § Indexers Mixed Mode Accessibility Events Operator Overloading Coercion Subclassing, Polymorphism Styles of Inheritance § Implementation Inheritance Interface Inheritance Abstract Classes Interfaces § Method Dispatch Multiple Inheritance Private Interface Inheritance Private Inheritance Accessibility Interface Reimplementation Abstract and Interface? Sealing Types and Methods Runtime Type Checking Namespaces: Organizing Types § Define a Namespace Namespace Resolution Special Types § Delegates § CTS Support Co- and Contravariance Anonymous Delegates Custom Attributes Enumerations § Flags-Style Limitations Helper Methods Generics § Further Reading §

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Runtime Type Checking

The CLR offers a variety of ways to perform dynamic runtime type checks that look for polymorphic compatibility between an instance and a type. Given an object o and a type T, you can use the castclass and isint IL instructions to check whether o is of type T or whether its type implements T if it’s an interface or if its type is a subtype of T, in which case it is safe to treat it as a T. The C# language surfaces these instructions with casting and its is and as keywords:

object o = /*...*/;
string s1 = (string)o; // Casting uses ‘castclass’
string s2 = o as string; // ‘as’ uses ‘isinst’
if (o is string) { // ‘is’ also uses ‘isinst’
   // ...
}

In this example, the cast uses the castclass instruction to dynamically check if the object instance o is of type System.String. If it’s not, a CastClassException will be thrown by the runtime. Both the as and is keywords use isinst. This instruction is much like castclass, but it won’t generate an exception; it leaves behind a 0 if the type doesn’t pass the type check. In the case of as, this means a null will result if the instance isn’t the correct type; in the case of is, this same condition results in a false.