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FIguRE 4-10 Employee s static Lookup method allocates and initializes a Manager object for Joe
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Thread Stack Heap Manager Object e (Employee) year (int32) = 5 Type object ptr Sync block index Instance fields Manager Object Type object ptr Sync block index Instance fields
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void M3() { Employee e; Int32 year; e = new Manager(); e = Employee.Lookup("Joe"); year = e.GetYearsEmployed(); e.GenProgressReport(); }
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Manager Type Object Type object ptr Sync block index Static fields GenProgressReport Employee Type Object Type object ptr Sync block index Static fields GetYearsEmployed GenProgressReport Lookup
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FIguRE 4-11 Employee s nonvirtual instance GetYearsEmployed method is called, returning 5
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The next line of code in M3 calls Employee s virtual instance GetProgressReport method . When calling a virtual instance method, the JIT compiler produces some additional code in the method, which will be executed each time the method is invoked . This code will first look in the variable being used to make the call and then follow the address to the calling object . In this case, the variable e points to the Manager object representing Joe . Then, the code will examine the object s internal type object pointer member; this member refers to the actual type of the object . The code then locates the entry in the type object s method table that refers to the method being called, JITs the method (if necessary), and calls the JITted code . For our discussion, Manager s GetProgressReport implementation is called because e refers to a Manager object . The result of this operation is shown in Figure 4-12 . Note that if Employee s Lookup method had discovered that Joe was just an Employee and not a Manager, Lookup would have internally constructed an Employee object whose type object pointer member would have referred to the Employee type object, causing Employee s implementation of GetProgressReport to execute instead of Manager s implementation .
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Thread Stack Heap Manager Object e (Employee) year (int32) = 5 Type object ptr Sync block index Instance fields Manager Object Type object ptr Sync block index Instance fields
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void M3() { Employee e; Int32 year; e = new Manager(); e = Employee.Lookup("Joe"); year = e.GetYearsEmployed(); e.GenProgressReport(); }
Manager Type Object Type object ptr Sync block index Static fields GenProgressReport Employee Type Object Type object ptr Sync block index Static fields GetYearsEmployed GenProgressReport Lookup
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FIguRE 4-12 Employee s virtual instance GetProgressReport method is called, causing Manager s over-
ride of this method to execute
At this point, we have discussed the relationship between source code, IL, and JITted code . We have also discussed the thread s stack, arguments, local variables, and how these arguments and variables refer to objects on the managed heap . You also see how objects contain a pointer to their type object (containing the static fields and method table) . We have also discussed how the JIT compiler determines how to call static methods, nonvirtual instance methods, and virtual instance methods . All of this should give you great insight into how the
4 Type Fundamentals
CLR works, and this insight should help you when architecting and implementing your types, components, and applications . Before ending this chapter, I d like to give you just a little more insight as to what is going on inside the CLR . You ll notice that the Employee and Manager type objects both contain type object pointer members . This is because type objects are actually objects themselves . When the CLR creates type objects, the CLR must initialize these members . To what you might ask . Well, when the CLR starts running in a process, it immediately creates a special type object for the System.Type type (defined in MSCorLib .dll) . The Employee and Manager type objects are instances of this type, and therefore, their type object pointer members are initialized to refer to the System.Type type object, as shown in Figure 4-13 . Of course, the System.Type type object is an object itself and therefore also has a type object pointer member in it, and it is logical to ask what this member refers to . It refers to itself because the System.Type type object is itself an instance of a type object . And now you should understand the CLR s complete type system and how it works . By the way, System.Object s GetType method simply returns the address stored in the specified object s type object pointer member . In other words, the GetType method returns a pointer to an object s type object, and this is how you can determine the true type of any object in the system (including type objects) .
Thread Stack Heap Manager Object e (Employee) year (int32) = 5 Type object ptr Sync block index Instance fields Manager Object Type object ptr Sync block index Instance fields
void M3() { Employee e; Int32 year; e = new Manager(); e = Employee.Lookup("Joe"); year = e.GetYearsEmployed(); e.GenProgressReport(); }
Manager Type Object Type object ptr Sync block index Static fields GenProgressReport Employee Type Object Type object ptr Sync block index Static fields GetYearsEmployed GenProgressReport Lookup
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