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void M1() { String name = "Joe"; M2(name); return; }

name (String) s (String) [return address] length (Int32) tally (Int32)

void M2(String s) { Int32 length = s.Length; Int32 tally; return; }

Now, let s start gearing the discussion toward the CLR . Let s say that we have these two class definitions:

internal class Employee { public Int32 public virtual String public static Employee } GetYearsEmployed() GetProgressReport() Lookup(String name) { ... } { ... } { ... }

internal sealed class Manager : Employee { public override String GetProgressReport() }

{ ... }

Our Windows process has started, the CLR is loaded into it, the managed heap is initialized, and a thread has been created (along with its 1 MB of stack space) . This thread has already executed some code, and this code has decided to call the M3 method . All of this is shown in Figure 4-6 . The M3 method contains code that demonstrates how the CLR works; this is not code that you would normally write, because it doesn t actually do anything useful .

void M3() { Employee e; Int32 year; e = new Manager(); e = Employee.Lookup("Joe"); year = e.GetYearsEmployed(); e.GenProgressReport(); }

FIguRE 4-6 The CLR loaded in a process, its heap initialized, and a thread s stack with the M3 method about

As the just-in-time (JIT) compiler converts M3 s Intermediate Language (IL) code into native CPU instructions, it notices all of the types that are referred to inside M3: Employee, Int32, Manager, and String (because of "Joe") . At this time, the CLR ensures that the assemblies that define these types are loaded . Then, using the assembly s metadata, the CLR extracts information about these types and creates some data structures to represent the types themselves . The data structures for the Employee and Manager type objects are shown in Figure 4-7 . Since this thread already executed some code prior to calling M3, let s assume that the Int32 and String type objects have already been created (which is likely because these are commonly used types), and so I won t show them in the figure .

Part II Designing Types Thread Stack Heap 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

void M3() { Employee e; Int32 year; e = new Manager(); e = Employee.Lookup("Joe"); year = e.GetYearsEmployed(); e.GenProgressReport(); }

Let s take a moment to discuss these type objects . As discussed earlier in this chapter, all objects on the heap contain two overhead members: the type object pointer and the sync block index . As you can see, the Employee and Manager type objects have both of these members . When you define a type, you can define static data fields within it . The bytes that back these static data fields are allocated within the type objects themselves . Finally, inside each type object is a method table with one entry per method defined within the type . This is the method table that was discussed in 1, The CLR s Execution Model . Since the Employee type defines three methods (GetYearsEmployed, GetProgressReport, and Lookup), there are three entries in Employee s method table . Since the Manager type defines one method (an override of GetProgressReport), there is just one entry in Manager s method table . Now, after the CLR has ensured that all of the type objects required by the method are created and the code for M3 has been compiled, the CLR allows the thread to execute M3 s native code . When M3 s prologue code executes, memory for the local variables must be allocated from the thread s stack, as shown in Figure 4-8 . By the way, the CLR automatically initializes all local variables to null or 0 (zero) as part of the method s prologue code . However, the C# compiler issues a Use of unassigned local variable error message if you write code that attempts to read from a local variable that you have not explicitly initialized in your source code .

4 Type Fundamentals Thread Stack Heap Manager Type Object e (Employee) year (int32) null = 0 Type object ptr Sync block index Static fields GenProgressReport Employee Type Object Type object ptr Sync block index Static fields GetYearsEmployed GenProgressReport Lookup

void M3() { Employee e; Int32 year; e = new Manager(); e = Employee.Lookup("Joe"); year = e.GetYearsEmployed(); e.GeProgressReport(); }