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6 Type and Member Basics
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So now, let s put this together to see how C# uses these different IL instructions:
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using System; public sealed class Program { public static void Main() { Console.WriteLine(); // Call a static method Object o = new Object(); o.GetHashCode(); // Call a virtual instance method o.GetType(); // Call a nonvirtual instance method } }
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If you were to compile the code above and look at the resulting IL, you d see the following:
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.method public hidebysig static void Main() cil managed { .entrypoint // Code size 26 (0x1a) .maxstack 1 .locals init (object V_0) IL_0000: call void System.Console::WriteLine() IL_0005: newobj instance void System.Object::.ctor() IL_000a: stloc.0 IL_000b: ldloc.0 IL_000c: callvirt instance int32 System.Object::GetHashCode() IL_0011: pop IL_0012: ldloc.0 IL_0013: callvirt instance class System.Type System.Object::GetType() IL_0018: pop IL_0019: ret } // end of method Program::Main
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Notice that the C# compiler uses the call IL instruction to call Console s WriteLine method . This is expected because WriteLine is a static method . Next, notice that the callvirt IL instruction is used to call GetHashCode. This is also expected, since GetHashCode is a virtual method . Finally, notice that the C# compiler also uses the callvirt IL instruction to call the GetType method . This is surprising since GetType is not a virtual method . However, this works because while JIT-compiling this code, the CLR will know that GetType is not a virtual method, and so the JIT-compiled code will simply call GetType nonvirtually . Of course, the question is, why didn t the C# compiler simply emit the call instruction instead The answer is because the C# team decided that the JIT compiler should generate code to verify that the object being used to make the call is not null. This means that calls to nonvirtual instance methods are a little slower than they could be . It also means that the C# code shown below will cause a NullReferenceException to be thrown . In some other programming languages, the intention of the code shown below would run just fine:
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Part II Designing Types
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using System; public sealed class Program { public Int32 GetFive() { return 5; } public static void Main() { Program p = null; Int32 x = p.GetFive(); // In C#, NullReferenceException is thrown } }
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Theoretically, the code above is fine . Sure, the variable p is null, but when calling a nonvirtual method (GetFive), the CLR needs to know just the data type of p, which is Program. If GetFive did get called, the value of the this argument would be null. Since the argument is not used inside the GetFive method, no NullReferenceException would be thrown . However, because the C# compiler emits a callvirt instruction instead of a call instruction, the code above will end up throwing the NullReferenceException. Important If you define a method as nonvirtual, you should never change the method to
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virtual in the future . The reason is because some compilers will call the nonvirtual method by using the call instruction instead of the callvirt instruction . If the method changes from nonvirtual to virtual and the referencing code is not recompiled, the virtual method will be called nonvirtually, causing the application to produce unpredictable behavior . If the referencing code is written in C#, this is not a problem, since C# calls all instance methods by using callvirt. But this could be a problem if the referencing code was written using a different programming language .
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Sometimes, the compiler will use a call instruction to call a virtual method instead of using a callvirt instruction . At first, this may seem surprising, but the code below demonstrates why it is sometimes required:
internal class SomeClass { // ToString is a virtual method defined in the base class: Object. public override String ToString() { // Compiler uses the call IL instruction to call // Object s ToString method nonvirtually. // If the compiler were to use callvirt instead of call , this // method would call itself recursively until the stack overflowed. return base.ToString(); } }
When calling base.ToString (a virtual method), the C# compiler emits a call instruction to ensure that the ToString method in the base type is called nonvirtually . This is required because if ToString were called virtually, the call would execute recursively until the thread s stack overflowed, which obviously is not desired .
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