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I found the "optional parameters" feature in C# 4.0 very interesting, so I tried to figure out how they made it happen. so I wrote a method like this: private static void A(int a = 5) { } Compiled it, then decompiled it in IL DASM, this is the IL code: .method private hidebysig static void A([opt] int32 a) cil managed { .param [1] = int32(0x00000005) // Code size 2 (0x2) .maxstack 8 IL_0000: nop IL_0001: ret } // end of method Program::A And it has got this in its metadata: (1) ParamToken : (08000002) Name : a flags: [Optional] [HasDefault] (00001010) Default: (I4) 5 So I followed the clue and wrote a method like this: private static void B([Optional, DefaultParameterValue(78)]int b) { } Compiled it and decompiled it, and I found that the C# compiler generated almost the identical MSIL code for method A and B(except for the name). As we can see there is no sign of attributes in the IL code and it felt wrong, so I wrote a custom attribute like this: [AttributeUsage(AttributeTargets.Parameter)] public class MyTestAttribute : Attribute { } Then used it in method C like this: private static void C([MyTest]int c) { } Compiled it and then decompiled it, and hah, I found this: .method private hidebysig static void C(int32 c) cil managed { .param [1] .custom instance void ConsoleApplication1.MyTestAttribute::.ctor() = ( 01 00 00 00 ) // Code size 2 (0x2) .maxstack 8 IL_0000: nop IL_0001: ret } // end of method Program::C The second line of the method body calls to the ctor of my custom attribute. So this leads to my doubts: What does [opt] mean? I mean the one that appears in front of method A and B's parameter. How come method C calls the constructor of the Attribute that

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