What's the magic of arrays in C#

Question

int[] a = new int[5]; string[] b = new string[1]; 

The types of both a and b inherit from the abstract System.Array, but there is no real classes in the built-in library(it seems that there are some runtime types, you can't find the type defination class of an int[]). Can you tell me what happens while compiling? And why did they(the c# team) make this design(I mean why it's not something like Array<T>,instead they are using an abstract class with compiler magics)?

Answer 1

Trying to reason this out within the .NET type system doesn't get you very far. There is core support built into the JIT compiler and the CLR to deal with creating arrays. A statement like this:

        var arr = new int[5]; 

Generates this IL:

  IL_0001:  ldc.i4.5   IL_0002:  newarr     [mscorlib]System.Int32 

Which the JIT compiler then translate into this machine code:

00000035  mov         edx,5                 ; arg2 = array size 0000003a  mov         ecx,6F535F06h         ; arg1 = typeof(int) 0000003f  call        FFD52128              ; call JIT_NewArr1(type, size) 

Core ingredients here are the dedicated IL opcode, newarr, instead of the usual newobj opcode that creates an instance of a class. And the simple translation to a CLR helper function that actually gets the object created. You can have a look-see at this helper function with the SSCLI20 source code, clr\src\vm\jithelpers.cpp. Too large to post here, but it is heavily optimized to make this kind of code run as fast possible, having direct access to the type internals available to CLR code.

There are two of these helpers available, JIT_NewArr1() creates one-dimensional (vector) arrays and JIT_NewMDArr() creates multi-dimensional arrays. Compare to the two overloads available for Type.MakeArrayType().