Question

System.Array serves as the base class for all arrays in the Common Language Runtime (CLR). According to this article:

For each concrete array type, [the] runtime adds three special methods: Get/Set/Address.

and indeed if I disassemble this C# code,

int[,] x = new int[1024,1024];
x[0,0] = 1;
x[1,1] = 2;
x[2,2] = 3;
Console.WriteLine(x[0,0]);
Console.WriteLine(x[1,1]);
Console.WriteLine(x[2,2]);

into CIL I get,

IL_0000:  ldc.i4     0x400
IL_0005:  ldc.i4     0x400
IL_000a:  newobj     instance void int32[0...,0...]::.ctor(int32,
                                                         int32)
IL_000f:  stloc.0
IL_0010:  ldloc.0
IL_0011:  ldc.i4.0
IL_0012:  ldc.i4.0
IL_0013:  ldc.i4.1
IL_0014:  call       instance void int32[0...,0...]::Set(int32,
                                                       int32,
                                                       int32)
IL_0019:  ldloc.0
IL_001a:  ldc.i4.1
IL_001b:  ldc.i4.1
IL_001c:  ldc.i4.2
IL_001d:  call       instance void int32[0...,0...]::Set(int32,
                                                       int32,
                                                       int32)
IL_0022:  ldloc.0
IL_0023:  ldc.i4.2
IL_0024:  ldc.i4.2
IL_0025:  ldc.i4.3
IL_0026:  call       instance void int32[0...,0...]::Set(int32,
                                                       int32,
                                                       int32)
IL_002b:  ldloc.0
IL_002c:  ldc.i4.0
IL_002d:  ldc.i4.0
IL_002e:  call       instance int32 int32[0...,0...]::Get(int32,
                                                        int32)
IL_0033:  call       void [mscorlib]System.Console::WriteLine(int32)
IL_0038:  ldloc.0
IL_0039:  ldc.i4.1
IL_003a:  ldc.i4.1
IL_003b:  call       instance int32 int32[0...,0...]::Get(int32,
                                                        int32)
IL_0040:  call       void [mscorlib]System.Console::WriteLine(int32)
IL_0045:  ldloc.0
IL_0046:  ldc.i4.2
IL_0047:  ldc.i4.2
IL_0048:  call       instance int32 int32[0...,0...]::Get(int32,
                                                        int32)
IL_004d:  call       void [mscorlib]System.Console::WriteLine(int32)

where the calls to the aforementioned Get and Set methods can be clearly seen. It seems the arity of these methods is related to the dimensionality of the array, which is presumably why they are created by the runtime and are not pre-declared. I couldn’t locate any information about these methods on MSDN and their simple names makes them resistant to Googling. I’m writing a compiler for a language which supports multidimensional arrays, so I’d like to find some official documentation about these methods, under what conditions I can expect them to exist and what I can expect their signatures to be.

In particular, I’d like to know whether its possible to get a MethodInfo object for Get or Set for use with Reflection.Emit without having to create an instance of the array with correct type and dimensionality on which to reflect, as is done in the linked example.

Answer 1

Look here, specifically section 14.2 on pages 63-65

http://download.microsoft.com/download/7/3/3/733AD403-90B2-4064-A81E-01035A7FE13C/MS%20Partition%20II.pdf

But the takeaway, and you can tell from the IL, is that they’re the getter and setter methods for dealing with arrays at the given index positions.

• A Get method that takes a sequence of
int32 arguments, one for each
dimension of the array, and returns a
value whose type is the element type
of the array. This method is used to
access a specific element of the array
where the arguments specify the index
into each dimension, beginning with
the first, of the element to be
returned.

• A Set method that takes a sequence
of int32 arguments, one for each
dimension of the array, followed by a
value whose type is the element type
of the array. The return type of Set
is void. This method is used to set a
specific element of the array where
the arguments specify the index into
each dimension, beginning with the
first, of the element to be set and
the final argument specifies the value
to be stored into the target element.

• An Address method that takes a
sequence of int32 arguments, one for
each dimension of the array, and has a
return type that is a managed pointer
to the array’s element type. This
method is used to return a managed
pointer to a specific element of the
array where the arguments specify the
index into each dimension, beginning
with the first, of the element whose
address is to be returned.

Edit: That’s pages 63-65 using the document’s page numbering. 73-75 in the actual PDF.