C# Array Indexer

Question

I was wondering: how is the C# array indexer implemented? You can index a C# array with basically every integer value from ulong to sbyte, does the internal implementation simply casts to a common type every time?

To be clear will this:

ulong i = 10;
var o = myArray[i];

be translated into something like:

ulong i =10;
var o = myArray[(int /*or whatver is the default type used*/)i];

?

Answer 1

The type of an array indexer is integer, so yes, the value will be converted to int. You can verify this by examining the IL code. Given this example:

var myArray = new[]{ 1,2,3 };
ulong i = 10;
var o = myArray[i];

This will compile into:

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       29 (0x1d)
  .maxstack  3
  .locals init ([0] int32[] myArray,
           [1] uint64 i,
           [2] int32 o)
  IL_0000:  nop
  IL_0001:  ldc.i4.3
  IL_0002:  newarr     [mscorlib]System.Int32
  IL_0007:  dup
  IL_0008:  ldtoken    field valuetype '<PrivateImplementationDetails>'/'__StaticArrayInitTypeSize=12' '<PrivateImplementationDetails>'::E429CCA3F703A39CC5954A6572FEC9086135B34E
  IL_000d:  call       void [mscorlib]System.Runtime.CompilerServices.RuntimeHelpers::InitializeArray(class [mscorlib]System.Array,
                                                                                                      valuetype [mscorlib]System.RuntimeFieldHandle)
  IL_0012:  stloc.0
  IL_0013:  ldc.i4.s   10
  IL_0015:  conv.i8
  IL_0016:  stloc.1
  IL_0017:  ldloc.0
  IL_0018:  ldloc.1
  IL_0019:  conv.ovf.i.un
  IL_001a:  ldelem.i4
  IL_001b:  stloc.2
  IL_001c:  ret
} // end of method Program::Main

The conversion happens at IL_0019 with conv.ovf.i.un instruction.

Converts the unsigned value on top of the evaluation stack to signed native int, throwing OverflowException on overflow.