Why accessing out-of-bounds indices of dynamic array doesn't raise AV?

Question

In the VCL, TByteDynArray is defined as a dynamic array:

type TByteDynArray = array of Byte;

But it seems that no index bounds checking is done:

var
  DataBytes: System.Types.TByteDynArray;
  i: Integer;
begin
  SetLength(DataBytes, 2);
  DataBytes[5] := 222; // Accessing index beyond set length.
  i := DataBytes[5]; // `i` is now set to "222".

The code above runs without errors.

Why isn't an AccessViolation raised, like with a static array? What is the point of SetLength if you can access and modify 65536 bytes of memory of the array variable regardless of the length set?

Answer 1

To detect array index out of bounds errors, set range check error on.

Range Checking

The $R directive enables or disables the generation of range-checking code. In the {$R+} state, all array and string-indexing expressions are verified as being within the defined bounds, and all assignments to scalar and subrange variables are checked to be within range. If a range check fails, an ERangeError exception is raised (or the program is terminated if exception handling is not enabled).

This is default set to {$R-}, but I suggest to set it on, at least through the development phase. It adds an extra overhead to the code, so this may be the reason it is off by default.

---

If you have a unit that is well tested and want to avoid range checking, add {$R-} at the top of the unit. This will override the project setting locally.

If you want to avoid range checking in a code block, this technique could be used:

{$IFOPT R+}
  {$DEFINE RestoreRangeCheck}
  {$R-}
{$ENDIF}

{- Some code here }

{$IFDEF RestoreRangeCheck}
  {$R+}
  {$UNDEF RestoreRangeCheck}
{$ENDIF}