Assign a dynamic array type to a TArray<T> variable

Question

I'm trying to assign a dynamic array type to a TArray<string> variable

type
  TMyStringArray = array of string;

function Test() : TMyStringArray;
begin
  ...
end;

...

var
  MyArray : TArray<string>;
begin
  MyArray := Test();
end;

On compiling, Delphi says:

[dcc32 Error] Unit1.pas(39): E2010 Incompatible types: 'System.TArray' and 'TMyStringArray'

This issue is covered in the documentation in the "Array Types and Assignments" section: "Arrays are assignment-compatible only if they are of the same type." TMyStringArray and TArray<string> are NOT the same type. — Remy Lebeau
– Remy Lebeau, Commented Feb 21, 2020 at 9:30
Just for the sake of completeness, it should be pointed out already here that TArray<string> is a dynamic array of strings; it's defined as array of string. — Andreas Rejbrand
– Andreas Rejbrand, Commented Feb 21, 2020 at 10:15

Fabrizio · Accepted Answer · 2020-02-21 08:11:54Z

2

I did it simply by using a type cast and it seems to work.

I would be glad to know I can fall into some problems doing this way

type
  TMyStringArray = array of string;

function Test() : TMyStringArray;
begin
  SetLength(Result, 2);
  Result[0] := 'Hello';
  Result[1] := 'World';
end;

procedure TForm1.FormCreate(Sender: TObject);
var
  MyArray : TArray<string>;
  i : integer;
begin
  MyArray := TArray<string>(Test());

  i := 0;
  while(i < Length(MyArray)) do
  begin
    ShowMessage(MyArray[i]);
    Inc(i);
  end;
end;

answered Feb 21, 2020 at 8:11

Fabrizio

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12 Comments

Andreas Rejbrand Over a year ago

Yes, this is correct. TArray<string> is defined as array of string, so it is a dynamic array of strings, just like array of string (obviously). So it is undesirable and strange that the compiler refuses to consider these types compatible without the cast.

Fabrizio Over a year ago

@AndreasRejbrand: I'm using Delphi XE7, does the newer versions accept the assignment without the type cast?

Andreas Rejbrand Over a year ago

I actually thought that was fixed in 10.3, but now I tried it, and it seems not. I still need to cast. (I know something changed very recently related to this, but I don't quite recall what it was.) But this is how the type system works. Try, for instance, to define type TDynStringArray = array of string; TDynStringArray2 = array of string;. They will not be assignment compatible without a cast...

Andreas Rejbrand Over a year ago

The best solution these days, I think, it to always use TArray<string> as the dynamic string array type. (It is better than array of string, for instance, because it can be used as a function return type. And it is better than any user-defined type defined as array of string because it is part of the core RTL, so all units agree on it.)

Remy Lebeau Over a year ago

@Fabrizio "does the newer versions accept the assignment without the type cast?" - in your example, no.

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