发现个delphi调用vc写的Dll中包含pchar参数报错奇怪现象

unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;

type
TForm1 = class(TForm)
btn1: TButton;
procedure btn1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;

var
Form1: TForm1;

implementation

{$R *.dfm}

function GetStr(s:PChar):PChar; stdcall; external 'MyDLL.dll';
procedure TForm1.btn1Click(Sender: TObject);
var
p  :PChar;
s:string;
begin
s:='123'; //procedure中的第一行语句不能直接调用GetStr,否则会运行报错,加上这行语句不报错了奇怪
p:= GetStr('63024823'); //接收必须是PChar,如果String可以编译通过,但运行报错
ShowMessage(p);
end;

end.