//我是这样测试的。
function GetSubStr(Str:array of char;BeginPos,Len:Integer):String;
begin
Result:=Copy(Str,Beginpos,Len);
end;
procedure TForm1.Button1Click(Sender: TObject);
var
Buffer:array[0..100] of Char;
begin
Buffer:='1a2s3dfasdfasdfasdfasdfasdfasdf';
ShowMessage(GetSubStr(Buffer,0,20));
end;