Delphi How to avoid Access violation when creating Tlist inside a Class definition [duplicate] - class

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I have made a Class representing a 2D array of strings using a type Tlist defined as
type
T2DimensionalStringlist = Tlist<Tstringlist>;
TwoDstringlistClass = class
private
TwoDimStringlist : T2DimensionalStringlist;
etc.
However I get an access violation when I call the constructor. All the constructor does is create the list using
TwoDimStringlist:= T2DimensionalStringlist.Create;
I haven't made many classes yet but the the full code of this one is at the bottom of this post.
Please can someone tell me what I have done wrong that would cause the access violation in the class constructor?
What I have tried
I've looked at this SO thread but it didn't really answer my question.
I've also tried turning the whole thing into a simple unit instead of a class and defining the list type and variable as globals eg
type
T2DimensionalStringlist = Tlist<Tstringlist>;
var
TwoDimStringlist: T2DimensionalStringlist ;
Then, if I create the list in the initialization with
TwoDimStringlist:= T2DimensionalStringlist.Create;
and free it in the finalization with
for i := 0 to TwoDimStringlist.Count - 1 do
TwoDimStringlist[i].free;
TwoDimStringlist.free;
then it works fine and I can use it as a 2D array of strings with various other procedures and functions like those in the class unit.
I've also tried moving the
type
T2DimensionalStringlist = Tlist<Tstringlist>;
to inside the class instead of outside but still got the error.
Here is the full code of the class
unit U_TwoDstringlist;
interface
uses
classes, //for Tstringlist
Generics.Collections; //for Tlist
type
T2DimensionalStringlist = Tlist<Tstringlist>;
TwoDstringlistClass = class
private
TwoDimStringlist : T2DimensionalStringlist;
public
function AddStringlist : integer; //returns index of newnly created stringlist
function AddString(ListIndex : integer; S : string) : boolean; //adds a string to the list at index ListIndex
function GetStringlist(ListIndex : integer): Tstringlist; //retuns the stringlist from index ListIndex
function ListCount : integer ; //returns number of used elements
function StringlistCount(ListIndex : integer) : integer ; //returns number of used elements in a particular stringlist
function GetString(ListIndex,StringlistIndex : integer) : string; //returns string at 2D index ListIndex,StringlistIndex
constructor Create; overload;
destructor Destroy; override;
end; //class
implementation
constructor TwoDstringlistClass.Create;
begin
TwoDimStringlist := T2DimensionalStringlist.Create;
end;
destructor TwoDstringlistClass.Destroy;
var
i : integer;
begin
for i := 0 to TwoDimStringlist.Count - 1 do
TwoDimStringlist[i].free;
TwoDimStringlist.free;
inherited;
end;
function TwoDstringlistClass.AddString(ListIndex: integer; S: string): boolean;
begin
try
TwoDimStringlist[ListIndex].add(s) ;
result := true;
except
result := false;
end;
end;
function TwoDstringlistClass.AddStringlist: integer;
begin
TwoDimStringlist.add(Tstringlist.create);
result := TwoDimStringlist.count -1;
end;
function TwoDstringlistClass.GetString(ListIndex, StringlistIndex: integer): string;
begin
try
result := TwoDimStringlist[ListIndex][StringlistIndex];
except
result := '';
end;
end;
function TwoDstringlistClass.GetStringlist(ListIndex: integer): Tstringlist;
begin
try
result := TwoDimStringlist[ListIndex] ;
except
result := nil;
end;
end;
function TwoDstringlistClass.ListCount: integer;
begin
result := TwoDimStringlist.count;
end;
function TwoDstringlistClass.StringlistCount(ListIndex: integer): integer;
begin
try
result := TwoDimStringlist[ListIndex].count
except
result := -1
end;
end;
end.
and the main form uses this class like this
implementation
var
My2dList : TwoDstringlistClass;
begin
procedure TForm1.btnCreateClick(Sender: TObject);
begin
My2dList.create;
end;
etc.