How to fix Access Violation at address in Delphi - oop

Question

I'm new to delphi and am trying to start OOP. However I get an access violation when using public property to set a private field.
type
User = class;
TData = class
private
CurrUser: User;
Connection: TFDConnection;
Query: TFDQuery;
procedure SetUser(newUser: User);
procedure SetConnection(newConn: TFDConnection);
procedure SetQuery(newQry: TFDQuery);
public
property CUser: User read CurrUser write SetUser;
property Conn: TFDConnection read Connection write SetConnection;
property Qry: TFDQuery read Query write SetQuery;
class procedure Login(uID: integer); static;
class procedure Logout(uID: integer); static;
class procedure ExitApp(); static;
end;
implementation
{$R *.fmx}
procedure TData.SetUser(newUser: User);
begin
CurrUser := newUser;
end;
procedure TData.SetConnection(newConn: TFDConnection);
begin
Connection := newConn;
end;
procedure TData.SetQuery(newQry: TFDQuery);
begin
Query := newQry;
end;
I expect to be able to set the Connection using that property however it gives me the access violation with any code that uses the property write:
TData.Conn.LoginPrompt := False;
TData.Conn.Connected := True;
var
TData: frmData.TData;
LoginForm: TLoginForm;
ErrorCount : integer;
implementation
{$R *.fmx}
procedure TLoginForm.ExitAppButtonClick(Sender: TObject);
begin
TData.ExitApp;
end;
procedure TLoginForm.LoginButtonClick(Sender: TObject);
var
companyPath : string;
nurseID : integer;
begin
if(UsernameInput.Text = '') or (PasswordInput.Text = '') or (PincodeInput.Text = '') then
begin
ShowMessage('Please enter your login details.');
Exit;
end;
try
TData.Conn := TFDConnection.Create(nil);
TData.Conn.Params.DriverID := 'MSAcc';
TData.Conn.Params.Database := 'D:\PulseDB\AlfaPersonnel\Pulse.mdb';
TData.Conn.LoginPrompt := False;
TData.Conn.Connected := True;
if(TData.Conn.Connected <> True) then
begin
ShowMessage('Could not connect, try again');
Exit;
end
else //When Connection
begin
TData.Qry := TFDQuery.Create(TData.Conn);
try
TData.Qry.Connection := TData.Conn;
TData.Qry.SQL.Text := 'SELECT * FROM NurseLogin WHERE Username=:uname AND Password=:pword AND PinCode=:pin;';
TData.Qry.Params.ParamByName('uname').AsString := UsernameInput.Text;
TData.Qry.Params.ParamByName('pword').AsString := PasswordInput.Text;
TData.Qry.Params.ParamByName('pin').AsString := PincodeInput.Text;
TData.Qry.Active := True;
if TData.Qry.RecordCount = 0 then ShowMessage('Details not recognised.')
else if TData.Qry.RecordCount = 1 then
begin
if TData.Qry.FieldByName('IsActive').AsBoolean then //If the user is active
begin
try
//Connect to the path
companyPath := TData.Qry.FieldByName('CompanyName').AsString;
TData.Conn.Params.Database := 'D\PulseDB\' + companyPath + '\Pulse.mdb';
TData.Conn.Connected := True;
ShowMessage('Connected to ' + companyPath);
finally
end;
end;
end;
finally
end;
end;
finally
end;
end;

Answer 1

You don't ever create an instance of your TData class. At some point you need to write:
TData := frmData.TData.Create;
Which is how you instantiate an instance. And you need to destroy it when you are finished also. Like this:
TData.Free;
That you did not instantiate an instance is the explanation for your access violation.
Some other issues:
Use the T prefix for types. Your variable should be named Data rather than TData.
Don't use global variables if at all possible.