Question

I’ve created new form instances like this:

var
  ClassToUse: TFormClass;

begin  
  ClassToUse := TfrmMyForm; //CREATED AT DESIGN TIME
.
.
  NewForm := ClassTouse.Create(NewTab); //NewTab is an instance of a tab
.
end;

The ABOVE code works fine.

But now I want to send a form as a string to a procedure that would create that form. Thus I changed the code to the following:

I’ve created new form instances like this:

var
  ClassToUse: TFormClass;

begin  
  ClassToUse := GetClass(pFormName); //pFormName is a string -- ERROR IS HERE!!
.
.
  NewForm := ClassTouse.Create(NewTab);  
.
end;

This gives the following error:

Error: Incompatible types: got "TPersistentClass" expected "TFormClass"

Perhaps I’m on the wrong page… What is the right way of implementing this?

Thanks!

Answer 1

The GetClass function returns TPersistentClass. You cannot assign one of those to a TFormClass. For exactly the same reason why you cannot assign a TPersistent reference to a TForm reference.

You can change the code to be like so:

var
  RegisteredClass: TPersistentClass;
begin  
  RegisteredClass := GetClass(pFormName);
  if RegisteredClass=nil then
    raise ESomeException.Create(...);
  if not RegisteredClass.InheritsFrom(TForm) then
    raise ESomeException.Create(...);
  NewForm := TFormClass(RegisteredClass).Create(NewTab);  
end;