background value if empty

问题

I have

property Background: TPicture read FBackground write SetBackground;

if nothing is ever assiged to it, what would the value be of background?

i have tried

if Background = NULL  then

begin
...
..
...
end;

回答1:

It depends. When the field is created in the constructor and FBackground is assigned, then use:

if FBackground.Graphic = nil then

Or:

if not Assigned(FBackground.Graphic) then

---

If the Graphic is assigned, then use:

if FBackground.Graphic.Empty then

---

And if both property and field are unassigned then use:

if FBackground = nil then

Or:

if not Assigned(FBackground) then

---

All of the above combined:

if (FBackground = nil) or (FBackground.Graphic = nil) or FBackground.Graphic.Empty then

来源：https://stackoverflow.com/questions/19439202/background-value-if-empty