Function to load PNG Image from ResourceStream returns nothing - image

Question

In Delphi 10.1 Berlin, I'm trying to change a picture on a form by loading a PNG image from a resource.
I've followed this:
Load image from embedded resource
and used a TWICImage to automatically handle different possible image formats.
In this case I specifically want to use a PNG for transparency.
For some reason the function I've created returns nothing.
However, if I call result.savetofile('test.png') within the function the resource is succesfully saved, which verifies that the resource exists in the EXE and has been found.
function LoadImageResource(NativeInstance: NativeUInt; ImageResource: string): TWICImage;
var
Strm: TResourceStream;
WICImage: TWICImage;
begin
Strm := TResourceStream.Create(NativeInstance, ImageResource, RT_RCDATA);
try
Strm.Position := 0;
WICImage := TWICImage.Create;
try
WICImage.LoadFromStream(Strm);
result := WICImage; //these return empty
result.savetofile('test.png'); //this succesfully saves the resource to disk
finally
WICImage.Free;
end;
finally
Strm.Free;
end;
end;
Outside of the function, if I attempt to assign the image by calling for example Image1.picture.assign(LoadFromResource(...)) or Image1.picture.graphic := LoadFromResource(...) nothing gets assigned. And If I then call Image1.savetofile('test.png') I get an access violation error.
What might I be missing?

Answer 1

The problem is that you are destroying the image that you return. It's important to understand that classes are reference types in Delphi. So after the assignment to Result, in your code, you still have only a single instance, but two references to that same single instance.
You need to remove the call to Free.
function LoadImageResource(Module: NativeUInt; const ResName: string): TWICImage;
var
Strm: TResourceStream;
begin
Strm := TResourceStream.Create(Module, ResName, RT_RCDATA);
try
Result := TWICImage.Create;
Result.LoadFromStream(Strm);
finally
Strm.Free;
end;
end;
A little tweak is needed to make the function exception safe:
function LoadImageResource(Module: NativeUInt; const ResName: string): TWICImage;
var
Strm: TResourceStream;
begin
Strm := TResourceStream.Create(Module, ResName, RT_RCDATA);
try
Result := TWICImage.Create;
try
Result.LoadFromStream(Strm);
except
Result.Free;
raise;
end;
finally
Strm.Free;
end;
end;
When you call the function it behaves like a constructor. It either succeeds and returns a new instance, handing over ownership to the caller. Or it raises an exception. Accordingly I would name the function CreateImageFromResource.