Each object is a "GDI Object" (of type HGDIOBJ). You use the same function, SelectObject, to choose your new object. The appropriate data type for your wanted object is just "H" + the object, for example "HPEN" or "HBRUSH". The H prefix indicates this is a Handle, which we've discussed previously. This just means you've got a pointer. What do you need to do with pointers? Yes, that's right, you need to allocate memory so it points to something, and deallocate that memory when finished. There are functions to do that, so lets look at them now.

Before using GDI objects you need to call the appropriate Create function. There are either of the form CreateXXX (e.g. CreatePen) or CreateXXXIndirect (e.g. CreateBrushIndirect). The first is pretty intuitive - you just say "someObj := CreateXXX({parameters});", and it creates the object as wanted. The "indirect" functions just require one extra step: filling out a structure. You use a LogXXX structure (e.g. LOGFONT) and fill out all its values. You then pass this structure into the CreateXXXIndirect function instead of the parameters directly. The Indirect functions are handy when there are lots of parameters in the CreateXXX version of your function.

After you've finished using your GDI object, you need to deallocate it. To do this, there's only one function you need to know about: DeleteObject. You use this for any HGDIOBJ, regardless. This means you have no excuses for forgetting this step, and will always do it in every program you make. Right? An (unnecessary) example: DeleteObject(SomeObj);

When you use a GDI function (e.g. FillRect) the DC uses the currently selected objects, as appropriate. It could use the Pen to draw a line, the Brush to fill an area, the region to define the clipping area (where drawing will take place), etc. Each of these is separate. However, you use the same function to change each attribute: SelectObject.

To change an attribute of the DC do this: Select your new object, use the DC, and select the old object again. It's important you follow this process. Other windows might use the same DC. If you don't restore the old value for your changed attribute these windows will get confused - for example, if they expect a grey brush and you change it to blue things could get messy. Luckily, SelectObject gives you the old value when you change the value, which allows you to easily restore it.

Here's a long-awaited example of using SelectObject:

var
  OldPen, NewPen: HPEN;
begin
  // Create the new pen first
  NewPen := CreatePen(PS_SOLID, 1, RGB(0,0,255));

  // Select the new pen, storing previous pen value in OldPen
  OldPen := SelectObject(SomeDC, NewPen);

  // use SomeDC somehow, e.g. LineTo
  
  // we're done - restore the old pen so things don't go wrong
  SelectObject(SomeDC, OldPen);
  DeleteObject(NewPen);
end

This code, of course, assumes you have already nabbed a DC called "SomeDC". Anyway, it demonstrates the basics. The first part is to create your new object. You then select it into the DC so any drawing functions will use your new object instead of the old one. At the same time you keep the old value. You use the DC, select the old value back again then finally clear up any created objects.

Releasing the DC
The final step is to release the DC. This follows the usual process - if you create something, you delete it. If you get something, you release it. Anyway, to release the DC you use the function ReleaseDC. Make sure you do this, because you want to be nice to Windows.

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