When reading source code which draws lines in Windows using GDI it is relatively common to see FillRect() being used despite the only purpose being to draw a line. But the end product of drawing a line with a width value and a filled rectangle are quite similar aren't they?
FillRect() is 1 function call, Using MoveToEx & LineTo requires 2
Which is more efficient when needing to draw a line, using FillRect() or MoveToEx() and LineTo()?
In the most common cases, FillRect will do the same thing as MoveToEx and LineTo for perfectly horizontal and vertical rectangles. Nowadays, there are so many layers of indirection between GDI and the screen that the performance difference is almost certainly not relevant.
Drawing operations in GDI typically depend on the current "state" of the device context (DC). Lines are drawn with whichever pen is currently selected into the DC. The pen determines the color, style (solid, dashed, etc.), thickness, end caps, etc.
FillRect, however, doesn't depend on much of the DC state. All drawing primitives depend on the mapping mode and clipping region, but, unlike lines, FillRect doesn't even depend on the selected brush, since you get to provide one right in the call.
Changing state (which objects are selected into the DC) can be a lot of work. If you know you want a horizontal line, 2 pixels thick, in blue, it's a tad easier to use FillRect than to first create a pen, select it into the DC, draw your line, select the pen back out, and then decide how to manage the lifetime of that pen (when do you delete it?). If the rest of the drawing is a bunch of dashed yellow lines with round endcaps, not having to keep switching state can make the code simpler.