Get taskbar placement from MONITORINFOEX.

MonitorInformation.cs

public enum TaskbarPlacement { Unknown, Left, Top, Right, Bottom }

private TaskbarPlacement GetPlacement(MONITORINFOEX monitorInfo)
{
    var m = monitorInfo.rcMonitor;
    var w = monitorInfo.rcWork;

    return (left: (m.left == w.left),
            top: (m.top == w.top),
            right: (m.right == w.right),
            bottom: (m.bottom == w.bottom)) switch
    {
        (left: false, true, true, true) => TaskbarPlacement.Left,
        (true, top: false, true, true) => TaskbarPlacement.Top,
        (true, true, right: false, true) => TaskbarPlacement.Right,
        (true, true, true, bottom: false) => TaskbarPlacement.Bottom,
        _ => default
    };
}