BackgroundBuilder/Services/TaskbarService.cs

using System;
using System.Runtime.InteropServices;

namespace BackgroundBuilder.Services
{
    /// 
    /// Implementation of  using P/Invoke (SHAppBarMessage).
    /// 
    public class TaskbarService : ITaskbarService
    {
        #region WinAPI
        [DllImport("shell32.dll")]
        private static extern IntPtr SHAppBarMessage(uint msg, ref APPBARDATA data);
        [StructLayout(LayoutKind.Sequential)]
        private struct APPBARDATA
        {
            public uint cbSize;
            public IntPtr hWnd;
            public uint uCallbackMessage;
            public uint uEdge;
            public RECT rc;
            public IntPtr lParam;
        }

        [StructLayout(LayoutKind.Sequential)]
        private struct RECT { public int left, top, right, bottom; }

        private const uint ABM_GETTASKBARPOS = 5;
        #endregion

        public int GetTaskbarSize()
        {
            var data = new APPBARDATA { cbSize = (uint)Marshal.SizeOf<APPBARDATA>() };
            if (SHAppBarMessage(ABM_GETTASKBARPOS, ref data) == IntPtr.Zero)
                throw new InvalidOperationException("Failed to retrieve taskbar position.");

            int thickness = IsHorizontal()
                ? Math.Abs(data.rc.bottom - data.rc.top)
                : Math.Abs(data.rc.right - data.rc.left);

            return thickness;
        }

        public bool IsHorizontal()
        {
            var data = new APPBARDATA { cbSize = (uint)Marshal.SizeOf<APPBARDATA>() };
            SHAppBarMessage(ABM_GETTASKBARPOS, ref data);
            int height = Math.Abs(data.rc.bottom - data.rc.top);
            int width = Math.Abs(data.rc.right - data.rc.left);

            return width > height;
        }
    }
}