AppBar using C#

Using the Code

First we must declare the needed structures and constants.
(I combine constants in enums).

We declare the RECT WINAPI structure as follows:

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

We declare the APPBARDATA SHELLAPI structure as follows:

[StructLayout(LayoutKind.Sequential)]
struct APPBARDATA
{
    public int cbSize;
    public IntPtr hWnd;
    public int uCallbackMessage;
    public int uEdge;
    public RECT rc;
    public IntPtr lParam;
}

Then, we declare the AppBar related constants enums as follows:

enum ABMsg : int
{
    ABM_NEW=0,
    ABM_REMOVE,
    ABM_QUERYPOS,
    ABM_SETPOS,
    ABM_GETSTATE,
    ABM_GETTASKBARPOS,
    ABM_ACTIVATE,
    ABM_GETAUTOHIDEBAR,
    ABM_SETAUTOHIDEBAR,
    ABM_WINDOWPOSCHANGED,
    ABM_SETSTATE
}
enum ABNotify : int
{
    ABN_STATECHANGE=0,
    ABN_POSCHANGED,
    ABN_FULLSCREENAPP,
    ABN_WINDOWARRANGE
}
enum ABEdge : int
{
    ABE_LEFT=0,
    ABE_TOP,
    ABE_RIGHT,
    ABE_BOTTOM
}

Next we declare the needed WIN32 and SHELL API functions import:

[DllImport("SHELL32", CallingConvention=CallingConvention.StdCall)]
static extern uint SHAppBarMessage(int dwMessage, ref APPBARDATA pData);
[DllImport("USER32")]
static extern int GetSystemMetrics(int Index);
[DllImport("User32.dll", ExactSpelling=true, 
    CharSet=System.Runtime.InteropServices.CharSet.Auto)]
private static extern bool MoveWindow
    (IntPtr hWnd, int x, int y, int cx, int cy, bool repaint);
[DllImport("User32.dll", CharSet=CharSet.Auto)]
private static extern int RegisterWindowMessage(string msg);

The next step is creating a function to register AppBar:

private void RegisterBar()
{
    APPBARDATA abd = new APPBARDATA();
    abd.cbSize = Marshal.SizeOf(abd);
    abd.hWnd = this.Handle;
    if (!fBarRegistered)
    {
        uCallBack = RegisterWindowMessage("AppBarMessage");
        abd.uCallbackMessage = uCallBack;

        uint ret = SHAppBarMessage((int)ABMsg.ABM_NEW, ref abd);
    fBarRegistered = true;

    ABSetPos();
    }
    else
    {
    SHAppBarMessage((int)ABMsg.ABM_REMOVE, ref abd);
    fBarRegistered = false;
    }
}

The last step is to create a function to position AppBar:

private void ABSetPos()
{
    APPBARDATA abd = new APPBARDATA();
    abd.cbSize = Marshal.SizeOf(abd);
    abd.hWnd = this.Handle;
    abd.uEdge = (int)ABEdge.ABE_TOP;

    if (abd.uEdge == (int)ABEdge.ABE_LEFT || abd.uEdge == (int)ABEdge.ABE_RIGHT) 
    {
    abd.rc.top = 0;
    abd.rc.bottom = SystemInformation.PrimaryMonitorSize.Height;
    if (abd.uEdge == (int)ABEdge.ABE_LEFT) 
    {
        abd.rc.left = 0;
        abd.rc.right = Size.Width;
    }
    else 
    {
        abd.rc.right = SystemInformation.PrimaryMonitorSize.Width;
        abd.rc.left = abd.rc.right - Size.Width;
    }

    }
    else 
    {
    abd.rc.left = 0;
    abd.rc.right = SystemInformation.PrimaryMonitorSize.Width;
    if (abd.uEdge == (int)ABEdge.ABE_TOP) 
    {
        abd.rc.top = 0;
        abd.rc.bottom = Size.Height;
    }
    else 
    {
        abd.rc.bottom = SystemInformation.PrimaryMonitorSize.Height;
        abd.rc.top = abd.rc.bottom - Size.Height;
    }
    }

    SHAppBarMessage((int)ABMsg.ABM_QUERYPOS, ref abd); 

    switch (abd.uEdge) 
    { 
    case (int)ABEdge.ABE_LEFT: 
        abd.rc.right = abd.rc.left + Size.Width;
        break; 
    case (int)ABEdge.ABE_RIGHT: 
        abd.rc.left= abd.rc.right - Size.Width;
        break; 
    case (int)ABEdge.ABE_TOP: 
        abd.rc.bottom = abd.rc.top + Size.Height;
        break; 
    case (int)ABEdge.ABE_BOTTOM: 
        abd.rc.top = abd.rc.bottom - Size.Height;
        break; 
    }

    SHAppBarMessage((int)ABMsg.ABM_SETPOS, ref abd); 
    MoveWindow(abd.hWnd, abd.rc.left, abd.rc.top, 
            abd.rc.right - abd.rc.left, abd.rc.bottom - abd.rc.top, true); 
}

This is a full listing of a sample AppBar form class:

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Runtime.InteropServices;

namespace Sample.AppBar
{
    /// <summary>
    /// Summary description for Form1.
    /// </summary>
    public class MainForm : System.Windows.Forms.Form
    {
        /// <summary>
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.Container components = null;

        public MainForm()
        {
            //
            // Required for Windows Form Designer support
            //
            InitializeComponent();

            //
            // TODO: Add any constructor code after InitializeComponent call
            //
        }

        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        protected override void Dispose( bool disposing )
        {
            if( disposing )
            {
                if (components != null) 
                {
                    components.Dispose();
                }
            }
            base.Dispose( disposing );
        }

        #region Windows Form Designer generated code
        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            // 
            // MainForm
            // 
            this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
            this.ClientSize = new System.Drawing.Size(960, 50);
            this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedToolWindow;
            this.Name = "MainForm";
            this.Text = "AppBar";
            this.Closing += new System.ComponentModel.CancelEventHandler(this.OnClosing);
            this.Load += new System.EventHandler(this.OnLoad);
        }
        #endregion

        #region APPBAR

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

        [StructLayout(LayoutKind.Sequential)]
        struct APPBARDATA
        {
            public int cbSize;
            public IntPtr hWnd;
            public int uCallbackMessage;
            public int uEdge;
            public RECT rc;
            public IntPtr lParam;
        }

        enum ABMsg : int
        {
            ABM_NEW=0,
            ABM_REMOVE=1,
            ABM_QUERYPOS=2,
            ABM_SETPOS=3,
            ABM_GETSTATE=4,
            ABM_GETTASKBARPOS=5,
            ABM_ACTIVATE=6,
            ABM_GETAUTOHIDEBAR=7,
            ABM_SETAUTOHIDEBAR=8,
            ABM_WINDOWPOSCHANGED=9,
            ABM_SETSTATE=10
        }

        enum ABNotify : int
        {
            ABN_STATECHANGE=0,
            ABN_POSCHANGED,
            ABN_FULLSCREENAPP,
            ABN_WINDOWARRANGE
        }

        enum ABEdge : int
        {
            ABE_LEFT=0,
            ABE_TOP,
            ABE_RIGHT,
            ABE_BOTTOM
        }

        private bool fBarRegistered = false;

        [DllImport("SHELL32", CallingConvention=CallingConvention.StdCall)]
        static extern uint SHAppBarMessage(int dwMessage, ref APPBARDATA pData);
        [DllImport("USER32")]
        static extern int GetSystemMetrics(int Index);
        [DllImport("User32.dll", ExactSpelling=true, 
            CharSet=System.Runtime.InteropServices.CharSet.Auto)]
        private static extern bool MoveWindow
            (IntPtr hWnd, int x, int y, int cx, int cy, bool repaint);
        [DllImport("User32.dll", CharSet=CharSet.Auto)]
        private static extern int RegisterWindowMessage(string msg);
        private int uCallBack;

        private void RegisterBar()
        {
            APPBARDATA abd = new APPBARDATA();
            abd.cbSize = Marshal.SizeOf(abd);
            abd.hWnd = this.Handle;
            if (!fBarRegistered)
            {
                uCallBack = RegisterWindowMessage("AppBarMessage");
                abd.uCallbackMessage = uCallBack;

                uint ret = SHAppBarMessage((int)ABMsg.ABM_NEW, ref abd);
                fBarRegistered = true;

                ABSetPos();
            }
            else
            {
                SHAppBarMessage((int)ABMsg.ABM_REMOVE, ref abd);
                fBarRegistered = false;
            }
        }

        private void ABSetPos()
        {
            APPBARDATA abd = new APPBARDATA();
            abd.cbSize = Marshal.SizeOf(abd);
            abd.hWnd = this.Handle;
            abd.uEdge = (int)ABEdge.ABE_TOP;

            if (abd.uEdge == (int)ABEdge.ABE_LEFT || abd.uEdge == (int)ABEdge.ABE_RIGHT) 
            {
                abd.rc.top = 0;
                abd.rc.bottom = SystemInformation.PrimaryMonitorSize.Height;
                if (abd.uEdge == (int)ABEdge.ABE_LEFT) 
                {
                    abd.rc.left = 0;
                    abd.rc.right = Size.Width;
                }
                else 
                {
                    abd.rc.right = SystemInformation.PrimaryMonitorSize.Width;
                    abd.rc.left = abd.rc.right - Size.Width;
                }

            }
            else 
            {
                abd.rc.left = 0;
                abd.rc.right = SystemInformation.PrimaryMonitorSize.Width;
                if (abd.uEdge == (int)ABEdge.ABE_TOP) 
                {
                    abd.rc.top = 0;
                    abd.rc.bottom = Size.Height;
                }
                else 
                {
                    abd.rc.bottom = SystemInformation.PrimaryMonitorSize.Height;
                    abd.rc.top = abd.rc.bottom - Size.Height;
                }
            }

            // Query the system for an approved size and position. 
            SHAppBarMessage((int)ABMsg.ABM_QUERYPOS, ref abd); 

            // Adjust the rectangle, depending on the edge to which the 
            // appbar is anchored. 
            switch (abd.uEdge) 
            { 
                case (int)ABEdge.ABE_LEFT: 
                    abd.rc.right = abd.rc.left + Size.Width;
                    break; 
                case (int)ABEdge.ABE_RIGHT: 
                    abd.rc.left= abd.rc.right - Size.Width;
                    break; 
                case (int)ABEdge.ABE_TOP: 
                    abd.rc.bottom = abd.rc.top + Size.Height;
                    break; 
                case (int)ABEdge.ABE_BOTTOM: 
                    abd.rc.top = abd.rc.bottom - Size.Height;
                    break; 
            }

            // Pass the final bounding rectangle to the system. 
            SHAppBarMessage((int)ABMsg.ABM_SETPOS, ref abd); 

            // Move and size the appbar so that it conforms to the 
            // bounding rectangle passed to the system. 
            MoveWindow(abd.hWnd, abd.rc.left, abd.rc.top, 
                abd.rc.right - abd.rc.left, abd.rc.bottom - abd.rc.top, true); 
        }

        protected override void WndProc(ref System.Windows.Forms.Message m)
        {
            if (m.Msg == uCallBack)
            {
                switch(m.WParam.ToInt32())
                {
                    case (int)ABNotify.ABN_POSCHANGED:
                        ABSetPos();
                        break;
                }
            }

            base.WndProc(ref m);
        }

        protected override System.Windows.Forms.CreateParams CreateParams
        {
            get
            {
                CreateParams cp = base.CreateParams;
                cp.Style &= (~0x00C00000); // WS_CAPTION
                cp.Style &= (~0x00800000); // WS_BORDER
                cp.ExStyle = 0x00000080 | 0x00000008; // WS_EX_TOOLWINDOW | WS_EX_TOPMOST
                return cp;
            }
        }

        #endregion

        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main() 
        {
            Application.Run(new MainForm());
        }

        private void OnLoad(object sender, System.EventArgs e)
        {
            RegisterBar();
        }

        private void OnClosing(object sender, System.ComponentModel.CancelEventArgs e)
        {
            RegisterBar();
        }
    }
}

For more information about creating AppBars, refer to the MSDN.

P.S.

Post in comments and questions about code parts. I plan to update the article descriptions based on your questions.