Winforms - Click/drag anywhere in the form to move it as if clicked in the form caption

Microsoft KB Article 320687 has a detailed answer to this question.

Basically, you override the WndProc method to return HTCAPTION to the WM_NCHITTEST message when the point being tested is in the client area of the form -- which is, in effect, telling Windows to treat the click exactly the same as if it had occured on the caption of the form.

private const int WM_NCHITTEST = 0x84;
private const int HTCLIENT     = 0x1;
private const int HTCAPTION    = 0x2;

protected override void WndProc(ref Message m)
{
    switch(m.Msg)
    {
        case WM_NCHITTEST:
        base.WndProc(ref m);

        if ((int)m.Result == HTCLIENT)
            m.Result = (IntPtr)HTCAPTION;
        return;
    }

    base.WndProc(ref m);
}

Here is a way to do it using a P/Invoke.

public const int WM_NCLBUTTONDOWN = 0xA1;
public const int HTCAPTION = 0x2;
[DllImport("User32.dll")]
public static extern bool ReleaseCapture();
[DllImport("User32.dll")]
public static extern int SendMessage(IntPtr hWnd, int Msg, int wParam, int lParam);

void Form_Load(object sender, EventArgs e)
{
   this.MouseDown += new MouseEventHandler(Form_MouseDown);  
}

void Form_MouseDown(object sender, MouseEventArgs e)
{                        
    if (e.Button == MouseButtons.Left)
    {
        ReleaseCapture();
        SendMessage(Handle, WM_NCLBUTTONDOWN, HTCAPTION, 0);
    }
}

The following code assumes that the ProgressBarForm form has a ProgressBar control with Dock property set to Fill

public partial class ProgressBarForm : Form
{
    private bool mouseDown;
    private Point lastPos;

    public ProgressBarForm()
    {
        InitializeComponent();
    }

    private void progressBar1_MouseMove(object sender, MouseEventArgs e)
    {
        if (mouseDown)
        {
            int xoffset = MousePosition.X - lastPos.X;
            int yoffset = MousePosition.Y - lastPos.Y;
            Left += xoffset;
            Top += yoffset;
            lastPos = MousePosition;
        }
    }

    private void progressBar1_MouseDown(object sender, MouseEventArgs e)
    {
        mouseDown = true;
        lastPos = MousePosition;
    }

    private void progressBar1_MouseUp(object sender, MouseEventArgs e)
    {
        mouseDown = false;
    }
}