When you show a .NET Form, by default the form will appear in the Windows Start bar and in the list of open windows shown when the user presses Alt+Tab.

Hide Form from Start Bar

To prevent a form from appearing in the Windows Start bar, add this statement to the form’s constructor:

this.ShowInTaskbar = false;

Hide Tool Window from Alt+Tab

To prevent a form from appearing in the list of windows shown when the user presses Alt+Tab, you can designate the form to be a tool window. Note that you can use SizableToolWindow or FixedToolWindow, and ShowInTaskbar must be set to false:

this.FormBorderStyle = FormBorderStyle.SizableToolWindow;
this.ShowInTaskbar = false;

Hide Borderless Form from Alt+Tab

However, if you want the form to be borderless, then you need to add the following statements to the form’s constructor:

this.FormBorderStyle = FormBorderStyle.None;
this.ShowInTaskbar = false;

AND you must add the following method to your derived Form class:

protected override CreateParams CreateParams
{
    get
    {
        CreateParams cp = base.CreateParams;
        // turn on WS_EX_TOOLWINDOW style bit
        cp.ExStyle |= 0x80;
        return cp;
    }
}