you are adding usercontrol in tabpage...
but you miss adding tabpage in tabcontrol1
void AddTab()
{
TabPage tbp = new TabPage();
tbp.Name = tabControl1.TabPages.Count.ToString();
UserControl1 cnt = new UserControl1();
cnt.Name = "Cnt" + tbp.Name;
cnt.Dock = DockStyle.Fill;
tbp.Controls.Add(cnt);
tabControl1.TabPages.Add(tbl);
}
Happy Coding!
:)