public partial class A : Form
    {
        public Form1()
        {
            InitializeComponent();
            ShowAnotherForm();
        }

        private void ShowAnotherForm()
        {
            Form B = new Form();
            Label load = new Label();
            load.Text = "加载中...";
            B.Controls.Add(load);
            B.ShowDialog(this);
        }
    }