private void MenuItem_Click(object sender, EventArgs e)
{
   ToolStripMenuItem menuItem = (ToolStripMenuItem)sender;
   Form form = (Form)Assembly.Load(“您的程序集名称”).CreateInstance(“窗体命名空间. “+ menuItem.Name); //注意: 窗体命  名空间后面一定要加一个点
   form.MdiParent = this; //如果是Mdi窗体的话请加这一行
   form.Show();
}

private void button3_Click(object sender, EventArgs e)
    {
      //获取单击的Button名称
      string btnname = ((Button)sender).Name;
      if (btnname == "button1")
      {
        //处理窗体
      }
      else if (btnname == "button2")
      {
        //处理窗体
      }
      else
      {
        //处理窗体
      }
    }

/// <summary> 
    /// 打开新的子窗体 
    /// </summary> 
    /// <param name="strName">窗体的类名</param> 
    /// <param name="AssemblyName">窗体所在类库的名称</param>  
    public static void CreateForm(string strName, string AssemblyName)
    {
      string path = AssemblyName;//项目的Assembly选项名称 
      string name = strName; //类的名字 
      Form doc = (Form)Assembly.Load(path).CreateInstance(name);
      doc.Show();
    }

    private void button1_Click(object sender, EventArgs e)
    {
      //获取单击的Button名称
      string btnname = ((Button)sender).Text;
      CreateForm("WindowsFormsApplication1." + btnname, "WindowsFormsApplication1");
    }