DropDownList Having CheckBoxes

<table>
      <tr>
            <td valign="top" style="width: 165px">
                  <asp:PlaceHolder ID="phDDLCHK" runat="server"></asp:PlaceHolder>
            </td>
            <td valign="top">
                  <asp:Button ID="btn" runat="server" Text="Get Checked" OnClick="btn_Click" />
            </td>
            <td valign="top">
                  <asp:Label ID="lblSelectedItem" runat="server"></asp:Label>
            </td>
      </tr></table><asp:HiddenField ID="hidList" runat="server" />

Step 2: Add below lines of code on page load.

protected void Page_Load(object sender, EventArgs e)
{
      DropDownList ddl = new DropDownList();
      ddl.ID = "ddlChkList";
      ListItem lstItem = new ListItem();
      ddl.Items.Insert(0, lstItem);
      ddl.Width = new Unit(155);
      ddl.Attributes.Add("onmousedown", "showdivonClick()");
      CheckBoxList chkBxLst = new CheckBoxList();
      chkBxLst.ID = "chkLstItem";
      chkBxLst.Attributes.Add("onmouseover", "showdiv()");
      DataTable dtListItem = GetListItem();
      int rowNo = dtListItem.Rows.Count;
      string lstValue = string.Empty;
      string lstID = string.Empty;
      for (int i = 0; i < rowNo - 1; i++)
      {
            lstValue = dtListItem.Rows[i]["Value"].ToString();
            lstID = dtListItem.Rows[i]["ID"].ToString();
            lstItem = new ListItem("<a href=\"javascript:void(0)\" id=\"alst\" style=\"text-decoration:none;color:Black; \" onclick=\"getSelectedItem(' " + lstValue + "','" + i + "','" + lstID + "','anchor');\">" + lstValue + "</a>", dtListItem.Rows[i]["ID"].ToString());
            lstItem.Attributes.Add("onclick", "getSelectedItem('" + lstValue + "','" + i + "','" + lstID + "','listItem');");
            chkBxLst.Items.Add(lstItem);
      }
      System.Web.UI.HtmlControls.HtmlGenericControl div = new System.Web.UI.HtmlControls.HtmlGenericControl("div");
      div.ID = "divChkList";
      div.Controls.Add(chkBxLst);
      div.Style.Add("border", "black 1px solid");
      div.Style.Add("width", "160px");
      div.Style.Add("height", "180px");
      div.Style.Add("overflow", "AUTO");
      div.Style.Add("display", "none");
      phDDLCHK.Controls.Add(ddl);
      phDDLCHK.Controls.Add(div);
}

Step 3:  
Place below javascript method in the aspx page  or you can place it in .js file and include it in the page.
showdiv method will be called on mouse over of div.
showdivonClick will be invoked on click of dropdownlist.
getSelectedItem will be called on click of checkbox and anchor.
check will be call on any click on the page basically it is used to hide the div on click of any where on the page apart from div.

<script language="javascript" type="text/javascript">
      function showdiv() {
            document.getElementById("divChkList").style.display = "block";
      }
      function showdivonClick() {
            var objDLL = document.getElementById("divChkList");
            if (objDLL.style.display == "block")
                  objDLL.style.display = "none";
            else
                  objDLL.style.display = "block";
      }      function getSelectedItem(lstValue, lstNo, lstID, ctrlType) {
            var noItemChecked = 0;
            var ddlReport = document.getElementById("ddlChkList");
            var selectedItems = "";
            var arr = document.getElementById("chkLstItem").getElementsByTagName('input');
            var arrlbl = document.getElementById("chkLstItem").getElementsByTagName('label');
            var objLstId = document.getElementById('hidList');
            for (i = 0; i < arr.length; i++) {
                  checkbox = arr[i];
                  if (i == lstNo) {
                        if (ctrlType == 'anchor') {
                              if (!checkbox.checked) {
                                    checkbox.checked = true;
                              }
                              else {
                                    checkbox.checked = false;
                              }
                        }
                  }
                  if (checkbox.checked) {
                        if (selectedItems == "") {
                              selectedItems = arrlbl[i].innerText;
                        }
                        else {
                              selectedItems = selectedItems + "," + arrlbl[i].innerText;
                        }
                        noItemChecked = noItemChecked + 1;
                  }
            }
            ddlReport.title = selectedItems;
            var Text = ddlReport.options[ddlReport.selectedIndex].text;
            if (noItemChecked == 1)
                  ddlReport.options[ddlReport.selectedIndex].text = lstValue;
            else
                  ddlReport.options[ddlReport.selectedIndex].text = noItemChecked + " Items";
            document.getElementById('hidList').value = ddlReport.options[ddlReport.selectedIndex].text;
      } 
      document.onclick = check;
      function check(e) {
            var target = (e && e.target) || (event && event.srcElement);
            var obj = document.getElementById('divChkList');
            var obj1 = document.getElementById('ddlChkList');
            if (target.id != "alst" && !target.id.match("chkLstItem")) {
                  if (!(target == obj || target == obj1)) {
                        obj.style.display = 'none'
                  }
                  else if (target == obj || target == obj1) {
                        if (obj.style.display == 'block') {
                              obj.style.display = 'block';
                        }
                        else {
                              obj.style.display = 'none';
                              document.getElementById('ddlChkList').blur();
                        }
                  }
            }
      }
</script> 

Step 4: btn_Click method will be used to get to get the selected checkbox status in the dropdownlist.

protected void btn_Click(object sender, EventArgs e)
{
      string strSelectedItem = string.Empty;
      CheckBoxList chk = (CheckBoxList)phDDLCHK.FindControl("chkLstItem");
      DropDownList ddl = (DropDownList)Page.FindControl("ddlChkList");
      for (int i = 0; i < chk.Items.Count; i++)
      {
            if (chk.Items[i].Selected)
            {
                  if (strSelectedItem.Length == 0)
                  {
                        strSelectedItem = chk.Items[i].Selected.ToString();
                  }
                  else
                  {
                        strSelectedItem = strSelectedItem + "," + chk.Items[i].Selected.ToString();
                  }
            }
      }
      ddl.Items.Clear();
      ddl.Items.Add(new ListItem(hidList.Value));
      lblSelectedItem.Text = strSelectedItem;
}
Step 5: Now add a method to get datatable which will be bind to checkboxlist.

public DataTable GetListItem()
{
      DataTable table = new DataTable();
      table.Columns.Add("ID", typeof(int));
      table.Columns.Add("Value", typeof(string));
      table.Rows.Add(1, "ListItem1");
      table.Rows.Add(2, "ListItem2");
      table.Rows.Add(3, "ListItem3");
      table.Rows.Add(4, "My ListItem Wraps also");
      table.Rows.Add(5, "My New ListItem5");
      table.Rows.Add(6, "ListItem6");
      table.Rows.Add(7, "ListItem7");
      table.Rows.Add(8, "ListItem8");
      return table;
}