Create Dynamic DropDownList

1. Create a DynamicDropDownlist.aspx and place a placeholder and two buttons with name btnCreate and btnRetrieve. 

<asp:PlaceHolder ID="PlaceHolder1" runat="server"></asp:PlaceHolder>
<asp:Button ID="btnCreate" runat="server" Text="Create DropDownList" />
<asp:Button ID="Retrieve" runat="server" Text="Button" onclick="btnRetrieve_Click" />

2. Create a class level Control variable and assign null to it.

Control cause = null;

3. Create CreateDropDownListControl method. This method will create dropdownlist, copy and paste below code.

public static Control GetPostBackControl(Page page)
{
      Control control = null;
      string ctrlname = page.Request.Params.Get("__EVENTTARGET");
      if (ctrlname != null && ctrlname != string.Empty)
      {
            control = page.FindControl(ctrlname);
      }
      else
      {
            foreach (string ctl in page.Request.Form)
            {
                  Control c = page.FindControl(ctl);
                  if (c is System.Web.UI.WebControls.Button)
                  {
                        control = c;
                        break;
                  }
            }
      }
      return control;
}

4. Create Page_Init method, copy and paste below code.

protected void Page_Init(object sender, EventArgs e)
{
      if (IsPostBack)
      {
            //Method to get the control which caused postback
            cause = GetPostBackControl(Page);
            //To make sure new dropdownlist get create on click of
create DropDownList
            if (cause != null && cause.ID.Equals("btnCreate"))
            {
                  CreateDropDownListControl();
            }
            //To re-create control in case of any postback excluding click of Create DropDownList
            else if (cause == null || !cause.ID.Equals("btnCreate"))
            {
                  CreateDropDownListControl();
            }
      }
}

5. OnSelectedIndexChanged method will return value of the selected dropdownlist index,Copy and paste below code. 

protected void OnSelectedIndexChanged(object sender, EventArgs e)
{
      DropDownList ddl = (DropDownList)sender;
      Response.Write(ddl.SelectedItem.Text);
}

6. btnRetrieve_Click method will return value of the all selected dropdownlist index,Copy and paste below code.

protected void btnRetrieve_Click(object sender, EventArgs e)
{
      int ddlCount = Convert.ToInt32(Session["Count"].ToString());
      for (int i = 0; i < ddlCount; i++)
      {
            Response.Write("Selected Dropdownlist values are " + Request.Form["ddl" + i.ToString()] + "</br>");
      }
}

Lets' put all together in DynamicDropDownlist.aspx.cs file.

using System;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class DynamicDropDownlist : System.Web.UI.Page
{
      Connrol cause = null;
      protected void Page_Init(object sender, EventArgs e)
      {
            if (IsPostBack)
            {
                  //Method to get the control which caused postback
                  cause = GetPostBackControl(Page);
                  //To make sure new dropdownlist get create on click of Create DropDownList
                  if (cause != null && cause.ID.Equals("btnCreate"))
                  {
                        CreateDropDownListControl();
                  }
                  //To re-create control in case of any postback excluding click of Create DropDownList
                  else if (cause == null || !cause.ID.Equals("btnCreate"))
                  {
                        CreateDropDownListControl();
                  }
            }
      }
      public static Control GetPostBackControl(Page page)
      {
            Control control = null;
            string ctrlname = page.Request.Params.Get("__EVENTTARGET");
            if (ctrlname != null && ctrlname != string.Empty)
            {
                  control = page.FindControl(ctrlname);
            }
            else
            {
                  foreach (string ctl in page.Request.Form)
                  {
                        Control c = page.FindControl(ctl);
                        if (c is System.Web.UI.WebControls.Button)
                        {
                              control = c;
                              break;
                        }
                  }
             }
             return control;
      }

      protected void Page_Load(object sender, EventArgs e)
      {
      }

      protected void btnRetrieve_Click(object sender, EventArgs e)
      {
            int ddlCount = Convert.ToInt32(Session["Count"].ToString());
            for (int i = 0; i < ddlCount; i++)
            {
                  Response.Write("Selected Dropdownlist values are " + Request.Form["ddl" + i.ToString()] + "</br>");
            }
      }

      protected void CreateDropDownListControl()
      {
            if (Session["Count"] == null)
            {
                  DropDownList ddl = new DropDownList();
                  ddl.ID = "ddl0";
                  ddl.AutoPostBack = true;
                  ddl.Items.Add(new ListItem("Test0", "0"));
                  ddl.Items.Add(new ListItem("Test1", "1"));
                  ddl.SelectedIndexChanged += new EventHandler
OnSelectedIndexChanged);
                  PlaceHolder1.Controls.Add(ddl);
                  Session["Count"] = "1";
            }
            else
            {
                  int ctrlCount = 0;
                  if (cause != null && cause.ID.Equals("btnCreate"))
                  {
                        ctrlCount = Convert.ToInt32(Session["Count"].ToString()) + 1;
                        Session["Count"] = (Convert.ToInt32(Session["Count"].ToString()) + 1).ToString();
                  }
                  else
                  {
                        ctrlCount = Convert.ToInt32(Session["Count"].ToString());
                  }
                  for (int i = 0; i < ctrlCount; i++)
                  {
                        DropDownList ddl = new DropDownList();
                        ddl.ID = "ddl" + i.ToString();
                        ddl.AutoPostBack = true;
                        ddl.Items.Add(new ListItem("Test" + i.ToString(), "0"));
                        ddl.Items.Add(new ListItem("Test" + (i + 1).ToString(), "1"));
                        ddl.SelectedIndexChanged += new EventHandler(OnSelectedIndexChanged);
                        PlaceHolder1.Controls.Add(ddl);
                  }
            }
      }

      protected void OnSelectedIndexChanged(object sender, EventArgs e)
      {
            DropDownList ddl = (DropDownList)sender;
            Response.Write(ddl.SelectedItem.Text);
      }

}

Happy Coding :)