hello expert, i have three dropdownlist option consist of two string and one datetime that convert into string. If i want to choose first and second dropdown without the third one which is datetime, it occur error "the string was not recognized as a valid datetime. there is an unknown word starting at index 0." but if i choose the third one itself or it combine with other dropdown list, it success. I want the user can choose the option with or without the date itself. Can you help me how to solve it ? this is my code in c#

using System;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Collections.Generic;

public partial class loginpage_searchdata : System.Web.UI.Page
{
    SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["MESTrans"].ConnectionString);
    SqlConnection con2 = new SqlConnection(ConfigurationManager.ConnectionStrings["MESCommEntities"].ConnectionString);
    SqlCommand cmd;
    SqlDataAdapter da;
    string query = "";

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            con.Open();

            query = "select jo_no,prod_line, need_by_date from CutPanelcard";
            cmd = new SqlCommand(query, con);
            SqlDataReader dr = cmd.ExecuteReader();
            while (dr.Read())
            {
                if (!string.IsNullOrEmpty(DropDownListSearchJO.SelectedValue))
                {
                    DropDownListSearchJO.Items.Add(dr[0].ToString());
                }
                if (!string.IsNullOrEmpty(DropDownListSearchLine.SelectedValue))
                {
                    DropDownListSearchLine.Items.Add(dr[1].ToString());
                }
                if (!string.IsNullOrEmpty(DropDownListSearchRec.SelectedValue))
                {
                    DropDownListSearchRec.Items.Add(dr[2].ToString());
                }
            }

            con.Close();
            gv1();
        }

    }

    private void gv1()
    {
        try
        {
            con.Open();
            query = "select * from CutPanelCard";
            cmd = new SqlCommand(query, con);
            da = new SqlDataAdapter(cmd);

            DataSet ds = new DataSet();
            da.Fill(ds);
            GridView1.DataSource = ds;
            GridView1.DataBind();

            con.Close();
        }
        catch (Exception ex)
        {
            Response.Write(ex.Message);
        }
    }

    /**  protected void DropDownListSearchJO_SelectedIndexChanged(object sender, EventArgs e)
      {
          try
          {
              con.Open();
              query = "select * from CutPanelCard where jo_no ='" + DropDownListSearchJO.SelectedItem.ToString() + "'";
              cmd = new SqlCommand(query, con);
              da = new SqlDataAdapter(cmd);

              DataSet ds = new DataSet();
              da.Fill(ds);
              GridView1.DataSource = ds;
              GridView1.DataBind();

              con.Close();
          }

          catch (Exception ex)
          {
              Response.Write(ex.Message);
          }
          GridView1.Visible = true;
      }**/

    /**protected void DropDownListSearchLine_SelectedIndexChanged(object sender, EventArgs e)
     {
         try
         {
             con.Open();
             query2 = "select * from CutPanelCard where prod_line ='" + DropDownListSearchLine.SelectedItem.ToString() + "'";
             cmd2 = new SqlCommand(query2, con);
             da2 = new SqlDataAdapter(cmd2);

             DataSet ds2 = new DataSet();
             da2.Fill(ds2);
             GridView1.DataSource = ds2;
             GridView1.DataBind();

             con.Close();
         }

         catch (Exception ex)
         {
             Response.Write(ex.Message);
         }

         GridView1.Visible = true;
     }**/

    protected void Button_Search_Click(object sender, EventArgs e)
    {
        try
        {
            con.Open();

                DataTable dt = new DataTable("UserLogin");

                    if ((!string.IsNullOrEmpty(DropDownListSearchJO.SelectedValue)) || (!string.IsNullOrEmpty(DropDownListSearchLine.SelectedValue)) || (!string.IsNullOrEmpty(DropDownListSearchRec.SelectedValue)))
                    {

                        query = "select [req_id],[prod_line] ,[jo_no],[buyer],[request_date],[need_by_date],[qty],[username] from CutPanelCard where jo_no=@jo_no OR prod_line=@prod_line OR need_by_date=@need_by_date";
                        cmd = new SqlCommand(query, con);
                        cmd.Parameters.AddWithValue("@jo_no", DropDownListSearchJO.SelectedValue);
                        cmd.Parameters.AddWithValue("@prod_line", DropDownListSearchLine.SelectedValue);
                        cmd.Parameters.AddWithValue("@need_by_date", Convert.ToDateTime(DropDownListSearchRec.SelectedValue));
                    }

                    SqlDataAdapter da = new SqlDataAdapter(cmd);
                    da.Fill(dt);
                    GridView1.DataSource = dt;
                    GridView1.DataBind();

                    con.Close();
        }

        catch (Exception ex)
        {
            Response.Write(ex.Message);
        }
        GridView1.Visible = true;
        Button_Confirm.Visible = true;
    }

    protected void Button_Confirm_Click(object sender, EventArgs e)
    {

        DataTable dt = new DataTable("CutPanelCard");
        dt.Columns.Add(new DataColumn() { ColumnName = "req_id", DataType = typeof(String) });
        dt.Columns.Add(new DataColumn() { ColumnName = "prod_line", DataType = typeof(String) });
        dt.Columns.Add(new DataColumn() { ColumnName = "jo_no", DataType = typeof(String) });
        dt.Columns.Add(new DataColumn() { ColumnName = "buyer", DataType = typeof(String) });
        dt.Columns.Add(new DataColumn() { ColumnName = "request_date", DataType = typeof(String) });
        dt.Columns.Add(new DataColumn() { ColumnName = "need_by_date", DataType = typeof(String) });
        dt.Columns.Add(new DataColumn() { ColumnName = "qty", DataType = typeof(String) });

        for (int i = 0; i < GridView1.Rows.Count; i++)
        {
            if (GridView1.Rows[i].RowType == DataControlRowType.DataRow)
            {
                CheckBox cbSelect = GridView1.Rows[i].FindControl("cbSelect") as CheckBox;
                if (cbSelect.Checked)
                {
                    DataRow dr = dt.NewRow();
                    Label lblReq = GridView1.Rows[i].FindControl("lblReq") as Label;
                    Label lblLine = GridView1.Rows[i].FindControl("lblLine") as Label;
                    Label lblJo_no = GridView1.Rows[i].FindControl("lblJo_no") as Label;
                    Label lblBuyer = GridView1.Rows[i].FindControl("lblBuyer") as Label;
                    Label lblReqDate = GridView1.Rows[i].FindControl("lblReqDate") as Label;
                    Label lblNeed_by_date = GridView1.Rows[i].FindControl("lblNeed_by_date") as Label;
                    Label lblQty = GridView1.Rows[i].FindControl("lblQty") as Label;

                    dr["req_id"] = lblReq.Text;
                    dr["prod_line"] = lblLine.Text;
                    dr["jo_no"] = lblJo_no.Text;
                    dr["buyer"] = lblBuyer.Text;
                    dr["request_date"] = lblReqDate.Text;
                    dr["need_by_date"] = lblNeed_by_date.Text;
                    dr["qty"] = lblQty.Text;

                    dt.Rows.Add(dr);  
                }

            }

        }

        Session["EmployeeData"] = dt;

        Response.Redirect("Confirmation.aspx", true);

    }
    protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
    {

    }

}

Posting 200+ lines of code for a simple problem is disrespectul of the time we spend to help you. Please show the relevant code for review.

Try

Convert.ToDateTime(DropDownListSearchRec.SelectedValue.ToString.());

Converting The value of DropDownListSearchRec to string will solve your problems

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