Not getting desired output

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C#5.0

ASP.NET4

dear sir i haave the code for inserting data into data in asp.net c# 4.5


here 's the code:
--------------------------------

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Data;
using System.Data.SqlClient;
using System.Web.UI.WebControls;

public partial class Register : System.Web.UI.Page
{
    SqlConnection con;
    SqlCommand command;
    SqlDataAdapter sda;
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void txtsubmit_Click(object sender, EventArgs e)
    {
        string register_id = txtReg_id.Text;
        string name = txtName.Text;
        string contact = txtcontact.Text;

        con = new SqlConnection("Data Source=TUSHAR\\SQL;Initial Catalog=clrroom;Integrated Security=True");
        con.Open();
        try
        {
            string insertq = "insert into Register(Registration_id, Name,Contact,Role)"+ "values(@Registration_id,@Name,@Contact,@Role)";
            command = new SqlCommand(insertq,con);
            command.Parameters.Add("@Registration_id",SqlDbType.VarChar).Value=txtReg_id.Text.Trim();
            command.Parameters.Add("@Name",SqlDbType.VarChar).Value=txtName.Text.Trim();
            command.Parameters.Add("@contact",SqlDbType.VarChar).Value=txtcontact.Text.Trim();
            command.Parameters.Add("@Role",SqlDbType.VarChar).Value= drdpRole.SelectedItem;
            command.ExecuteNonQuery();

        }
        catch(Exception ex_)
        {
            Response.Write("Error occured"+ ex_.Message + "");
        }
        finally
        {
            con.Close();
        }

    }
}

What I have tried:

sir
i have ignored it after first execution of this page but shows this message ="Error occuredFailed to convert parameter value from a ListItem to a String."