An expression of non-boolean type specified in a context where a condition is expected, near 'User_name'.

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

namespace store_2
{
    public partial class regc : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

            if (IsPostBack)
            {
                SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["registrationconnectionString"].ConnectionString);
                con.Open();
                string checkuser = "select count(*)from the customertable where the User_name='" + TextBoxusername.Text + "'";
                SqlCommand com = new SqlCommand(checkuser, con);
                int temp = Convert.ToInt32(com.ExecuteScalar().ToString());
                if (temp == 1)
                {
                    Response.Write("User alredy exixts");
                }
                con.Close();
            }
        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            try
            {
                SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["registrationconnectionString"].ConnectionString);
                con.Open();
                string insertQuery = "insert into customertable(User_name,Name,Gender,Adress,City,State,Pin_code,mobile_no,Email_id,Password,product_id,Product_Quntity) values (@User_name,@Name,@Gender,@Adress,@City,@State,@Pin_code,@mobile_no,@Email_id,@Password,@product_id,@Product_Quntity)";
                SqlCommand com = new SqlCommand(insertQuery, con);
                com.Parameters.AddWithValue("@User_name", TextBoxusername.Text);
                com.Parameters.AddWithValue("@Name", TextBoxname.Text);
                com.Parameters.AddWithValue("@Gender", TextBoxname.Text);
                com.Parameters.AddWithValue("@Adress", TextBoxadress.Text);
                com.Parameters.AddWithValue("@City", DropDownListcity.SelectedItem.ToString());
                com.Parameters.AddWithValue("@State", DropDownListstate.SelectedItem.ToString());
                com.Parameters.AddWithValue("@Pin_code", TextBoxpincode.Text);
                com.Parameters.AddWithValue("@mobile_no", TextBoxmobno.Text);
                com.Parameters.AddWithValue("@Email_id", TextBoxemail.Text);
                com.Parameters.AddWithValue("@Password", TextBoxpassword.Text);

                com.ExecuteNonQuery();
                Response.Redirect("admin.aspx");
                Response.Write("Registration Is Successful");
                con.Close();
            }
            catch (Exception ex)
            {
                Response.Write("Error:" + ex.ToString());
            }
        }
    }
}